My C68 climate control in my '94 Corvette seems to have an issue with the outside A/C temperature reading. Though it doesn't happen all the time, at least once a day the outside temp reading will go haywire and the reading will increase/decrease 5 to 10 degrees (i.e. reads 75 deg, drops 5 degrees or so, quickly increases back to 75 or higher). Where should I start to look as far as determining the cause? I recently bought a reconditioned C68 control unit and outside temp sensor, but could it possibly be the HVAC programmer as well? Any help/advice would be appreciated.

 

The outside air temperature sensor feeds into the programmer. So it could be a problem. I would clean the connections in the connector on the programmer as well as any connections up by the sensor.

It looks like the sensor resistance is read bythe programmer, then goes to the head by a digital data link. So I would not go there first if all other HVAC functions are operating.

You could temporary replace the sensor with a fixed resistor of equal resistance of the sensor at ambient temperature. That would be one way to eliminate one item.

But I have to ask, what was the original problem where you replaced the sensor and the head.
Was it the same problem? Did this function work OK with the original parts?

 

I haven't replaced any parts yet. I bought them as backup replacements. This problem only happens once. I could drive my car 4 times in a day and it will act up only once. The only other known issue I have with the hvac is sometimes I get the flashing light on my climate control, then it stops after a minute. The trouble code is "01", the temp door, but everything else functions fine.

 

The temperature blend door position feedback voltage goes to the programmer. If the programmer does not like where the door is per feedback voltage, then you get a code 1 or 2 depending which end of the doors range it does not like. Sometimes the the motor assembly go bad. But given these two symptoms, I would definitely start by cleaning the connector and edge connector pins on the programmer.

You might want to consider the resistor idea just as a point of elimination. Or since you got a replacement sensor, put it in.

The programmer is a bi**h to get it in and out to work on it but all you need is time. I would give it a half day of grief and have the beer cold afterwards.

 

Thanks for the tip(s). I'll make sure my garage fridge has a 12 pack handy before I bury myself underneath the dash.

 

Thats an understatement....

It's a helluva a days work for a SINGLE friggin 10mm bolt....
Puttin it back right is even more fun !

If it comes to that, a for sale sign is cheaper than the 6 or so chiropractor appts that you'll pay for.

I had a similar issue and a new sensor cured it. The outside temp sensor is a pellet type that depends on sensitive resistance levels to send accurate info to the system. A dirty connection, loose connection or even rotted wire will make it do odd things. I also relocated mine to a spot in the nose over the air-dam so it got fresh air and did not suffer from heat-sink where it was mounted further up in the nose area.
It used to rise 10 degrees sitting at a stop light...then back to normal as soon as I'd start to roll again.

The control head is a 2 part circuit board that has one board seated into the other, so the socket gets loose and things get weird.Thats more constant than what you describe.

The BCM handles power distribution, and the programmer manages vents, BUT, it is part of the loop for the data, but it rarely fails. Its 99% vac mechanical with electrical control. Save that thing for last...

One more important detail, the programmer MUST be installed level in its original position or it won;t work right. I am not sure how or why, but if its not mounted correctly, it just won;t work.

 

Let me add when I worked on my motor problem I cheated at bit. Rather than disconnecting the hoses and assembly on the programmer to get the box out, I opened the box and unplugged the connectors and took the board out. When I was making voltage measurements I had it out of the box but connected with the one connector. Check my thread and see the pictures.

http://forums.corvetteforum.com/c4-t...blem-94-a.html

Just to put your mind at ease on getting it out, it's work but not the worst. What was a real pain was getting the cruise control module out. Now that will make you cry. I'm not posting those pictures.

Troubleshooting first in the easy to get places. Only dig into the car as a must go situation. Do all that's possible from the exterior.

Never heard that about mounting the programmer. Don’t know if you could get it in another position anyway. The only logical item I can think of is if not properly positioned, the vacuum hoses might kink and vents would not work properly.

 

Someone told me that the little vacuum servos were very weak and the solenoids lacked the strength to operate them if they were tilted...I dunno.
I did have trouble with mine, went back and got it as level as possible and its been fine since....

The programmer was kind of like traffic school...

Been there and have no desire to go back unless I've GOT to..

 

Wow. I appreciate all the info. I guess I'll start by replacing the outside temp sensor with the spare I bought and just go from there. My a/c works strong and is cold, just have that temperature "bug" to resolve.
Judging by the comments you guys made about the programmer, I guess that and the heater core is what GM built this car around.