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freezing up

raylo on Thu July 10, 2008 7:25 PM User is offline

Year: 1994
Make: Chev
Model: S-10 P/U
Engine Size: 4.3L
Refrigerant Type: 134a
Ambient Temp: 82
Pressure Low: 22
Pressure High: 155
Country of Origin: United States

I have done several a/c jobs over the years and never come across this.

I just redid my system with a reman R4 compressor(from local discount parts house), new condenser & new accumulator (from AcKits), new suction/disch hose set. Alls I kept was evap and liquid line. Also all new o-rings. System was clean but I flushed it anyway. Evacuated and it held vac overnight. Charged with 30 oz 134a IAW placard in engine compartment. Also installed 8 oz PAG 150.

Running a/c on max it freezes up the condensation on the evaporator, accumulator, and suction line big time. Looking at the low pressure that sort of makes sense since low 20s psi is solidly in the freezing zone. But it never gets low enough to cycle the compressor off. It did cycle as expected during the initial charge so I know the low pressure switch on the accumulator is functioning. I also unscrewed the pressure switch while the a/c was running as a rough test and it did indeed disengage the clutch. So I think the switch is good.

I understand that this is usually a sign of low charge but I am not detecting any obvious leaks (electronic detector). And if so why doesn't pressure get low enough (20 psi?) to cycle the compressor off?

I may be getting a little bit of a detect near the compressor but can't get by the clutch to get very close to the shaft to really be sure if the seal is leaking. All the line connex are solid, no sign of leakage at all.

The pressures I noted above were while revving the motor to 2500 rpm and seem very low to me. Leak/low charge or just a lame weak compressor? Are there any othe causes of freezing that I need to check before returning the POS compressor and getting a new OEM unit?

One other note: I used a smart VOV orifice tube. I don't think this should be a factor.

Edited: Thu July 10, 2008 at 7:53 PM by raylo

chris142 on Thu July 10, 2008 9:22 PM User is offline

You can get an adjustable cycling switch. Will need to cut the old plug off and crimp/solder on different connectors. Then you can adjust it so that it cycles off wherever you wish.

Chick on Thu July 10, 2008 9:23 PM User is offlineView users profile

Your cycling switch is probably set to around 19 to 21 psi, so until you get the low side to drop that low, it won't cycle, try windows up, ac on max, and low fan speed, see if it cycles then, and note the pressure when it does.. Frost on the low side is normal,. it's freezing the water in the air as it transfers heat. The Vov may be contributing to it, but as long as your vent temps are cold, who cares.. But you don't want the evap to freeze to a block of ice, I'm not sure, but you may have an adjustable cycling switch. look for a screw in between the two prongs of the switch, to raise the cycle off pressure turn clockwise one turn..Let us know what you find..

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Chick
Email: Chick

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Freedoms just another word for nothing left to lose

GM Tech on Thu July 10, 2008 10:00 PM User is offline

bad cycling switch, period...

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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......

raylo on Fri July 11, 2008 5:55 AM User is offline

Thanks guys, perhaps I'll pick up a new switch today and give it a try. They're cheap and easy to replace. I really think the swicth is working OK, though, since it did when I charged the system. I had to add a can or so before the comp would initally cycle "on" to continue charging. Then it cycled less and less as the charge built, just as expected. I did not record the exact cycling pressure but I believe it is/was around 20.

BTW the evap does freeze into a block of ice if I give it long enough. It leaves huge pools of meltwater after I park it and leave it sit for an hour. I've not had it so bad yet that it actually blocked airflow over the evaporator. Perhaps it would with a longer drive.

And I still wonder about the compressor perhaps not being strong enough to create a high enough pressure differential to pull the low side down below the trip point. I have tried low load (a/c on max with blower fan on slow) and it still doesn't cycle. The high side pressure looks low to me, too, but I don't mess with this stuff often enough to remember precisely past jobs. I should keep a log book of readings/conditions whenever I do an a/c job for reference.

Edited: Fri July 11, 2008 at 6:43 AM by raylo

GM Tech on Fri July 11, 2008 8:54 AM User is offline

Your switch should cycle "off" at 22 psi-- the old switch will cycle- but at too low a set point-- get the right switch for your truck-- heck you may even have one for a van on there now-- they are all different because of the settings- but they will all thread on and plug in-- look for a yellow band, or a brown band, or a gray band around the base of the switch-- An S-10 I believe takes a yellow band- as I remember- would need to look it up to be sure...

-------------------------
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......

raylo on Fri July 11, 2008 10:30 AM User is offline

I am original owner and no one else has worked on the a/c ever. So it is the OEM switch and unless GM built it wrong it should be the correct item. It has a brown colored plastic band at the base. The system worked fine before my recent work. It cooled well and didn't freeze. It was just that the the compressor was getting noisy and I wanted to avoid the black death scenario. I had done that install way back when, too and used the same OEM LP switch. I am going to get a new one to try even though I don't think that is the problem....

raylo on Fri July 11, 2008 2:47 PM User is offline

Well, the new switch is in and the compressor is cycling. I did some low load testing in the driveway and it cycles off at about 23.5-24 psi with the new switch. I repeated the test with the OEM switch and it would never cycle off by itself but when it pulled down to 21-22 psi I opened the throttle gently and was able to observe it cycle off at 20 psi as the rpm increased.

So it works and doesn't freeze with the new switch but the questions linger for me. Assuming GMTech's mention that 22 psi is the correct setpoint is the new switch cycling at too high a pressure? Reason I ask that is that the center vent temp at steady state only gets down to 47 or so. Bottom line is I am happy it isn't freezing anymore but am disappointed with the warmish temps. Seems we should be able to get these 134a systems to blow down into the low-mid 40s and that a real 22 psi switch would make that happen. It was doing that before with the old switch but wasn't cycling enough to keep from freezing up.

I am going to search around and try to find some more detailed specs on these pressure switches. I am also going to research the adjustable pressure swicth mentioned above. Maybe that is the way to go?

Edited: Fri July 11, 2008 at 2:54 PM by raylo

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