MrPDJ

OK Guys,
I'm trying to troubleshoot an A/C problem that has been bugging me for a couple of years. My drivers side doesn't blow as cool as my pasenger side (usually a 10F to 15F difference) I took it into a shop, but they just said it was a little low and added some more refrigerant (didn't go for a test drive). This seemed to help a little bit, but didn't fix the problem.

I've given up on the shop and I'd like to tackle this problem myself. I've got the helms service manual, but I'm still stuck.

To summarize the problem, at idle, the A/C system performance seems to be reasonable, but as soon as I start driving the compressor discharge pressure takes a nose dive

Conditions today at idle:
50% Rel. Humidity
90F
Low Side: 30
High Side: 230

Conditions today at speed (~30 mph) (pretty much the same in 2nd or 4th gear):
Low Side: 30
High Side: 150-160

Any thoughts here? Low coolant charge? Compressor going out? Any other checks I should perform? I know that the increaed air flow across the condenser will drop the discharge pressure, but I wouldn't expect it to be that dramatic.

GONZZO

The readings U posted at idle was the cat just started or had it been running were the doors open or closed was the A/C system in recirculate or taking in outside air??? the reason I am asking is I want to know the what the temp was at the a/c evaporator, this dicates low side pressure. The hi side pressure will go up at idle and is usually controlled by a pressure switch that turns the radiator fan on and off based on the pressure. Driiving down the road the pressure will drop because more air is passing over the condenser. To do a check without any expensive equipment is to put the a/c system in recirculate, put fan onto hi speed. Get two pencil thermometers put one at the return air vent under the dash the other one goes at the outlet vent on the passenger side. The passenger vent temp should be 15 - 20 degrees lower than the reading at the return vent if the system is operating properly. Make sure both thermometers are good.

MrPDJ

Previous readings were in RECIRC with both doors closed. System was operating for ~10 minutes before I took the readings.

I've got two thermometers and I'll try your suggestion and report back.

rgtkst

i think mine is doing the same thing yours is. at idle, sitting still, my a/c blows nice and cold. when i get moving the thing starts to loose its cold air. like it is turning itself off or something. i understand they will shut off at WOT and kinda figured it might be something to do with that sensor maybe going bad and cutting it out any time i push the pedal to go. post up what you find out so i can get mine fixed too

GONZZO

rgtkst
There is a pressure switch on the low side, at idle due to the compressor not turning fast the low side pressure can be above the cut out of the switch, but as soon as you start driving down the road the a/c compressor will start to run faster causing the low side pressure to drop below the cut out setting and the compressor clutch will disengage until the pressure rises above the cut in setting. Possibly you could be low on refrigerant. Try as I suggested above using the two thermometers.

Good Luck

MrPDJ

I haven't had time to do the thermometer test yet, but I've been reading more about the C5 A/C system and I have a new theory that explains the most symptoms of my A/C. Let me know what you think about it.

This article has helped me formulate this theory.


The compressor in the Vettes (and some other GM vehicles) is a V7. It is equipped with a refrigerant control valve that regulates the stroke (refrigerant flow rate) of the compressor. With a variable stroke compressor, the compressor always runs and the control valve adjusts to obtain optimum performance. GM went this route because it requires less power than the fixed capacity compressors (fuel economy) that cycle on and off.

My theory is that my control valve is stuck in a less than optimum position (i.e. less than full flow capability). When at idle, the reduced airflow across the condenser causes the compressor discharge pressure to increase. With a high (approx. normal) discharge pressure, a decent amount of refrigerant can pass through the system, but not the proper amount that would be flowing if the valve were working. This produces the mediocre results that I get at idle (driver side warmer than passenger side.) When I first start to drive (or rev the engine without moving) both the compressor speed increases and pumping capacity increase. This is why I get a short burst of increased a/c performace (temps almost the same) as I pull away from a light.

However, once I get moving the compressor discharge pressure drops off significantly because of the airflow. This in turn, reduces the refrigerant circulation rate. With the control valve stuck, the compressor can't increase the stroke of the compressor to circulate more refrigerant through the system.

Anyone ever replace one of these control valves on a compressor? If this is it, the control valve is cheaper than a compressor.

GONZZO

thanks, you just educated me. If the compressor was running in a reduced capacity mode then the low side pressure would go up (less compressor capacity same heat load) the low side pressure you posted indicated the evaporator was not seeing any real heat. I never asked but do you have dual climate control ? If so there are post on this forum with members having similar problems, the solution seem to be reindexing your a/c actuaors. These actuators control the dampers in the air system to maintain proper supply air temperature.

Good Luck

MrPDJ

I have been suspicious of the dampers for a while now, but I thought I would exhaust troubleshooting of the refrigerant/compressor system before ripping into the dash to hunt for an actuator problem.

From the posts that I've read, the actuators appear to be functioning properly. For example, with the A/C off, the temperature out of both sides is equal. I don't have any A/Ccodes set and I've tried resetting them before with no change to output. I've also tried indexing the driver's side temperature actuator as one member suggested, but this didn't make a change.

I need to double check my pressure numbers again. After I posted my numbers, I went back to do some more checking and they were different. Low side was like 50-60. It was getting dark and the mosquitos were out, so I didn't spend much time out there.

Something else that I have noticed is that when my A/C switches from RECIRC to A/C, I can feel/smell the humidity coming up on the drivers side. I sometimes think that some air is bypassing the evaporator and coming straigt into the car.

I've been putting up with mediocre A/C for 3 years now, and I'm ready to fix it. I'll keep reporting on my progress.

brookman

www.ackits.com has a great forum with some very good techs that post. Ask your question over there also.

MrPDJ

OK,
I finally gave up on trying to trouble shoot without evacuating and recharging the system, so I rented a vacuum pump from a local tool rental place and evacuated the system. Pulled it down to 29 "Hg, closed the valves and it held for 1 hour.

Hopefully this indicates that there are no leaks (or they are very small). Added 1.5 cans of R134 before I realized that my high side hose fitting was not attached well and was leaking. (I later found some oil on the floor, I'm hoping that this was from the leaking fitting and not something worse. I haven't seen any new drips...knock on wood)

I then, had to re-evacuate since I didn't know how much I actually charged to the system vs leaked out to the air. Then, added 2 oz of oil and 26 oz (two cans + a little bit more) of R134a.

Low side pressure was ~31, high side was ~260 at 1000 rpm if I remember correctly (90F amb). Jumped up to ~42 psi when in fresh mode and full fan. They system is blowing much colder now on both sides (~47 from both vents).

If these conditions hold up (no oil leaks), I'm going to call it done for now. If things deteriorate, I'll know that I've got a problem somewhere.