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V7 Gm questions

CobraGuy on Thu April 21, 2011 10:13 AM User is offline

Year: 2001
Make: Pontiak
Model: Fire Chicken
Engine Size: LS1
Refrigerant Type: 134A
Ambient Temp: 80
Pressure Low: 33
Country of Origin: United States

I have a couple of questions that I hope you folks can guide me in the right direction. I haven't played much with the GM variable displacement compressors. But the a/c system in this car is lacking. The best duct temp I can get is in the high 40's with an ambient temp in the high 80's...and this is driving down the interstate. The condenser has been removed and all fins cleaned. No sign of compressor damage anywhere. System evacuated for an hour at appx 29" then charged to factory spec of 24 oz. Static pressures are good for the ambient temp. High side pressures seem fine. But the low side wants to stay around 33 psi...sometimes I've seen 30...but that's rare. With all the fixed displacement Ford's I've done, I use an adjustable cycling switch to lower the low side pressures to the mid 20's and the things perform extremely well with duct temps that will hit the 30's here in AZ. (freezing of the evap is not an issue) So I have some questions:

Is there a control valve out there that will lower my low side to where I want it? I've seen charts, heard everything was consolidated, etc etc. Very confusing. If nothing new, is there an application I could pull one from in the bone yard?

Is there a way to modify the control valve to change the pressures?

I have seen a "mod" to the control valve that forces the compressor to full stroke. Has anyone had any experience with this? I realize a cycling switch would need to be wired in.

Finally, is there a compressor out there that is fixed in displacement, but would fit where the V7 does with the side mounting bolts, etc.?

Thanks for your time folks.

GM Tech on Thu April 21, 2011 11:40 AM User is offline

So what is wrong with 40's vent temps? Bet this was on high fan.....lower the fan speed and use recirc and you can get into the 30's

Variables use a control valve that keeps suction pressure just above evap freeze point----GM already thought of what you want to do-- sure you can put in another lower set point control valve, but everytime it rains you'll be mad that the a/c doesn't blow-- because your evap is snow white (froze). When GM built your car, they did not know it was going to AZ- it may just as easily went to humid prone FL.

Variables are designed to run at high 20's low 30's suction pressure-- irregardless of heat load-- (unless of course it is extremely hot out)

F- cars were not designed with a/c in mind-- they wanted styling first-- then let the a/c designers come in and figure out how to design around with what little room is left-over after the stylist leaves. Any car maker will tell you styling sells--- and nobody buys a shoe box looking car just because the a/c works great!

A cycling switch on the variable compressor will do nothing at all- unless you lock it into full stroke- which it will be at anyway most of AZ time. (anytime it is over 90 degf ambient- or anytime suction pressure is above 35psi)

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Edited: Thu April 21, 2011 at 11:45 AM by GM Tech

CobraGuy on Thu April 21, 2011 11:48 AM User is offline

High 40 degree temps are absolutely unacceptable in ANY automotive a/c system with 80 degree ambient temps. (and extremely low humidity) Wait and see what they are when it's 118 outside.

I have many Ford systems with mid 20 low side temps and duct temps in the 30's...with ambient temps well into the 100's. Same with GM vehicles with fixed displacement and adjustable cycling switches. I have never, ever frozen an evaporator in Arizona. If so, I would simply adjust the cycling switch a tad...but I've never had to do that. A 134A system with 25 psi on the low side just won't freeze up an evaporator here. At least I have yet to see it.

If I could get this to run in the 20's for low side pressures, I would be satisfied. I can't get it there. You mention lower set points...and that is what I would like to pursue. Do you have a control valve that will accomplish that? I can't seem to find just what I'm after. You seem to have a lot of experience with this system and your guidance would be appreciated.

Can't argue with your assessment of F cars...but there are a LOT of them with duct temps in the 30's. So I know it can be just may take quite a bit to get there.

Thanks again for your help.

TRB on Thu April 21, 2011 11:55 AM User is offlineView users profile

Just curious what are your high side pressures at idle and 1500 rpm?


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GM Tech on Thu April 21, 2011 12:09 PM User is offline

Your V-7 is at full stroke anytime the suction presure is above 35 psi-- that is just how it works-- destroking only occurs when suction drops to the 20's- to avoid freeze-up-- so the only time a lower set valve will help is when you are not at full stroke which would equate to less than 90 or so ambient - (maybe even 80) - monitor your suction pressure while driving- anytime it dips, it will be destroking-if you want to test and fool it - don't pull a complete vacuum- this was done on J cars in mid 90's (as a factory snafu) and vent temps went to high teens- and freeze-up was a major issue- only problem is that head pressure will rise immensly as well.

