On my 1991 C4 I have a couple of problems with my acc that I would appreciate to have some help to solve.

One problem is that the compressor never shuts off. Cold air comes out of the outlets, there will build up ice on some of the tubing and it works "fine" but it never shuts off.

As a result of that I use all the other settings on the control panel which does not activate the compressor. Doing that it seems that the fan speed is allways very high. 5 or more to any temperature setting but 27 degrees centigrade. If I use 27 fan speed will eventually go down to 4 in the auto setting. Manually I can set it to whatever I like.

Any ideas?
Can anybody tell me where the sensors are located and how I check them?

Rgds

Dag from Sweden

 

The low pressure cycling switch is probably bad. Pull off the connector and see if the compressor stops. If it does, the switch is not working and closed all the time.

What year car is it?

 

Hi pcolt94,

Thanks for your fast reply!

I didn't suspect the low pressure switch would cause this but I will surely have a look at it.
When I was looking for a Corvette to buy, I came across one where the ac wouldn't start. They suspected that there wasn't enough "gas" in the system and started the compressor by shortening the low pressure switch to show me that was the cause.
My car is 1991 and I will post pictures of the engine so you can guide me.

Rgds

Dag

 

You may want to consider buying the FSM (factory service manual). You will probably have other items that have to addressed over time.

 

Hi George6488,

I will do that eventually. Right now I'm trying to sort out some minor issues before the Corvette season ends here in Sweden.

Rgds

Dag

 

Actually, you might want to learn how mobile a/c or just a/c works.

Refrigerants have a temperature for any given pressure. The R12 that came in your '91 is 32 degrees at 30 psi. Much below that, it makes ice cubes out of the humidity in the air. The low pressure switch keeps it from doing that by opening the clutch circuit. You don't know whether it's working right without knowing what the pressure is, but there's a good probability that it's broken, or something in the wiring is keeping the circuit completed to ground.

 

I am very keen on learning and understanding how an a/c works so please don't stop.

My car has been left alone for a couple of days now (the a/c for longer than that) and I just checked that there is continuity in the low pressure switch when the harness is disconnected. Don't know yet if that means anything.
If there is not enough refrigerant in the system I suppose the switch should shut off the compressor in order not to brake it. Since my compressor never shuts off - would that mean that the pressure in my system is to high? I bet the system has been topped up during these years.

Rgds

Dag

 

The cycling switch opens when the pressure gets to low (below 22 psi) so the evaporator does not freeze up. If there is not enough Freon in the system the compressor it will usually cycle on and off to fast.

If there is way too much Freon in the system, it can make the compressor always stay engaged and never cycle off. This is due to the cycling switch never opening and keeping the compressor running. You would need a set of gauges to determine this. However with that much Freon in the system the pressure will never get low hence evaporator icing is probably reduced which seems not to be the symptom you have.

A set of gauges will tell it all. But low pressure cycling switches are a frequent part that goes bad. I actually had it happen to me few year back. The good news is they are cheap to buy and easy to replace. The system does not have to be opened, it just screws on and off.

Even if you replace the cycling switch on a chance it's bad, I would still get the pressures checked just to know what they are. If you replace the switch and the compressor seems to cycle normal and it blows cold, chances are you are in the ball park for the correct amount of Freon.

 

Paul,

This morning I pulled off the connector to the cycling switch and the compressor stoped just as you said it would.
I value your info on replaceing the switch and will do that on chance since I believe that there is enough refrigerant in the system to make it work properly.
I will have the pressure checked eventually.

Thanks for your help! I will come back with a progress report.

Rgds

Dag

 

For R12, the cutoff threshold is 25 psi; for R134 it's 22.5 psi. If it's set at 22.5 psi for R12, it will make ice. You really need a manifold gage set. Most new switches are now set for the R134 threshold, and if you're using R12, you will need to raise it using the adjustment screw between the terminals .

 

I was thinking R134 and just wrote 22 psi. It's obviously an R12 system and 25 psi is my corrected answer.

 

Paul and SunCr,

I really appreciate all your help! Thank you so much!

Today I ordered and picked up a low pressure cycling switch. Before I picked it up I browsed the Corvetteforum and found lots of useful information. I read about R134 conversions and different cotoff thresholds. I read about a guy who opened the adjusment screw half a turn and suddenly got cold air coming out of the ducts. Adjustment? Interesting!
Here in Sweden you have to pay like 80 bucks for a switch so I thought I should fiddle around with the adjustment screw on the old switch before I installed the new one. If you can open the screw in order to get the compressor to work more then you should be able to do the opposite. I closed it one turn and nothing happened. One more turn and still nothing. The compressor was on all the time. One more turn without any change. Ok, the switch's gone south. I will replace it later. Just when I was about to close the lid the compressor suddenly stoped. Then it came on again. So I continued adjusting the screw until I unfortuneately lost count of the turns. I ended up with this:
The compressor is on for about 6 seconds and then off for about 13-14 seconds. Outer temp was 57 F and the acc was set for 73 F. The fan speed slowly came down and finally stabilized at 4. Does these figures make sence to you guys?
I am happy with the result and hope it will stay that way but I realize that I need to have the pressure checked and possibly the switch replaced.

Cheers

Dag

 

The information you furnished is not really anything you can sink your teeth into and make a technical determination. Sounds good…….sounds bad really doesn't mean anything. Checking the pressure with a set of gauges is the only real way of knowing what is going on and what is right and correct. You really don’t know where you are with a shot in the dark.

