Automotive Air Conditioning Information Forum (Archives)

Provided by www.ACkits.com

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

Do I have a "lemon" compressor?

av8or1 on Sat October 10, 2009 1:42 AM User is offline

Year: 1985
Make: Mazda
Model: RX7
Engine Size: 1.3
Refrigerant Type: R134a
Ambient Temp: 78
Pressure Low: 70
Pressure High: 0
Country of Origin: United States

Hi Y'all,

Hmmmmmmmm....I've run into a scenario that, as a DIYer, I've never seen before. I bought an RX7 in July. 'Was told that the compressor had a bad seal, thus no working A/C. Fine. I took it off, drove to the auto parts shop, they had it looked at by a compressor rebuilding shop (that I wasn't aware existed at the time). The rebuilders said that it did have a bad seal and that they had the parts to rebuild it if I wanted or else I could buy a reman. So I opted for a rebuild for about $90 less than a "new used/rebuilt" compressor. Fine.

I get the thing re-installed, put in a new dryer and expansion valve (plus a new evap core) and vacuum the system. It holds pressure overnight. So after work today I go to put refrigerant in it. The first can appears to be going ok, seeing around 50 - 70 psi low side, high side around 50, steadily rising to 100. Compressor is engaged the whole time, isn't cycling on and off, which I was a little surprised over. Hmmmmmm....ok. Can empty, closed the two low side valves (high side valves were always closed), disconnected the can. Low side reading goes to 0. Hmmmm....ok. I was told that the compressor had some oil in it after the rebuild and to put oil in after the first can of refrigerant. Ok, so I connect up the 3 oz (2 oz oil, 1 oz R134a) can of PAG R134a compressor oil. 'Seems to be going in ok, low side pressure is only around 30, high side has decreased to around 50. Hmmmmm...ok. 'Disconnect can of oil after shutting off both low side valves. A bit of oil spurts out. Not much, but I guess the can wasn't completely empty. I'd waited, so this was a head scratcher. Maybe I hadn't waited long enough for the can to empty....hmmmmmmm....ok. So I go to hook up the second can of refrigerant and notice that the low side pressure is HIGH, like 75 or 80 psi. I look and the compressor isn't engaging. I look inside and the A/C light is on, just as it had been before. Windows down, recirc on (which equals "max A/C" in an RX7), so....what??!

After already breaking one can tap, then not being able to use that can of refrigerant, then needing to go to the parts store for a replacement can AND tap, I'd had enough for one night. I secured everything and closed up shop.

So.....I need help guys. I realize that there could be an electrical issue at work here, but before I go tearing off into that, is it possible that I somehow blew the clutch in the newly-rebuilt compressor? It doesn't seem likely, but I'm not an A/C expert, so...of course anything is possible, I understand that. 'Just doesn't seem likely. Maybe the problem all along was due to an electrical issue? Hmmmmmm...don't know about that as a possible explanation either.

Sheesh. BTW, I searched the FAQ but didn't find anything that addressed this scenario, so I decided to post. 'Hope y'all don't mind.

Thanks,

Jerry

HECAT on Sat October 10, 2009 8:33 AM User is offline

I bought an RX7 in July. 'Was told that the compressor had a bad seal, thus no working A/C. Fine. I took it off, drove to the auto parts shop,

1. Testing? It does not appear that you confirmed system functions electrically or a shaft seal leak before removal.

they had it looked at by a compressor rebuilding shop (that I wasn't aware existed at the time). The rebuilders said that it did have a bad seal and that they had the parts to rebuild it if I wanted or else I could buy a reman. So I opted for a rebuild for about $90 less than a "new used/rebuilt" compressor. Fine.

2. Rebuilt means a new shaft seal and anything else minimally needed. These can also be found at auction sites. New shaft seal, fresh paint, and amazing prices that seem too good to be true.

3. Remanufactured means a lot more time and materials go into the process to return OE like performance and specifications.

I get the thing re-installed, put in a new dryer and expansion valve (plus a new evap core) and vacuum the system. It holds pressure overnight.

4. Clean? Assuming you found the system empty from a leak. You make no mention of doing anything to try and verify or flush the waste oils and other potential contaminants such as metal debris, leak sealers, etc. Only the gas leaked out, everything else (whatever it is) remains in the system.