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

iceman2555 on Thu April 21, 2011 2:19 PM User is offlineView users profile

If suction pressures are in the 33 psi range, one would be looking at refrigerant temps of app 37 degrees. Since low pressures are measured after the accumulator inlet and the refrigerant would have been converted to a gas at this point and this requires absorption of heat and thus would indicated a evap temp of app. 31 to 34 degrees. The posted pressures would then indicate a possible blend door problem that is contributing to the elevated vent temp. Also keep in mind that the vent temp is not the same temp of the air exiting the evaporator. The difference may be as much as 5-10 degrees, depending upon heat load on the windshield/dash cover and possible loss of duct sealing materials.
Also,what were the test conditions for the posted vent temperatures? Stated that the system was charged with 24 was the system charged? By gauges and cans or with a metered/weight procedure?
The statement that 'static pressures' were good for the temps...what does this mean? Static pressures are not an indication of system charge.....this system could have two oz or two lbs of refrigerant and the 'static' pressures would be the same. Static pressures indicate the presence of refrigerant...not the amount.
There were no postings of high it is possible that these were not available....they would help.
Cleaning the condenser is great....not only the fins but the inner flow orifices also. Is it possible to obtain the inlet and outlet temps of the condenser....keep in mind that the condenser on this vehicle flows in a reverse direction. The discharge (inlet) is on the bottom of the condenser and the liquid (outlet) is on the top. Post this information. Keep in mind that all pressures and temps are only valid when the system is completely and fully charged.
Let us know a bit more.

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

CobraGuy on Fri April 22, 2011 8:51 AM User is offline

Good questions...I'll record a more complete data set and post it. I know the high sides were in the 170 range when I first started to check this. That's with an 80 degree ambient temp in the shop.

Iceman, I am confused about your static pressure statement. GM actually gives specs in their manuals regarding static pressures vs ambient temp in order to do a quick check for the proper charge.

Dougflas on Fri April 22, 2011 3:30 PM User is offline

If you take a refrigerant system that holds 2 lbs as an example. If there is 1 lb of refrigerant in it, the pressure on the gauge will read the same as if the system contained 1.75 lbs of refrigerant. This is with the system not running and has equalized. All you can tell is that there is refrigerant in the system.

CobraGuy on Thu April 28, 2011 9:31 AM User is offline

Ok...some answers.

My recovery station will not just recovers. So I charge from that bottle with a scale. However, that time, the machine was in use and all I had were two 12 oz cans. I charged them both through the high side port (not running of course) by simply warming the cans a bit. Then pulled everything from the hoses through the low side. I purged the hoses of course before ever starting. Subsequent charges were done with a bottle and scale. I had a full charge by the way when I used the cans. The weight was correct when I recovered it. Now, I started with low side pressure around 33 psi and duct temps in the high 40's. I pulled the control valve and made sure it was clean and not damaged in any way. Then I installed a .067 orifice tube and my low side pressures dropped to about 30 psi and the duct temps dropped to around 46 degrees. Shop temp was 88 degrees...but these temps are the same whether in the shop or on the highway...which surprised me a bit. So yesterday, I changed the orifice tube to a .062. Low side pressures dropped to 28 to 29 psi and duct temps are in the 44 to 45 degree range. Again shop temp was 88 degrees. Starting to get closer to where I can live with it...but still not where it really should be. I'm thinking about going to a .057 orifice tube and see what it does.

Now, here's the kicker. In the shop...the high side pressures are staying about the same with all the orifice tubes. They start out around 170 psi, and I've never seen them over 185 no matter how long I've run it in the shop. This is measured coming out of the condenser. (top line) The condenser and radiator were recently removed for some engine work and they were both thoroughly cleaned and fins straightened. They are both in great shape as far as that is concerned. No sign of catastrophic compressor damage anywhere, so I hesitate to condemn the condenser as plugged, etc. It appears to be the original compressor...but who really knows. I have a brand new Delphi compressor on the shelf, but I hate to throw it on there with what I know so far. I don't think it needs it...but the high side pressures does have me thinking a bit. I usually see them a bit higher than that, but I'm also not used to this V7 either.

One other item...I clamped off the heater core hose just to make sure the blend door wasn't contributing to this. No difference in duct temp. All tests were done after the car had been sitting in the shop over night with no sun load on the dash, etc. (That's huge I know...just ask Acura and their God how do they get away with that mess!)

I was going to modify the fan controls so they come on high anytime the a/c is on and get rid of that strategy the PCM uses to control them. But there is no noticeable difference in duct temp between around town, the shop, or the highway. Evidently the low speed is enough. Very surprising to me.

Oh yes...have you heard about modifying the control valve (soldering it closed) then adding an adjustable cycling switch to the system? This puts the compressor in full stroke 100% of the time regardless of load. I have a shop here who has done this to their own cars with outstanding results. They say you can now feel the compressor cycle, and they are waiting to see how long term affects are on the compressor and clutch. But it's a thought to keep in the back of our mind...

GM Tech on Thu April 28, 2011 10:58 AM User is offline

Since you like to experiment, why not change the set point on the control valve? push it together more, or pull it apart more will change the control points. That's how they are set originally by a press fit adjustment- measure overall length, then put one in a vice and squeeze it together about .003" and test results- then try just the oppposite based on your previous results...then you have learned how to full stroke a compressor....

