Automotive Air Conditioning Information Forum (Archives)

wptski on Mon May 27, 2013 8:19 PM

Year: 1978
Make: Chevy
Model: Corvette
Engine Size: 350
Refrigerant Type: R12
Country of Origin: United States

Car was stored in 2001, started last summer but had no brakes and no AC.

IIRC, had the same issue in 2001, AC clutch cycles ON/OFF shortly after turning then AC ON. Believe it or not, low pressure switch is still made! Picked up a generic switch from a auto parts store(Ranco 9533N400 REV. A). This switch has a cut in and cut out adjustment screws which aren't going to be accessible after installing which is going to be fun anyway. Does one assume that it's adjusted correctly or should I try to check/adjust, if so, how? I put it in the fridge which is around 38-40F and it didn't open but in the freezer a short time, it did but the outside case wasn't cold. I have a thermal imager. Is that a hollow tube with fluid or what inside? The old switch had the end bent right around, tight. You'd think if it was hollow, it would break!

I tried some ice in water with a fairly accurate glass thermometer with the tube end in the water. It got down to 33F which seemed to be the lowest I could get and still stayed closed. IIRC, a ice slurry is the proper way?? Any comments?

BTW, I have five 12oz cans of R12 in case it's low also.

WyrTwister on Tue May 28, 2013 9:56 AM

You can mix ice and salt to get temps lower than 32 degrees F . Maybe ice and alchol (sp) .

God bless
Wyr

mk378 on Tue May 28, 2013 10:22 AM

It's a temperature switch, and it seems to be working properly. The tube on the switch is a sealed system containing a volatile liquid, and its internal pressure (proportional to temperature) operates the switch contacts. It doesn't measure pressure of refrigerant in the A/C system.

The compressor has to cut off before the evaporator reaches the freezing point, or condensed water will freeze on the outside. The ice then blocks the airflow and the A/C system stops working.

Those temperature switches did go bad a lot, but rapid cycling of the compressor is usually a pressure issue tripping the high pressure cutout switch in the high side line or the back of the compressor.

wptski on Tue May 28, 2013 1:27 PM

It wasn't rapid cycling but maybe every 10 seconds. There isn't any high side pressure switch in this system and I had power through the low pressure switch to the evaporator switch but nothing on the other side going to the clutch. The evaporator pipe to which the switch tube was attached to wasn't even cold to the touch. I have an electrical diagram book which shows the switch opening up at 32F.

The suggested salt water did work. Water at 30F or lower, the new switch wouldn't open up. I grabbed the old switch and within seconds, it opened up. I noted the screw positions on the new switch and tried each one up to a full turn each way, still nothing but if you put it in the freezer for a short time, it opens up. It appears that the new one is at least way out of specs but I'm unsure if it could be adjusted that much.

Back on the old switch. It stayed open till around 43F. Would that be a bit too high? It got banged around a lot during removal too. It also has a one screw on the bottom, maybe for an adjustment??

Edited: Tue May 28, 2013 at 1:30 PM by wptski

WyrTwister on Tue May 28, 2013 1:46 PM

This is just a guess , but I would think , with the compressor clutch off , and the interior fan blowing , it would not take too long for the evaporator switch to reach 43 degrees F ?


God bless
Wyr

wptski on Tue May 28, 2013 1:57 PM

Quote
Originally posted by: WyrTwister
This is just a guess , but I would think , with the compressor clutch off , and the interior fan blowing , it would not take too long for the evaporator switch to reach 43 degrees F ?

God bless

Wyr
That could be but it appears that it "was" opening up way too soon before I removed it.



Edited: Tue May 28, 2013 at 1:57 PM by wptski

wptski on Tue May 28, 2013 7:08 PM

I was curious about the new switch so I placed some heavily salted water in the freeze and trial/error it to see when it would open up. At 28.5F in didn't but at 23.7F it did so somewhere between it opened up but didn't close till 48F.

WyrTwister on Tue May 28, 2013 11:40 PM

If I am understanding all or most of this , it sounds like the new switch may not work ?

God bless
Wyr

wptski on Wed May 29, 2013 7:56 AM

Quote
Originally posted by: WyrTwister
If I am understanding all or most of this , it sounds like the new switch may not work ?

God bless

Wyr
I understand that this might be a bit hard to follow this and odd that somebody is testing a new part. It's only two screws but in such a difficult spot in this vehicle to get to, I wanted to verify that it's working properly and plus it's adjustable. It'll work but by the time it opens up the evaporator would be a block of solid ice! Way out of adjustment or defective, I'm not sure which.

It was purchased at O'reilly's on the 26th at their hub store, no other location had it. They only stocked one and it looks like it wasn't replaced yet. I was going to take it back after they had another in stock to try another and make just one trip but maybe that's not going to happen.

wptski on Fri May 31, 2013 9:21 AM

The post by mk378 was correct! I bypassed the switch and the evaporator pipe started to frost up. I stumbled upon this really good thread all about that switch: Evap Switch. When I said that the pipe wasn't cold was because it was cold lower on the pipe covered with that black putty and doesn't travel that high before turning the clutch OFF. I may be low on Freon or some other issue.

I returned the new switch and reinstalled the old one. I slid in a bead type K-type temperature sensor under one of the hose clamps to monitor the temperature and will connect gauges next.