5. Where did the oil go? Some of it is in the compressor, condenser, filter, and hoses; and with a leaking system, a high percentage can also end up in the evap. As refrigerant levels drops the oil migration is compromised. For the most part, oil migration with liquid refrigerant (miscible) continues fairly well without compromise. But inside the the evap is where the proper charge is needed to maintain adequate gassing/boiling activity to "splash" the oil on through this loop lube circuit. As this activity diminishes within the evap the oil begins to pool.

So after work today I go to put refrigerant in it. The first can appears to be going ok, seeing around 50 - 70 psi low side, high side around 50, steadily rising to 100. Compressor is engaged the whole time, isn't cycling on and off, which I was a little surprised over.

6. Compressor runs with no charge in the system? This is not good. It should not run unless you override a functioning low pressure cut out circuit. The first can should be charged into vacuum with engine off.

Hmmmmmm....ok. Can empty, closed the two low side valves (high side valves were always closed), disconnected the can. Low side reading goes to 0.

7. A good sign that maybe the compressor is trying to "suck"

Hmmmm....ok. I was told that the compressor had some oil in it after the rebuild and to put oil in after the first can of refrigerant. Ok, so I connect up the 3 oz (2 oz oil, 1 oz R134a) can of PAG R134a compressor oil.

8. Compressor came with "some", you added 2, and there was a lot left in there. So what is the factory prescribed oil quantity, and how much do you have in there now?

'Seems to be going in ok, low side pressure is only around 30, high side has decreased to around 50. Hmmmmm...ok. 'Disconnect can of oil after shutting off both low side valves. A bit of oil spurts out. Not much, but I guess the can wasn't completely empty. I'd waited, so this was a head scratcher. Maybe I hadn't waited long enough for the can to empty....hmmmmmmm....ok. So I go to hook up the second can of refrigerant and notice that the low side pressure is HIGH, like 75 or 80 psi. I look and the compressor isn't engaging. I look inside and the A/C light is on, just as it had been before. Windows down, recirc on (which equals "max A/C" in an RX7), so....what??!

9. If the compressor is off, you may just be seeing the static pressure of the partial charge.

So.....I need help guys. I realize that there could be an electrical issue at work here, but before I go tearing off into that, is it possible that I somehow blew the clutch in the newly-rebuilt compressor?

10. Electrical testing seems appropriate at this point. "Rebuilt" compressor probably still has your original clutch that has not been operational for a while, this is why I would recommend Reman or New as you proceed.

Hope this helps, not tying to be critical; just pointing out my observations from your post.

-------------------------



HECAT: www.hecatinc.com You support the Forum when you consider www.ackits.com for your a/c parts.

FLUSHING TECHNICAL PAPER vs2.pdf 

bohica2xo on Sat October 10, 2009 10:07 AM User is offline

11. When & how was this unit convereted to 134a?

.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

av8or1 on Sat October 10, 2009 2:48 PM User is offline

Quote
Originally posted by: bohica2xo
11. When & how was this unit convereted to 134a?

In 2001, Rotary Performance, Dallas, TEXAS. That's all I know, unfortunately.

av8or1 on Sat October 10, 2009 3:08 PM User is offline

Quote
Originally posted by: HECAT
1. Testing? It does not appear that you confirmed system functions electrically or a shaft seal leak before removal.

Yes, you are correct, I should have done more testing. All I did was to press the A/C button (vehicle running), see that nothing happened (meaning that the idle didn't increase when the compressor engaged, etc.) and then verify that the system had no pressure via my set of manifold gauges. At that point I "bought the story" I was given. Hrrmph! Shoulda done some more electrical checking I suppose, my bad.

Quote
2. Rebuilt means a new shaft seal and anything else minimally needed. These can also be found at auction sites. New shaft seal, fresh paint, and amazing prices that seem too good to be true.
3. Remanufactured means a lot more time and materials go into the process to return OE like performance and specifications.


Quite possibly, yes. This outfit supposedly provides a one-year warranty with their rebuilt compressors. I called and asked what all they did to mine and the guy went off into this long schpiel of what they do on a typical rebuild, as he couldn't remember mine specifically, which I wasn't too surprised about. Although I've never had one of these critters apart, his description made sense and was more involved that simply replacing a seal, so I "bought that story" too. And it wasn't cheap at around $300, so the "too good to be true" thing doesn't seem to apply here, but what do I know? :-/

Quote
4. Clean? Assuming you found the system empty from a leak. You make no mention of doing anything to try and verify or flush the waste oils and other potential contaminants such as metal debris, leak sealers, etc. Only the gas leaked out, everything else (whatever it is) remains in the system.

That's true, all I did after getting everything hooked back up was to vacuum the system for about an hour with a standard vacuum pump.

