I have been having problems with my climate control lately. The a/c works fine, however the outside temperature consistently reads about 30 degrees fahrenheit low. Normally this is not a problem as I'm not terribly concerned about knowing what the outside temperature is but the problem is that when it gets down to about 60 degrees or so, the climate control thinks its below freezing and prevents the compressor from cycling. Although I don't need the A/C on at that temperature, I do sometimes need the defrost mode to dry the air and keep the windshield from fogging up. I understand that the control head takes readings from two sensors and displays the lower number in order to rule out any bias caused by engine heat. I have unplugged each of these separately with no change in temperature reading. I figured that it is unlikely that both sensors failed so I replaced the control head with a used one I bought on Ebay. Unfortunately, the outside temperatures still read low and what's worse, the replacement head failed to turn the a/c on even though the outside temp read: 46 degrees fahrenheit. Should I assume this is a head problem or is there another control module I need to look at. Thanks for the help.

Audifan1989

 

There are two head units used, analog and digital. 89 models should use the digital version. The same head units were used on high end GM vehicles like Cadillac’s, Buicks, etc. and to some respects are interchangeable. The system is also called the Harrison CCOT system and uses more than just two temperature sensors. The outside temperature sensor on my 91 is located behind the grill next to the left headlight. Often the plastic holder will break with time and the sensor bounces along until it gets damaged or shorted. Also check the connectors as they have a tendency to corrode and give false resistance readings. Also check the multi function sensor as this too has a tendency to corrode badly. The other problem is the flap servo. When I replaced these two parts things started working more normally.

 

Thanks for the tip. It did turn out that the sensor by the drivers headlight was faulty. Last time I disconnected it I didn't wait long enough for the head unit to sample (and adjust the temp reading) using the other sensor.

Mike