Ok, on to the next subject with the HVAC on my '93: The subject pretty much spells it out. When the system is off and I'm driving down the road, I get LOADS of heat from the foot level vents, and I think a little from the defroster vents, but hard to tell for sure. In fact, I almost want to say that I get MORE heat when it's off, just with no fan forcing it out. Any ideas? As usual, I'm assuming this will entail ripping half the car apart to fix... Am I right?

 

Blend door that controls heat is not closing ( fully) allowing air across the heater core so you get hot air flow.

 

Being new to C4's, I wasn't aware that they were like C3's when it came to repairs. Step 1 of any job on the C3 is to dismantle half the car to get to the faulty part.

 

Ah, blend doors... I thought my absolute-sorry-excuse-for-a-vehicle Grand Cherokee was the only one with blend door issues! I will start taking EVERY NUT AND BOLT out of the interior until I get to the blend door!

 

Most cars with Elec A/c use a blend door as a method to mix hot and cold air to get the programmed temp
If you remove the BCM from engine side of ducting ,you can look in and see if door moves.





Location of blend door;
on later cars like yours I believe the door is direct to blend motor ; no linkage as shown in '89 setup

 

C4's were built different to C3
The heater core was put on the production line and the car built around it

 

I've never driven much of anything with the HVAC completely off, but I'm not so sure that isn't a Default position for an Electronic Unit nor have I seen it spelled out any Service Manual. The Trouble Tree's for these Units are for when you've got some Button activated other than Off. All cars provide fresh air to keep you from suffocating and suing the Manufacturer. Now with a Manual set up, the cable will keep the temp door closed. However with an Electronic Unit, it wouldn't surprise me if the fresh air intake pops it open, though the last position should be retained in Memory for whatever temp you had it at. Remember too that if it's still controlling the door based on inputs from the Inside and Outside Temp Sensors, you're going to get heat with lower outside air temps. You might just get the Binary Data out of the CCM Module and see what the numerical representation is for door position and compare it to Outside and Inside Sensor Input or after it's off, turn it back on to verify that it returns to whatever the previous setting was. Otherwise, the Economy setting should provide outside air, though that may still be heated if it's particularly cold out. Assuming it works in Economy or any other selection, I don't think it's worth tearing it apart. The Engineers designed the Vette's HVAC to be on most of the time.