Quote
5. Where did the oil go? Some of it is in the compressor, condenser, filter, and hoses; and with a leaking system, a high percentage can also end up in the evap. As refrigerant levels drops the oil migration is compromised. For the most part, oil migration with liquid refrigerant (miscible) continues fairly well without compromise. But inside the the evap is where the proper charge is needed to maintain adequate gassing/boiling activity to "splash" the oil on through this loop lube circuit. As this activity diminishes within the evap the oil begins to pool.

Thanks for the information regarding the evap core, I didn't know that about the evap core. Hmmmmmmm. Well again, I replaced my old one with a "new" one, though it was pulled from another RX7. I wanted to buy new, but Mazda doesn't make this part any longer (##@%^&$%&!@#&) so I had no choice but to use what I had or get this other one. It appeared newer and "less used" than mine, was cleaner and didn't have any leaks, so I went with it. Hmmmmmm.......

Quote
6. Compressor runs with no charge in the system? This is not good. It should not run unless you override a functioning low pressure cut out circuit. The first can should be charged into vacuum with engine off.

How do you do that exactly? The compressor wasn't running before I put in the first can of refrigerant. This has a receiver/dryer instead of an accumulator, so I'm not sure where the low pressure switch is gonna be located. That's next on my list of things to check.

Quote
7. A good sign that maybe the compressor is trying to "suck"

That doesn't sound good. What does that mean then?

Quote
8. Compressor came with "some", you added 2, and there was a lot left in there. So what is the factory prescribed oil quantity, and how much do you have in there now?

Well I was told that the compressor had 1 oz. I added 2. My understanding is that the system should have 5. So I was going to add another can and a half.

Quote
9. If the compressor is off, you may just be seeing the static pressure of the partial charge.

Don't understand that, sorry.

Quote
10. Electrical testing seems appropriate at this point. "Rebuilt" compressor probably still has your original clutch that has not been operational for a while, this is why I would recommend Reman or New as you proceed.

Believe me, I would have bought "new" if I could have. However two parts stores told me that it was not possible to buy "new", that it was reman only or else rebuild what I have. The second option was possible according to the rebuilder shop because they claimed that the compressor was still in good enough shape (yeah - hmmmmm) and it was a bit cheaper, though not by much.

I had planned to work on the electric side of things today and see how far I got. Or didn't. ;-)

Quote
Hope this helps, not tying to be critical; just pointing out my observations from your post.

Nah, no worries, thick skin here; I did ask the question and I DO NOT claim to be an A/C expert, just a DIYer, which probably makes the pros cringe to hear, I realize that. I ask only that you understand that in this day and age I'm trying to save a little money where I can, which is on labor. I'll put myself to work to save a bit, but I won't skimp on parts. In my mind I had done the best I could parts-wise, given the limitations posed on me.

'Preciate the help.

Jerry

HECAT on Sat October 10, 2009 9:24 PM User is offline

I missed that the evap was changed. Did you clean this used one? What about the condenser?

If I purchased a used car with seller claiming "it works, just needs a shaft seal and a charge". I would assume the worst; like the system was converted to 134a (properly?) 8 years ago, and that he possibly shot a half a dozen cans of refrigerant with leak sealers in it, before he gave up. I would make sure that all the components were either replaced or flushed clean and completely dry.

To charge first can: When the vacuum process is completed, remove the yellow hose from vacuum pump, attach can tap, purge line, and (with engine off) release the first can of refrigerant into the vacuum. Sometimes you can get enough in this way to allow for the clutch to engage; and not even have to override a low pressure cut out. It is good to have some charge in there to begin moving oils so as not to starve the compressor of lube on start up.

The fact that the low side went down to zero, indicates the compressor is making some suction (i.e. "suck").

Static pressure (HP & LP are near the same) is just the refrigerant gas pressure in a system when not operating (clutch not engaged).

Understand the desire to use your own abilities and to do this with a limited budget. But throwing hours and hours away, banging your head against the wall, and doing it over and over again; is just not the most economical way to do it right.








-------------------------



HECAT: www.hecatinc.com You support the Forum when you consider www.ackits.com for your a/c parts.

FLUSHING TECHNICAL PAPER vs2.pdf 


Edited: Sat October 10, 2009 at 9:29 PM by HECAT

bohica2xo on Sun October 11, 2009 12:46 PM User is offline

A used car with a "conversion" that is not working means starting from scratch. Every time.

