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Clutch Slipping Revisited

Underdog on Tue August 05, 2008 8:23 PM User is offline

Year: 1986
Make: Chevy
Model: Celebrity
Refrigerant Type: 134
Ambient Temp: 85
Pressure Low: 30
Pressure High: 275

I'm back. So I got it to work but still have the clutch slipping issue. Got the charge in quicker as Iceman sugested and all appeared well. 85 degrees ambiant 30 lbs and 275 at idle. Didn't get a vent temp but it blows nice and cold. Problem is that when you hold the revs up around 2500 for a while, the clutch starts slipping. I checked the pressures and at the higher RPM the high side climbs slightly over 300, the low drops to 25 and the clutch starts slipping. I don't know what the normal high side is for this model car and when the compressor should cycle. It's obviously still engaged at 300+ when it starts slipping. I guess the thing I need to know first off is if over 300 is normal or is the pressure switch bad not cycleing the compressor. Or, is there a problem with this rebuilt compressor?? It will sit & idle blowing cold air but you can't drive it.

GM Tech on Tue August 05, 2008 8:37 PM User is offline

over 300 psi is not normal-- are your electric coolings fans working? The high pressure cut-out switch will cut it out at 425 psi-- I have seen that yr and model have the fan switch and high pressure switch on the rear of the compressor have their connectors reversed- plugged into the wrong switches-- do you have two switches on rear of compressor? has anyone fiddled with the plug in connectors? One is a fan switch that turns on cooling fan- other is high pressure cut-out as I remember

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......

Underdog on Tue August 05, 2008 9:00 PM User is offline

Cooling fan works fine and is cycling corectly. One switch on back of compressor, red, one terminal. As I understand, more of safety excess head pressure switch. The high side cycling switch is mounted on the high side line close to the charging port. So, I guess what I need to know is at what pressure is the cycling switch supposed to cut out. If you were thinking the high side is too high, would that point to a bad switch?

Thanks , Jim

GM Tech on Tue August 05, 2008 9:31 PM User is offline

What engine you got 2.8 6 cyl? or 2.5 4 cylinder???-- the later has a V-5 compressor-- and is not designed to cycle off of low pressure- like the H-6 of the former.

Cycling when designed for it is done by low pressure side switch- usually on accumulator-- if a system "cycles" off of high side- then that is bad - too much pressure for any number of reasons.

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......

retrofit on Tue August 05, 2008 9:50 PM User is offline

Wouldn't that be a DA6 compressor (not recommended for use with R134a)? I thought the HR6 was introduced in the 1988 model year.

edit: Just reread. Rebuilt compressor could possibly be a HR6.

Edited: Tue August 05, 2008 at 10:00 PM by retrofit

Underdog on Tue August 05, 2008 10:42 PM User is offline

It's a 2.8 V-6. Compressor I believe to be an HR6. It seems to me that they went to a different compressor in 87. Should also mention that I put in a blue orfice tube. I thought that may be causeing the high side pressure but also thought the switch should cycle before it became a problem.

Edit: It just came to me about it cycling off the low side. Perhaps the switch I'm seeing on the high side is for the cooling fan. It comes on and the pressure noticeably drops. I'm starting to think more about the smaller orifice tube.

Edited: Tue August 05, 2008 at 10:55 PM by Underdog

retrofit on Tue August 05, 2008 10:51 PM User is offline

Did you replace or readjust the cycling switch?

Underdog on Wed August 06, 2008 7:10 AM User is offline

Originally posted by: retrofit
Did you replace or readjust the cycling switch?

No, only parts replaced were the compressor w/clutch, the blue orifice, acumulator and all o-rings. Rest of system is original. Really frustrating to be this close to having it working, but not.