The new switch is probably set correctly and I would not touch it. If the system works well with the new cycling switch, that would be a better indication cycling is correctly. It's actually a know good part and reference point.

 

Paul,

Of course you are right. These figures are nothing you can make a technical determination from. But I would like to know if my results sounds fairly good or way off. How low should the fan speed be? My girlfriend is complaining about the "storm" coming out of the outlets. I know I can lower it manually but the auto setting should work comfortably. Or?

If I feel the return pipe from the evaporator I can sense the temperature difference when the compressor comes on and off. Is that a sign of low level of refrigerant?
Came to think of the possibility that some technician at some point loosened the adjustment screw in order to sell the car with a functioning acc.

My 2004 Audi A4 doesn't shut the compressor off and on so frequently but my previous car, a 1997 Audi A3, did. Also the cooling fan was on very often like in the Corvette now. The A3 acc didn't work when I bought the car so I had repaired at an athourized dealer.

The new switch is, as you say, most lightly set correctly. But to what? R12 or R134? Also I don't know what's in the system.
The car is 20 years old and I'm quite sure some repairs has been done to the system since the return pipe from the evaporator, the one with the switch in question, has abrasion marks after contact with top fender part under the bonnet. It was poorly routed after a repair of something. The marks doesnt seem to be that old so a pretty good guess is that the system is filled with R134. I can't tell if it also was converted at that time.
If I replace the switch and get the "not cold enough" feeling, it is probably set for R12 and I can allways open the adjustment screw half a turn.

I guess what I'm saying is either to have the system completly checked up and properly set up by a proffesional or settle for the system as it is. Perhaps with the new switch and a backyard top up. The later I can get from a proffesional working after closing hours. Real stuff (R134) and proper equipment.
There is a difference between the acc season in Orlando and Stockholm and my car will probably be in the garage from mid october to april so perhaps the later option will do.

Still it bugs me a bit not to have system work as it should and I dont like that my shortcomings is money to others. I will probably end quite well educated in acc area.

Oh, by the way - this morning the auto light in the acc control panel started to flash. I set it to the "air in the face" setting where the compressor is not actuated. Still flashing. I shut it off and the to auto again. Then the system appreared to work as it should.

Cheers

Dag

 

I think you should take it to a shop. You're not going to be able to fix it by guessing and changing parts and you are taking a chance of making a really expensive mistake.

 

I actually don’t know who runs the A/C in auto mode. Most everybody runs it in manual mode for better control, but each to his own.

You question if the system has R12 or R134 in it. If it was converted the fittings should tell you. If they are the screw on type, it's probably R12. If it is the clamp on fittings (especially the Low side) with the kind of mushroom top, then it's R134.

If you bought a switch for a 91, then it is probably set to R12 setting.

 

Factory specs are no more than 8 cycles per minute at 70 degrees. Cycling is hard on the compressor (which is why GM no longer uses a cycling orifice system) and realistically, a fully charged system shouldn't cycle at all when it's 70 or better. At 50 degrees, it shouldn't exceed this limit - most will be off longer than on. Without gages and not knowing the temp of the air across the Condensor, you can't really adjust the switch. Note - the last switch I bought was set for R134 and with the lower threshold, it made ice until I adjusted it to the correct pressure for R12. I didn't realize it until it was below 70 degrees and I only happen to notice it because I was working on something else.

 

Paul,

I'm not sure which fittings you mean. Please have look at my photos, perhaps you can tell me from them.
Don't know why but I get the feeling that the system not is untouched.

Regarding the auto setting. On my Audi I have the Multitronic tranny. I can, in a way, shift it manually but why should I? It does a much better job than I ever could manually. The Multitronic accelerates faster both than the traditional automatic gearbox and the stick so perhaps I'm a bit spoiled with really good auto modes.
With only three sensors in the acc system I think it's possible you can do a better job yourself. I know my girlfriend complains about to much air coming out the outlets on the passenger side. Normally I use auto for both the fan and for the direction of the air but I'm not too satisfied with the performance.

You think the switch is set for R12? I wonder why the didn't ask what refrigerant I have. I guess there are cars still running with R12 but most of them would have converted or topped up with R134 since R12 is long gone.

Cheers

Dag

 

No more than 8 cycles at 70?
Perhaps I don't have correct pressure in the system but my unscientific adjustment seems to have adapted the switch quite well to the situation.
If GM stopped using the orifice system, I guess so did Audi. I don't hear the same on and off sounds.
70 or better - do you mean 70 or more? What do you mean by not cycling at all? On all the time?
What do you mean with "At 50 degrees, it shouldn't exceed this limit"?

As I said earlier: "The compressor is on for about 6 seconds and then off for about 13-14 seconds. Outer temp was 57 F and the acc was set for 73 F". That means the compressor comes on every 20 seconds. That's three cycles a minute. Not to bad.

I guess I have to check my switch to see what pressure it is set for before I install it. Then depending on behaviour, I can guess what's in the system. R12 or R134.

Cheers

Dag

 

I don’t see any pictures that you posted so I am posting 2 pictures. They are both fittings on the low pressure side. One is for R12 and the other is R134. You should be able to identify which one you have.



R12 Fitting


R134 Fitting