Conversions many times have way too much oil in them. Other times not enough. Failed dessicant containment, air, sealer, etc. - you are likely to find all sorts of things in a secondhand conversion.

The only way to know you have a proper oil charge in a system is to flush to bare metal & start fresh.

FYI you have a TXV system, the compressor does not "cycle" like a CCOT system - it should shut down when the evaporator temperature falls too low.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

av8or1 on Sun October 11, 2009 2:12 PM User is offline

Quote
Originally posted by: bohica2xo
A used car with a "conversion" that is not working means starting from scratch. Every time.

I'm learning this...thanks for the tidbit, as an engineer I can understand why you'd make such an assertion.

Quote
Conversions many times have way too much oil in them. Other times not enough. Failed dessicant containment, air, sealer, etc. - you are likely to find all sorts of things in a secondhand conversion.

Could you help me define "secondhand conversion"? Does that mean any conversion not done by a dealership or A/C-only shop?

Quote
The only way to know you have a proper oil charge in a system is to flush to bare metal & start fresh.

Well I've never flushed a system, but have begun to read the document on this website, so I may attempt it if the electrical checking that I will attempt next fails to identify any problems. I like the idea of starting fresh, actually.

Quote
FYI you have a TXV system, the compressor does not "cycle" like a CCOT system - it should shut down when the evaporator temperature falls too low.

Funny you mentioned that, but that was actually a thought that I had. If the evap core temp drops too much or heaven forbid it freezes then it would make sense that the compressor turns off and won't engage again. This was part of the reason I decided to suspend work, take a step back, do some more research and then come back at it again later. Could you help me here? What is a CCOT system? Are you referring to the accumulator/orifice tube system? What happens in the CCOT system when the evap core temp falls too low?

Thank y'all again for the help.

Jerry

iceman2555 on Sun October 11, 2009 3:15 PM User is offlineView users profile

With so many hands being in this repair, seems the best and most logical approach would be to go back to scratch.
The compressor seems to be pumping....and cash has already been invested so stay with this item. Although a bit more research would have located several companies that offer a true 'REMANUFACTURED' compressor and some that even offer NEW units. There are two units listed as possible replacements, one a Denso and two Sandens. All are available new with and without clutches. New evaps are also available and a very reasonable cost.
Suggestion to have the refrigerant recovered....the system disassembled and totally flushed to clean all residual lubricants and possible debris. If the old compressor (rebuilt) is maintained, insure that it is flushed also...using several oz of appropriate lubricant and rotations of the drive to through clean the compressor. Refill and drain several times to insure proper cleaning. Drain and add the correct amount of the proper lube back into the compressor.
This system may have a tube and fin condenser or a multi flow serpentine unit. If the latter is used, it is very difficult to properly clean without specific equipment. Multiple flow orifices make it very difficult to properly clean the condenser. There are certain procedures that will aid in proper cleaning of the condenser.
Re assembly with a NEW Rec/Drier/Filter and a suggestion for a new TXV. If the TXV is replaced attempt to locate an OE unit. Some of the 'off shore' TXV's are pure junk.
Decide whether or not to recharge with 12 or convert to 134a. Personally, this should be left as a 12 system...better performance. However, if the choice is 134a, consider a new universal Parallel Flow HI Efficiency condenser as a replacement. This alone should be of great benefit for system performance over the older 12 design unit.
Add the correct type and amount of lubricant for the system. 25-30 % in the compressor (suction side), max 1 oz in R/D/F, the remainder can be added to the evap prior to reassemble.
Evac and recharge the system. Suggest to recharge with the correct equipment so that a true known amount of refrigerant can be added to the system.
After the system is 'brought back' to close to OE spec's then a proper evaluation of the system may be obtained.
Consider a new fan clutch as part of this repair...esp if the current unit is older than 3 yrs or 50k miles. If unable to determine the status of the fan clutch....replace the darn thing...they are not that expensive.


-------------------------
The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

bohica2xo on Sun October 11, 2009 7:01 PM User is offline

A "secondhand" conversion is any conversion that you did not do yourself.

You will see the words "Death Kit" here a lot. People selling cars go to a chain store & get a conversion kit. A couple of fittings, wham-bam 134a, oil & air all at once. It will kill the system - a Death Kit.

Even a competent engine shop may know nothing about A/C. Converting refrigeration systems is not a simple 2 step process, even though it has been sold as such.

CCOT stands for Cycling Clutch Orifice Tube. The evaporator freezes repeatedly with CCOT, it thaws out when the system cycles off - it is a bang-bang control system, and CRAP could be used to identify it as well.

