HVAC Controller...again

[TheEngineer]

My 96 Bonneville was having a problem for a while with the HVAC controller failing to function properly. I would turn it on one day and it would blow all hot. Turn it on later and it would blow all cold. Back and forth, no control whatsoever from me. Finally, it just started working again. Up until then I thought something was mechanically wrong, but now I believe it has to be an electronic issue with the controller. Further confirmation of this is that I just did some work that required the battery to be disconnected and suddenly the problem is back. How often do these controllers go bad? Is this a common issue? GM Parts Direct is quoting this as a $500 part, which I have no intention of spending. The car isn't worth that much. Is there a way to calibrate these things?

[willwren]

Can we confirm which climate control you have? Manual knobs or ECC (Electronic Climate Control)?

[TheEngineer]

Electronic.

[willwren]

A typical failure after battery disconnect/reconnect on this generation is a failed airmix actuator. The plastic gears snap and break when the system re-initializes.

Verify this by removing the glovebox liner (very easy to do) and watch through the small round hole in the dash support bar as you cycle from heat to cold repeatedly. I assume your ECC display is flashing as if it has an error?

If the airmix actuator isn't bad, I'm afraid it's a tossup between the ECC head unit itself or the HVAC controller. Unless you have access to a Tech2, you'll probably have to shotgun this one.

[TheEngineer]

A typical failure after battery disconnect/reconnect on this generation is a failed airmix actuator. The plastic gears snap and break when the system re-initializes.

Verify this by removing the glovebox liner (very easy to do) and watch through the small round hole in the dash support bar as you cycle from heat to cold repeatedly. I assume your ECC display is flashing as if it has an error?

If the airmix actuator isn't bad, I'm afraid it's a tossup between the ECC head unit itself or the HVAC controller. Unless you have access to a Tech2, you'll probably have to shotgun this one.

Yep, it's flashing. And I thought a mechanical failure was the problem before until the problem just went away and everything worked fine, including the flashing stopping. Even now it seems to alternate between supplying only full hot air, or working properly. It's luck of the draw when I start the car. I will take a look to make sure the mechanicals are alright, but I would think if it works sometimes, the gears must be intact.

[willwren]

Or a gear is cracked/slipping, or the actuator arm is not retained properly.

[Sirius]

Yep, it's flashing. And I thought a mechanical failure was the problem before until the problem just went away and everything worked fine, including the flashing stopping. Even now it seems to alternate between supplying only full hot air, or working properly. It's luck of the draw when I start the car. I will take a look to make sure the mechanicals are alright, but I would think if it works sometimes, the gears must be intact.

If I were placing bets, it would be on the airmix actuator (mechanical) and I'd probably have over a 90% chance of winning. My '93 SSE did this too - disconnect battery, get the flashy ECC display, erratic heat control, then after a period of time all seems well. It gets progressively worse then, one day, kaput. I believe, in your case, the gear WillWren speaks of is compromised but not completely defunct.

Do as Will said and watch the movement of the arm, it should be smooth and consistent. No jerking or hesitation and should stop at the same extremes every time (set ECC to max then min temps). ANY anomalous motion is indicative of a bad actuator. Even if the motion seems proper it could still be faulty. With the car off and using a long skinny screwdriver (or something similar), reach through the hole and try to move the arm manually. You shouldn't be able to move it. Don't go Hercules on it, but even under moderate pressure it shouldn't move. If it moves at all, it's hosed.

It is very rare for the ECC head unit to be faulty, and only slightly less rare that the ECC controller is bad. Concentrate on the airmix actuator to begin with, I think.

Here's a link that may help. There's a picture of a broken gear near the end. Might help you get a handle on what's going on. http://trialsnuts.com/ecc.pdf

[TheEngineer]

Very nice writeup. Hopefully I can get a chance to look at it this weekend. For now, it is flashing, but after a little hesitation at startup, it seems to be working.

Any handy writeups like that for replacing a leaky evaporator?

[Sirius]

I dunno. Check around in Tech Info, there's lots o' good stuff in there.

[TheEngineer]

Probably no one cares, but I figure an update to this can't hurt.

So I planned to take apart the glove box to watch the action of the mechanical parts but...SOMEONE locked the box and then proceeded to lose that key. So I need to get a new one and in the meantime can't open my glove box. However, before I could get the key, the display stopped flashing and it is now working properly again. No issues whatsoever. And as a side bonus, I bought a can of stop leak for the leaking AC evaporator and it actually worked! So now I have a fully functioning HVAC system again for the bargain price of $20 for the stop leak. Not too shabby. Someday when I get that key I will take a look at the mix door in action, but for now I'm leaving well enough alone.