johnboy123 on Wed August 06, 2008 9:29 AM User is offlineView users profile

Your pressures are off. sometimes retro fits and be a bear of a job. lets start from stratch. recover your refrigerant - pull the orifice tube and make sure it is clean better yet replace it. (white for gm). don't mess with the low pressure switch yet--we are not sure if it has or has not been reset. evac the system -reload you system with approximately one half the recommended amount required for the R12- i.e. if the R12 system (since i don't have my book here) recommends 36oz start with 18 oz of R34A - we are going to assume that the ambient temperature is 80 to 85 degrees (outside air) the higher the ambient temperatures the higher the pressures and visa versa. set your blower motor speed the next lower setting than the high speed. turn on your air conditioner - what we are looking first is at what pressure does the compressor cycle on and off- if it is approximately 40 psi on your low side guage then we need to reset the low pressure switch - if it cycles between 18 - 28 psi then it has already been reset- the low pressure switch will be located on the accumulator/drier- if it has not been reset - disconnect the electrical plug - take a 5/8 open end wrench and remove the low pressure switch (when re installing it be sure not to overtighten you will break it and cause a leak) look inside the switch and you will see a screw head take a flat head screw driver and turn it 1/2 turn to the left and reinstall the switch. Here is our goal - 30 to 40 psi on the low side pressure - 180 to 230 psi on the high side - vent temperatue about 40 degrees at 1500 rpm's - add refrigerant until those goals are met not to exceed the original R12 specifications. my experience is that you can not depend on the old majic formula. if that goal cannot be reached you need to check the air flow through the condensor - make sure there is nothing blocking it such as oil, bent fins, fan not working, etc. good luck

Underdog on Wed August 06, 2008 10:09 AM User is offline

Thanks, I'm evacuating the system now. I took the blue tube out and replaced with the GM white one. I suspect it since the system worked before on 134. Only reason I got into the system was because of a leak and I was lead to believe the blue Ford orifice was a trick to get more cooling. I'll post back after I get it recharged.

Underdog on Wed August 06, 2008 1:58 PM User is offline

Ok, with 30 oz in at idle.. 80deg ambiant, 35 and 225 at idle. Reving up goes 30 and 300. Vent 50 degrees. Took it for a ride and vent fluctuates between 45 and 47 degrees. At 55 mph I could feel the compressor cycling fairly often so I assume at speed the low side is dropping below 30. Idleing stationary, it doesn't cycle. I put about another 10oz in and took for another drive. Vent temps are the same but compressor staying on a little longer. The acumulator and lines are cold and sweating quite a lot. I'm now wondering if the evaporator could be partialy blocked with dirt or mold. Given the age of the car, I wouldn't be surprised.

mk378 on Wed August 06, 2008 2:02 PM User is offline

Just to clarify, are all the fans on when the pressure is 300 with the car sitting still? They should be. If not that's why the pressure is so high. Seems like it may just be a fan problem since you're getting decent cooling on the road.

Edited: Wed August 06, 2008 at 2:02 PM by mk378

Underdog on Wed August 06, 2008 4:30 PM User is offline

Yes fan is on. Comes on about 225lbs if I remember correctly. There is only one fan on the left half of the rad though. Evidently worked OK as designed for R12. I'm thinking it may be possible to get it better with a larger, full width fan but I don't know that I want to dump any more money into it. You can gather from my other posts that this was my dads car. Gave it to my girl friend a couple years ago. I've mentioned getting something newer but she absolutely loves this car. Giving a woman a station wagon is not a great idea. I never know what she's hauling in next! LOL

BTW, Want to thank all for thier thoughts on this project. I've had my own autobody shop since 78 and changed many a busted condenser but always took them to someone for recharge. My best guy retired several years ago. So, at the ripe age of 55 I decide to study on ac systems. I've learned a ton from this site. Thank You, Jim

NickD on Wed August 06, 2008 7:16 PM User is offline

Slipping clutch is a very dangerous condition, causes vast amounts of excessive heat that can fry the grease out of your pulley bearing and even ruin you compressor seal.

Are you sure you are getting the full battery voltage, around 14.2 volts to the coil? And is the coil good? Applying even 12.6 volts to the coil, the current flowing to it should be around 4 amperes, much higher, and turns in the coil can be shorted that results in a lower magnetic field. By leaving the voltage applied, the coil heats up increasing the resistance of the coil so as it heats up, the observed current should decrease. Very common for a cold coil to look good, but that heat also causes expansion shorting turns, in that case, the current will increase as it heats up. Also check the gap between the pulley and the clutch, should be 20 mils, 30 is okay, but larger than that causes problems. But mostly pull in problems can get some slippage as more force is on the return springs.

When you switched between vacuum and charging, did you change hoses? Very easy to get air in the system that way that explains high pressure with poor cooling, also a lot of extra compressor load as air really doesn't like to be compressed. There are ways around this, some in the FAQ section.

GMTech has a charging station that doesn't have this problem.

Underdog on Thu August 07, 2008 8:01 AM User is offline

That's a good point. When I had the initial slipping at high pressure, I discovered that I hadn't tightened the alternator belt and the discharge light was on. Since I went back to the white orifice and tightened the belt, It seems to be staying engaged.
I'm charging with cans and a gauge manifold. Everytime I switched cans I would bleed the manifold by depressing the shrader valve on it. Vacuumed the system for a half hour and it held at 25 inches. Thanks again.

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