The TXV system meters refrigerant into the evaporator based on the suction line temperature. When a sensor in the evaporator core sees temperatures below freezing it will stop the compressor until the evaporator core temperature rises above freezing. When operation in warmer temps, TXV systems usually run more or less continuously.

You should get a Mitchell or Alldata subscription for that car, and have a look at the electrical system. There should be a couple of pressure switches in series (High & Low cutouts) as well as a thermoswitch in the evaporator core. There may be a throttle switch to drop the compressor at WOT, or a coolant temperature switch as well - it all depends on make, model & year.

I agree with Ice, the conversion will have more sucess with an upgraded condensor.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Karl Hofmann on Sun October 11, 2009 7:08 PM User is offlineView users profile

I don't know if you have a Lemon Compressor or not, but does it work a bit like an Orange squeezer?

-------------------------
Never knock on deaths door... Ring the doorbell and run away, death really hates that!

NickD on Mon October 12, 2009 6:11 AM User is offline

Any vehicle made prior to 1994 and in some cases 1995 is suspect of having a poor conversion made. Or simply topped off with R-134a that was extremely common. Funny, I was dealing with a guy trying to sell an RX-7 and was told the same thing, so I offered him a thousand less, he found another sucker to buy it. It's all a question of getting what you are paying for and the resale value of BS is next to nil.

Government gets away with screwing me for what they charge me for services I ain't getting, can't seem to do anything about that, but try to draw the line there with other folks. There are the screwers and the screwees, difficult to meet an honest person anymore.

av8or1 on Wed October 21, 2009 12:29 AM User is offline

Quote
Originally posted by: iceman2555
With so many hands being in this repair, seems the best and most logical approach would be to go back to scratch.
In the end I'll be coming close to that, yes.

Quote
The compressor seems to be pumping....and cash has already been invested so stay with this item. Although a bit more research would have located several companies that offer a true 'REMANUFACTURED' compressor and some that even offer NEW units. There are two units listed as possible replacements, one a Denso and two Sandens. All are available new with and without clutches. New evaps are also available and a very reasonable cost.
Boy but I would like to see where you did your research and learn from it! I called 5 local parts stores, 3 local Mazda dealerships and all of them told me that I cannot buy a NEW compressor for it (only remanufactured or rebuild my own) and that the evap core unit was/is a "discontinued part." So ... help? Where did you find this stuff exactly?

Quote
Suggestion to have the refrigerant recovered....the system disassembled and totally flushed to clean all residual lubricants and possible debris. If the old compressor (rebuilt) is maintained, insure that it is flushed also...using several oz of appropriate lubricant and rotations of the drive to through clean the compressor. Refill and drain several times to insure proper cleaning. Drain and add the correct amount of the proper lube back into the compressor.
Well the update for everyone on this forum is that I gave in and relented, taking the car to my trusted mechanic to have him take a look-see at it. As it turned out, I discovered on my own that the A/C fuse was ok but that the cutout switch (which on my system is between the receiver/dryer and the TXV) and the evap core temp sensor both had poor electrical connections despite my passing first attempts to make them better. 'Corrected that properly and the compressor began engaging again. Then I noticed that it wouldn't hold a charge. So I have a leak. Crap.

"Ok then, let's go back to square 1", I then decided. Naturally, as a DIYer I don't have the necessary equipment to flush anything and can't justify the expense. This was the reason I decided to take it to my mechanic. He confirmed all of the above, so at least I was on the right track, so to speak. He, of course, adopted the position of flushing everything, just as folk on this forum have. So we'll see.

Quote
This system may have a tube and fin condenser or a multi flow serpentine unit. If the latter is used, it is very difficult to properly clean without specific equipment. Multiple flow orifices make it very difficult to properly clean the condenser. There are certain procedures that will aid in proper cleaning of the condenser.
Well I've actually learned the diff between the two after coming to this forum, so thanks. And mine is a tube-n-fin type condenser. I can imagine how the serpentine would be more difficult because of the narrower passages however.

Quote
Re assembly with a NEW Rec/Drier/Filter and a suggestion for a new TXV. If the TXV is replaced attempt to locate an OE unit. Some of the 'off shore' TXV's are pure junk.
Actually you hit the nail on the head with this. In addition to the leak, my mechanic told me that when the system does have a charge it doesn't "blow very cold air." He said that most commonly this is a problem with the TXV and asked about the one I had on there. I told him how it was physically bigger than the one I took off of it but that how the parts store swore up and down that it was/is the correct one. He asked that I bring back the original to him, so I'm gonna do that during lunch tomorrow.

