I’m a bit late getting in on this thread, but I do have a little to add…AND I have a theory. I too, have sat there with a flashlight watching the motion of my airmix door through that little hole in the dash brace.
My car is a ’93 and I was able to read out the error codes. They told me my HVAC programmer had gone the way of Teddy Roosevelt, so I replaced (and upgraded) with a 95 unit. I still had the flashing ECC display after this was done. Turns out the airmix actuator was the culprit, so the error codes were slightly more than useless. My ECC was doing some whacky stuff too.
A little more history:
Prior to this, it seemed whenever I disconnected my battery I’d get the flashy ECC display and the car would blow heat when I wanted A/C. [During the last failure, it blew cold when I wanted heat (winter in Utah) – how it knows to do opposite of what you need, I don’t know!]. I would ignore the flashy dash for a couple weeks and it would fix itself (my favorite method of troubleshooting). That is until the last failure (seemingly caused by another battery disconnect) this winter - my wife drove the car, blowing cold air, for a couple months until I used the redneck repair as a temporary solution - it was apparent it wasn't going to fix itself. I just recently replaced the airmix actuator and the ECC works like a champ again. I have drafted some procedures to do this…I’ll post a link later next week after the Gods have reviewed it.
Not so brief theory:
The point of the history is to lend credence to my theory. Here’* what I think is happening – why disconnecting the battery hoses up the airmix door and calibration is required. The failure mode for these actuators is a big plastic gear that drives the rod that connects to the airmix door. This gear is pressed onto a serrated shaft and after time will split.
I think, after it starts to split, this gear works OK until the battery is disconnected. As Jeff speculates, the actuator “forgets” where it’* at and when it goes through it’* little startup routine (going full cold, then full hot, then to set position) it pushes too long in the cold position and the gear slips on the shaft. Now when it drives to the hot position (the calibration position), it doesn’t open the door all the way. This is sensed by the ECC system and it sets the “calibration required” code. Because the gear can still grip fairly well, as the system is started up daily it eventually gets moved back to the correct position. (Note: there are no limit switches on the airmix door, I think it’* all done within the actuator motor circuitry). That’* why mine would “fix” itself after a while.
Once that gear splits and strips enough, just moving the door will cause the shaft to slip in the gear. This is the point that, no matter what you do, you can’t get the ECC to stop flashing. You can confirm this failure mode by taking a screwdriver and attempting to move the airmix door arm, with the airmix actuator rod still connected. If you can move this arm easily, your actuator is toast. Don’t turn into the Incredible Hulk and really reef on that arm, you might damage a perfectly good item. But if the arm moves with just a little force, you’ve found your huckleberry.
More information for anyone troubleshooting ECC problems!