This is why they tell you never to use a dropped control valve- it changes the set point.

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

mk378 on Thu April 28, 2011 11:10 AM User is offline

After some point (which you have probably reached), smaller OT's decrease your system's ultimate BTU capacity, which may become an issue in hot weather.

Also to consistently find the control valve's set point, run the interior fan on low and rev up the engine. All of these tests should have been done with the engine revved up to 1500 rpm.

CobraGuy on Fri April 29, 2011 10:31 AM User is offline

All readings weren't necessarily checked at 1500 rpm...but they were off idle. I can be more precise. By the way, you may be correct about the system capacity. With the .062 orifice, it looked good in the shop, but out on the road, it wasn't as good as the .067. Temps were in the mid 90' a bit warmer than the other day, but not much. I'm also going to install a water control valve. No use having engine coolant in that box if it's not needed. They make a valve that is designed for the LSx engines, so I'll toss one in.

GM Tech...I really didn't know that about the control valves!! That is GREAT information! I can see where they are press fit and I even entertained the idea of pulling one apart and playing with spring pressures and shims. But there are two or three springs in there and I could end up with a mess. Do you know which way to move the valve to change the set point? In other words, if I press it together, does it lower or raise the set point? Thank you very much for this information. I can't wait to play around with it.

CobraGuy on Sun May 01, 2011 9:21 AM User is offline

Interesting lessons here. I rounded up some control valves and took a few measurements. My valve I had in the car was the longest. Now, there are two spots on the RCV that are press fit. One at the very end (small end) and one in the middle. If you shorten the valve at the middle junction (press it together deeper), it lowers the high side pressures, but has little effect on the low side. If you press the small end in deeper, it raises the low side pressures. That really surprised me since sealing off that end port supposedly forces full stroke constantly. So in order to lower low side pressures by adjusting the valve, you evidently have to lengthen that small end. I see no way to do that and test that theory. No way to grab that small end and pull it. But it was fun to play with and see what did what.

Next thing I may try is to actually seal off the small end and see if it truly makes a difference as claimed and pulls the compressor into constant full stroke. Only way that would work is to install a cycling switch...which would be very easy by installing an adjustable Ford switch in the compressor ground circuit. Full PCM control would still be functioning that way. I'm not sure that is an answer...or even wise...but it will be fun to play with. OE engineering is nothing more than a huge compromise for all the different situations and buyers they encounter. Tailoring things to fit individual preferences is what true car crafting and building is all about. Anyone can replace parts...this is fun.

GM Tech on Sun May 01, 2011 11:22 AM User is offline

In manufacture of control valves, the small end is not the set point adjustment, only the two primary body halves.....

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

iceman2555 on Mon May 02, 2011 9:59 PM User is offlineView users profile

If you wish to make this a 'constant' full stroke compressor...simply remove the two middle orings.....this should allow for loss of system pressure differences. However, know that the 'V' series is not designed for constant full stroke operation and compressor longevity will will fail earlier.
I seriously doubt the cooling issue is the compressor...or the control valves.....dropping the suction pressures will make the compressor de-stroke earlier....reducing cooling efficiency of the entire system.
The 'Firebird' and 'Camaro' utilize a special mounting arrangement and I know of no other GM unit that will 'bolt on'. This is not to say that a fixed displacement unit could not be 'modified' to fit....but not one 'out of the 'box' that will work. I live in S FL....there are two Camaro's in the family...both have 'V' OE and the other aftermarket and both work very well. We do not of course have your heat load....but we do have the extra humidity that off sets this.
One other issue....what are the inlet and outlet temps of the condenser....keep in mind that the condenser in this vehicle actually flows in a reverse mode...the inlet (discharge) is on the bottom and the outlet (liquid) is on the top. Also a variable will maintain basically the same pressures...not matter what operational displacement maybe...this is one of the more difficult aspects of 'pressure' testing a system.
A high side pressure reading may not be a true measure of condenser flow.....check the inlet and outlet temps...but the system should be properly charge first.

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

CobraGuy on Thu May 05, 2011 9:10 AM User is offline

That's good information Iceman. I have seen a write up on sealing the port in the end of the valve by some means...solder...weld...etc to achieve a constant stroke compressor. Yours seems much easier and doesn't compromise the valve if someone wanted to change back.

I've done some homework and there are actually a few companies that make brackets to bolt a Sanden to the factory location. It's very clean. But then you are stuck with having to make new hoses to fit the Sanden. Not that big a deal I guess if a person wanted to change things to a cycling system. There is also a kit that moves a Sanden compressor to the upper right side of the engine on an LSx. It too is clean, but now you have a compressor where you really don't want one...and that kit was expensive.

I was shocked at the difference the different control valves made to the high side pressures. It was dramatic. I don't know how anyone can do any type of diagnosis work using pressures with these systems the way the pressures are internally controlled. Sounds like the condenser temps may be the way to go. I'll have to take those measurements and let you guys know what they are. Thanks for the information.

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