Quote
Evac and recharge the system. Suggest to recharge with the correct equipment so that a true known amount of refrigerant can be added to the system.

Agree completely, but hard to do for a DIYer. Those automatic evac/recharge machines are expen$ive. I did see one guy use a free-form-stand-alone scale and an old-compressed-air-looking tank full of refrigerant once...maybe that might be an acceptable alternative.

Quote
Consider a new fan clutch as part of this repair...esp if the current unit is older than 3 yrs or 50k miles. If unable to determine the status of the fan clutch....replace the darn thing...they are not that expensive.

Already have the part, it was gonna be the last thing I did. 'Just didn't mention it on this forum. Yet.

Thanks,

Jerry

av8or1 on Wed October 21, 2009 12:41 AM User is offline

Quote
Originally posted by: bohica2xo
A "secondhand" conversion is any conversion that you did not do yourself.
Hi again bohica2xo. Thank you for your replies in this thread. I have saved all of the pertinent information that I have learned from y'all in this forum into my car technical notes document for future reference.

Needless to say I have learned a lot about A/C repair during the course of this "project." Which actually was one of my goals at the outset. I mean this car is not my daily driver, I do not need the A/C to work right now, and the vehicle is older. I didn't mind spending a little cash to (a) repair the system and (b) learn something in the process, so I've done just that. No regrets. I've learned a LOT thus far. Again, I've never claimed to be an A/C repair technician. 'Just trying to learn from them. ;-)

Quote
You will see the words "Death Kit" here a lot. People selling cars go to a chain store & get a conversion kit. A couple of fittings, wham-bam 134a, oil & air all at once. It will kill the system - a Death Kit.
Hahaha! Ok, good information, thanks!

Quote
Even a competent engine shop may know nothing about A/C. Converting refrigeration systems is not a simple 2 step process, even though it has been sold as such.
And how. I can't tell you the number of folk who have told me that; and these were/are guys who I thought knew a lot about the endeavor. As an engineer (though of a different discipline) I cannot understand how a conversion from one system to a different system, though they perform the same function, could be simplified to such a base degree.

Quote
CCOT stands for Cycling Clutch Orifice Tube. The evaporator freezes repeatedly with CCOT, it thaws out when the system cycles off - it is a bang-bang control system, and CRAP could be used to identify it as well.
Great stuff, thanks! I gotta admit though, your reference to "CRAP" threw me off...what am I missing with that?

Quote
The TXV system meters refrigerant into the evaporator based on the suction line temperature. When a sensor in the evaporator core sees temperatures below freezing it will stop the compressor until the evaporator core temperature rises above freezing. When operation in warmer temps, TXV systems usually run more or less continuously.
Yeah, I noticed the temp sensing coil that was/is strapped and insulated to the low side exit from the evap core. I presume that a TXV system runs continuously in warmer temps because the core really never freezes like it will in a CCOT system, yes?

Thank you again everyone. When my mechanic and I get everything worked out, I'll post a final summary of the situation that will be intended to help other newbies like me, should they stumble across this thread in the future.

Jerry


Edited: Wed October 21, 2009 at 12:43 AM by av8or1

bohica2xo on Wed October 21, 2009 1:40 PM User is offline

Jerry:

You are not missing much... "CRAP" - you know, a steaming coil of unwanted stuff from the back end of a dog?

Back when A/C was an expensive option in a luxury car, the system had to perform - buyers would not pay that kind of money for a marginal system. Vehicles were equipped with both a TXV and a POA valve. Precise, machined parts with calibrated responses.

The TXV meters liquid refrigerant into the evaporator based on the refrigerant gas discharge temperature. If more heat is being removed, more refrigerant is sent through. This is a continous process, with the CV of the valve being variable within a wide range.

The POA valve throttles the suction of the compressor. It holds the evaporator suction pressure just above freezing to prevent ice formation. Under high heat loads it can be wide open, and at low heat loads the evaporator core stays at 33f, and the compressor suction may be in partial vaccum. Un-needed refrigerant is stored in the receiver as a liquid. Continous control, smooth & repeatable.

The CCOT system is a bang-bang control, with a compromise orifice size. At times the orifice is too big - excess refrigerant is slopped into a bucket in the engine bay (accumulator) where it becomes gas again, while cooling the engine bay. At other times the orifice is too small, giving reduced cooling. The system is controlled by a limit switch, that under some conditions it hits repeatedly. The hysteresis in the system causes the evaporator temperature to go through wide swings in tempearture, which can often be felt in the vent temperatures. It is the equal of only driving your car at idle or full throttle - not very effecient. Imagine maintaing a 65mph cruise with only WOT & idle.

