Weird HVAC condition
Thread Starter
Instructor
Joined: Oct 2012
Posts: 236
Likes: 2
From: Tampa FL
Weird HVAC condition
Took the 91' out to a cars and coffee in the AM (Yea!) and my display for the outside temp stayed @47F. In the afternoon it went to about 70F and the display still read 47F, AC worked fine and the heat worked in the AM.
Looking through the FSM, A code 03 or 04 could be set if the outside temp sensor is open/shorted/or malfunctioning. Did an resistance check on it and it was good. The FSM says nothing about the temp being stuck at a certain temperature.
I disconnected the battery and reconnected (didn't want to dive under the dash unless I had to) and then the display read a correct 75F. Seems I reset the control module by the gas pedal...
Is this common? Is there a condition that causes this? Thanks homies.
Looking through the FSM, A code 03 or 04 could be set if the outside temp sensor is open/shorted/or malfunctioning. Did an resistance check on it and it was good. The FSM says nothing about the temp being stuck at a certain temperature.
I disconnected the battery and reconnected (didn't want to dive under the dash unless I had to) and then the display read a correct 75F. Seems I reset the control module by the gas pedal...
Is this common? Is there a condition that causes this? Thanks homies.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 207
From: Orlando FL
I don’t think this is a common occurrence. Digital circuits can on occasion hang or get stuck on a digital bit. I would just move forward and use the car and keep an eye on it.
There is also a data stream between the programmer and control head. Just something else to consider. But my first thought would be to start at programmer also.
There is also a data stream between the programmer and control head. Just something else to consider. But my first thought would be to start at programmer also.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
On 1990 and later cars all the intelligence is inside the HVAC programmer. The HVAC controller is just a bunch of switches and the display. Communication is bi-directional.
'86 to '89 C68 systems are the opposite: all the smarts are in the control head and the programmer just operates the HVAC duct doors and the blend door motor. Communication is one way: from the control head to the programmer.
I've had weird things happen with my system. About 10 years ago it suddenly decided to "remember" the outside temperature that it was reading when the car was shut off. It takes about 5 minutes to slowly drift to the correct outside temperature. Well, sorta correct, as it's always several degrees too high. New sensor didn't fix it. I've had the unit apart several times and found nothing obvious.
'86 to '89 C68 systems are the opposite: all the smarts are in the control head and the programmer just operates the HVAC duct doors and the blend door motor. Communication is one way: from the control head to the programmer.
I've had weird things happen with my system. About 10 years ago it suddenly decided to "remember" the outside temperature that it was reading when the car was shut off. It takes about 5 minutes to slowly drift to the correct outside temperature. Well, sorta correct, as it's always several degrees too high. New sensor didn't fix it. I've had the unit apart several times and found nothing obvious.
Thread Starter
Instructor
Joined: Oct 2012
Posts: 236
Likes: 2
From: Tampa FL
Maybe it was programmed by microsoft, it just needed a re-boot.
Seriously, the computer uses the sensor for the auto climate function. It also uses the interior sensor and the sun load sensor. It did give a trouble code when I disconnected the outside sensor though. Thanks guys.
Seriously, the computer uses the sensor for the auto climate function. It also uses the interior sensor and the sun load sensor. It did give a trouble code when I disconnected the outside sensor though. Thanks guys.
