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1992 Reatta strange compressor cycling

newton5 on Wed May 05, 2010 9:46 PM User is offlineView users profile

Year: 1992
Make: Buick
Model: Reatta
Engine Size: 3.8L
Refrigerant Type: R-12
Ambient Temp: 80
Pressure Low: 40
Pressure High: 180-200
Country of Origin: United States

This one has me scratching my head. It's a 92 Reatta with EATC.
I was delighted when the customer said he had the factory service manual. Unfortunately, the majority of the EATC diag. info is contained in a later "Final Edition Service Manual".

The compressor cycles regularly at about 10 sec. on, and 10 sec. off. When the compressor cycles off, the pressures are:
-40psi and dropping
-180-200 and climbing slightly

The manual shows a low pressure switch that opens at 8psi (??), a low side temp sensor and a high side temp sensor. The switch and sensors are inputs to the BCM (Body Computer Module) The switch also supplies power to the compressor relay coil, but at 8psi opening it's obviously not a cycling switch.

Is anyone familiar with this system. I suspect it's similar to other high line GM vehicles of that era, but I have little experience with the electrical part of the system on those cars. Is there a self test or diag mode on the operator panel that might be useful?

The High side temp Sensor is located in the liquid line about 18" from the evaporator and the Low Side Temp sensor is located right at the output of the orifice tube on the evaporator inlet tube. I'm guessing the BCM compares the 2 readings to control the compressor clutch. After that I'm lost.

Any wisdom or thoughts on how I should proceed.

GM Tech on Wed May 05, 2010 10:11 PM User is offline

Works just like a Caddy system-- or an Aurora-- no cycling switches-- just a low pressure switch in case of no refrigerant. You are correct, it uses an algorithm to compare low side input refrigerant temps to high side refrigerant temps- then decides when to run the compressor. The key to the system is having the correct refrigerant charge in it-- (I suggest pull and weigh refrigerant, then weigh back in correct amount)- if charge level is low-- then look at compressor shaft seal as primary leak location. Those early 90's cars were also prone to evaporator leaks--does air flow out vents good? look at evap air inlet face through blower motor module hole (remove module) if there isoily greasy build-up on evap-- then evap is leaking- collects dust dirt and plugs up air flow....

Gotta love those Caddys-- they are in a world of their own......

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......

newton5 on Wed May 05, 2010 11:16 PM User is offlineView users profile

Thanks Tech.
I'm actually just getting back to this vehicle and the cycling issue, my apologies for failing to mention that.

The car came to me with a "poor cooling" issue early last fall that turned out to be poor airflow due to a mouse nest in the evaporator case. It was also short cycling at that time.
After cleaning the evaporator case I recovered and recharged, assuming it was cycling because it was low on refrigerant. The amount I recovered was within 3oz of the specified charge amount. The airflow from the vents is now very good. The short cycling still remains, though, and the cycle times remained constant before, after and up to now.

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