The POA valve has not exactly dissapeared. It turns up in luxury models like Lexus frome time to time. GM developed a very elegant solution, called the V5 / V7 compressor series. They maintain evaporator / suction pressure by variable displacment of the pump. By destroking the pump at light loads it becomes very effecient. A V7 coupled with a TXV gives performance that duplicates the old A6 / TXV / POA systems.

As A/C became standard equipment in cheap cars, all expensive parts were eliminated. A 20 dollar TXV was replaced with a 60 cent orifice. The 40 dollar POA just went away. CRAP performance became the norm.

B.


-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

av8or1 on Wed October 28, 2009 10:32 PM User is offline

Hi bohica2xo,

Thank you for the brief automotive A/C history lesson! I've saved this off to my technical document too. Good information. I had never even heard of a POA valve prior to this discussion!

I had a question about something Hecat said in an earlier post :

Quote
For the most part, oil migration with liquid refrigerant (miscible) continues fairly well without compromise. But inside the the evap is where the proper charge is needed to maintain adequate gassing/boiling activity to "splash" the oil on through this loop lube circuit. As this activity diminishes within the evap the oil begins to pool.

Can you explain this to me a little further? I find the "splash" thing kinda confusing. I thought that the oil travelled through the system due to the pressure and also the flow of the refrigerant. Naturally now that I think about it, that almost seems "too simple" somehow. Anyway, I'd like to understand this splashing-in-the-evap-core thing better than I do at this point...

UPDATE:
-----------

Well the latest is that my mechanic put his sniffer to work and then told me that I had a leak in the evap core box. I took it off and whoa! but was that miserable excuse for a TXV leaking. So that's where the leak was/is. I've put an OEM TXV on order, but they sent the wrong part, making me wonder if the correct part has also been discontinued like the evap itself has been. I plan to take the original TXV to the dealership and see what develops from there. In addition, my mechanic wanted me to replace the rubber section of the suction hose. I wrangled with it a bit and in the end I had to remove the entire thing from the evap exit right up to the compressor. I had to do that because the fitting along the firewall in front of the driver wouldn't budge. I didn't wanna strip the nut and look stupid, so I just took the whole line down to the local A/C repair shop.

I asked them to replace the hose, which only extends about 18 inches aft of the compressor as the rest of the line is solid aluminum, and to also chop off the unmovable fitting and replace it with two new fittings plus an O-ring. I then asked them to explain how they would do all of this stuff. Long story short version is that it was an educational day at the repair shop. The guy took some time to explain how they hydraulically crimp the hose to the fittings and the fittings to the aluminum line. He also explained how they would cut off the bad fitting and weld in two new ends. I requested that they keep the overall length/width/however-you-wanna-look-at-it the same and they told me "of course".

They also had some cut-open compressors on display and I got a good explanation of how these things work. I'm hardly an expert at this point, but at least I understood everything he was telling me. I feel as though I could rebuild a compressor now, albeit with an instruction manual in case (ahem...when) I get lost in the process. ;-) At a minimum I left knowing a lot more than when I entered the store, so all was good.

So my mechanic and I are waiting on the TXV, I'm gonna put the suction hose back on tomorrow night sans the evap core, and when the TXV finally arrives, I'll sinch everything up, take it back to the shop and we'll see what happens from there. And BTW, in case you're wondering my mechanic is the best because he lets me work on my stuff as much as I am able so long as I don't hold him up too much in the process, which I don't. I work on everything at home. It's more shuffling (eg driving) back-n-forth to the shop, but much more interesting, educational, fun and rewarding in the end.

I'll keep everyone updated with the hope that it helps someone at some point in the future.

Thanks,

Jerry

HECAT on Thu October 29, 2009 6:33 AM User is offline

Jerry,

As any refrigerant in liquid form (liquid refrigerant and oil mixed) is pushed under pressure through the orifice (OT or TXV) it becomes a liquid mist. A proper charge allows for metered refrigerant to remain liquid as it enters the bottom of the evap (this is known as a flooded evap). If the charge is too low the refrigerant will begin to gas at the orifice point which is way too early, as it is not flooding the evap.

