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Weird Heating/Cooling issue with AC Off


I’ve got an issue with my Alero that has me stumped.

So right now where I live the outside air temp is cool, maybe 50 degrees. My car has an AC On/Off button, and a temperature dial. With my AC off, and my temperature dial on full cold, once the car gets warmed up for a few minutes my vents act as if I am in full heat mode blowing hot air. If I nudge the temperature dial one click (it has about 20 positions), suddenly it will start blowing outside temp air. Sometimes it stays this way the whole ride, sometimes it will randomly go back to hot air again, sometimes it will cycle at random intervals. If I nudge the dial sometimes it will fix it quickly, other time I have to nudge it again a few times.

At first I thought it was an AC issue, because I had this happen to me with the AC on when it was a hot day. But my AC can make ice cold air no problem. I checked the refridgerent and its good. With the AC on I get the same symptoms as above, but instead of ambient air I get ice cold air when its working, then randomly heat air. I really feel like the heat level is appropriate for actually being in Heat mode, not just some ambient heat from the car or something.

So I’m pretty stumped. How does the temperature dial work, is it a potentiometer than the computer uses to dictate how much hot or cold air is allowed into the cabin? Is it a mechanical swiper of some sort? I’m very knowledgeable about electronics (my career is an electronics technician), so if its anything to do with that I can fix it, I just don’t know what that knob is actually doing and what other components behind the scenes make it all work. I understand the heat exchanger, the evaporator, how all that works in relation to compressor/refridgerant for cold and engine temp/coolant/thermostat for heat, but no idea what actually happens to only allow one or the other to enter the cabin and why my car is screwed up.

Thanks in advance for any help!

I’m not sure what type of switch the temp dial actually is but its effect is to command a blend door to move. The blend door is a motor driven panel in the ducting that directs more or less air past the heater core. They have a motor and sensor so the HVAC computer knows where it is based on the temp dial’s position. If the temp dial has numbers on it, in degrees F, there is also likely a sensor or sensors in the car to provide feedback to the body computer that controls the blend door. The blend door actuator is a pretty likely culprit as is the switch. The blend door actuator is available from Rock Auto for about $40. The switch? Might be a GM dealer of junkyard part only 'cause Oldsmobile.

It sounds like the water valve is opening and closing, but your description makes me suspect that the problem is not in the valve itself but in the temperature control knob connections. Can you take the temperature control knob apart and make sure everything looks tight, and perhaps spray some tuner cleaner through it?

So an update and I think I know the problem.

I found a video and popped the temperate control module out from the dash and confirmed it is electronically controlled and the three knobs are three potentiometers. When I took the knob covers off the potentiometers, the temperature one is really loose while the other two are rigid. When I push down on the potentiometer, everything works fine. When I let go it goes into full heat mode. Repeated about 10 times consistent results.

So I think what’s going on is the potentiometer has one (or multiple) broken solder joints on the board and so when its not making contact, the computer is seeing an open so high resistance and that must be what makes it go full heat mode. When it is making contact it is seeing the appropriate resistance and doing whatever position it is in correctly.

I’m gonna try to get the board off from the dash tonight and resolder it. I’ll let you guys know if it works!

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Tentatively my idea seems to have been correct and fixed it. I’ll wait a week before I feel its good to go.

I took the circuit board out and that pot was definitely wiggling. I couldn’t remove the board from its housing because it was riveted on, so I could only solder it from the wrong side and hope it made its way to the other side of the board. Thankfully I have very nice tools for this for my primary job! After I was done it was rock solid, and I measured it and it was operating correctly.

Put it back together and been working so far no issues. We’ll see!