1990 Air conditioner compressor
Thread Starter
8th Gear
Joined: Feb 2005
Posts: 8
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From: Millersville MO
1990 Air conditioner compressor
Can someone tell me if its normal or not for my compressor to cycle on and off all the time while in "AUTO" mode. (Electronic Climate Control) It also does it in the "HEAD & FEET" slection. I know this may be vauge,but I don't know what else to put right now.
Team Owner
Joined: Aug 2000
Posts: 40,156
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From: San Pedro CA
are you saying the lights on the panel are changing? or the air is moving up and down?
If the lights are changing, there is a problem with the programmer.
if the air is moving up and down, thats normal in auto mode, keeps all air at the same temp. works quite well, really.
If the lights are changing, there is a problem with the programmer.
if the air is moving up and down, thats normal in auto mode, keeps all air at the same temp. works quite well, really.
Team Owner
Joined: Oct 2000
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From: 63.8% of all statistics are made up on the spot.
St. Jude Donor '08-'10
the way I understand the workings of it is that after you pick the temp that you want and set the auto mode the car will turn the a/c on or the heat whichever is needed to get the inside of the car to the temp that you have picked. If the air temp is higher than the temp you have picked it turns on the a/c. When the inside temp reaches that point it turns the a/c off and if the inside temp goes up it will turn it back on again. Same thing with the heat except in reverse.
Thread Starter
8th Gear
Joined: Feb 2005
Posts: 8
Likes: 0
From: Millersville MO
Air conditioner compresser
What I was trying to say was, no matter what the temp. setting,the compressor cycles on and off. Almost like clock work... 20 seconds on... 10 seconds off all the time. The lights remain on the settings that have been slected, no problem there. Thanks for responding. Any help is appreciated. 
Le Mans Master
Joined: Dec 2000
Posts: 7,839
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From: San Diego, Ca
Basically, if the compressor didn't cycle or have some means of controlling operating pressure, the Evaporator would ice over and once air flow is blocked, there is no way for it to absorb heat. The freeze protection device in the system is the Low Pressure Switch. It opens or breaks the electrical circuit to the compressor clutch when it senses a low side pressure of 25 psi (R12 systems - R134 is set at 22.5 psi).
Pressure is dependent on ambient air temp and charge or the amount of gas in the system. Lower temps mean lower pressures and at 50 degrees or below, the compressor may not come on until engine heat warms things up a bit and at these temps, the compressor is going to cycle with a great deal more frequency. At 70 degrees or above, pressures are generally high enough to keep the compressor from cycling at all. GM usually specs all of it's a/c systems to 8 cycles or less per minute at 70 degrees.
Excessive cycling can be caused by a low charge or a plugged up system. After '89, most GM systems will stop working with a low charge and generate a trouble code. A plugged up system is bit more problematic. Both conditions should be diagnosed with a manifold gage set and most a/c shops charge very little to do it for you (the last Pep Boys ad I so wanted 29 Bucks).
Pressure is dependent on ambient air temp and charge or the amount of gas in the system. Lower temps mean lower pressures and at 50 degrees or below, the compressor may not come on until engine heat warms things up a bit and at these temps, the compressor is going to cycle with a great deal more frequency. At 70 degrees or above, pressures are generally high enough to keep the compressor from cycling at all. GM usually specs all of it's a/c systems to 8 cycles or less per minute at 70 degrees.
Excessive cycling can be caused by a low charge or a plugged up system. After '89, most GM systems will stop working with a low charge and generate a trouble code. A plugged up system is bit more problematic. Both conditions should be diagnosed with a manifold gage set and most a/c shops charge very little to do it for you (the last Pep Boys ad I so wanted 29 Bucks).