As the proper amount of metered liquid then enters the evap, the refrigerant encounters the cabin heat load and "boils off" to a gas, as it does this it removes (carry away) heat from the air blown over the evap (latent heat of evaporation). The TXV varies its orifice size according to this heat load; the OT is fixed.

The "gassing off" (evaporation, boiling) of a refrigerant causes the oil and refrigerant to separate (distillation). If the system charge and oil loads are correct, by design, this "boiling" activity will be strong enough for the proper amount of oil to continue on through the refrigeration loop (thus my use of the term "splash").

If the system charge is low, "boiling" occurs early and not at the designed level of activity, therefore you get weaker vent temps (poor thermal transfer) and poor oil migration. So with a low charge or as a leak worsens, the amount of oil continuing on to feed the compressor is compromised as it begins to pool in the evap.

The OT system uses an accumulator after the evap that can handle some (accumulate) oil overcharge; the TXV system does not, and thus the proper oil and refrigerant charge has become even more critical. But neither system can compensate for low refrigerant (oil migration) or low oil loads other than to self destruct from lack of lubrication.

It is much more complicated that this, just tying to offer a simple explanation.



-------------------------



HECAT: www.hecatinc.com You support the Forum when you consider www.ackits.com for your a/c parts.

FLUSHING TECHNICAL PAPER vs2.pdf 


Edited: Thu October 29, 2009 at 4:09 PM by HECAT

NickD on Thu October 29, 2009 7:06 AM User is offline

It's my understanding that oil in the refrigerant circuit does not cool, but kind of required to keep the compressor from seizing. They knew this even a hundred years ago, so designed compressors with oil sumps and even a sight glass to quickly view the oil level. But that cost a couple of bucks more, so just mix the oil with the refrigerant with no way to check it. Not much different than a much cheaper two cycle engine where it's the owners responsibility to use the correct quality and quantity of that now, way overpriced two cycle oil.

While I have ridden with people that won't let you eat or drink in their cars from fear of dropping a crumb on the carpet, their underhood looks like a disaster. Filthy, and with an underhood like that, can't find those small AC oil leaks, so a disaster is soon to happen. Well, that is the way AC systems are made today, preventative maintenance is checking for those mild oil leaks, but can't do that with a greasy filthy engine compartment. Seen some so bad, don't even want to touch them, fear of picking up some strange disease. It's just a problem waiting to happen.

av8or1 on Sat November 14, 2009 2:14 AM User is offline

Thank you Hecat, for the explanation. I was able to follow all of that without a problem. I'd like to drill deeper and understand the distillation phenomenon a bit better; ergo, what is it exactly that is "strong enough" to keep the oil moving through the system when the proper amount of refrigerant and oil exist, etc. My guess would be pressure (and maybe a bit of suction from the compressor via the suction hose? eh - seems like a stretch), but ... anyway. The concept is pretty simple, the details are where "the fun" lies. :-)

UPDATE:
Well I now have a working A/C system! Woooo-hoooo! As it turned out the TXV for my 85 RX7 has also been discontinued, like the evap core, and so my mechanic found a suitable replacement from a shop out of Houston (I'm in Austin). Supposedly they only sell to automotive repair shops, so DIYers like me don't have access to their products. Oh well. This new TXV "fit the bill" as the expression goes and doesn't leak like the POS I got from the auto parts store. So I got the evap box back from my mechanic, cinched everything back together and took it back to him for charging and evaluation. Boy but was that FUN! He let me hang around in the shop (he's a freelancer type) and help him charge it, etc.! I had used those machines in the past but it was still interesting to do it again.

And BTW, a word of advice to other DIYers ... I realize this general principle is kind of "duh!" type of knowledge, but I'll state it anyway. if you take something in for repair at an A/C shop, check it over before using it. I had this shop cut off a fitting on the suction hose that had probably never been taken apart and then weld on two new fitting ends that would unscrew and come apart. I had also requested that they put an o-ring in there for me. Just out of being anal, I decided to unscrew the fitting and verify that there was an o-ring in there. I understand that people can forget sometimes, so...and sure enough, disconnecting these two sections of the suction hose revealed that there was no o-ring. So I put one on and re-connected them before reinstalling the suction hose.

So there you have it. Overall I have enjoyed this project and learned a lot, for which I am appreciative. As previously mentioned this was one of my goals from the outset: to learn. So thank you to everyone who responded to my thread and shared their expertise; I appreciate you too. I took some photos to share with y'all in the interest of helping others but can't find a way to upload them.

Thanks again!

Jerry

Back to Automotive Air Conditioning Forum

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.