Carnivore

I have a 1990 C4 Corvette with the C68 Electronic Climate Control. For the second time, I need to replace the Blend Door Actuator. In the 1990 Corvette, the Blend Door Actuator sits on top of the Heater Core Housing Case; a major project to replace due to its location. I have heard that some owners have replaced the 1990 Heater Core Housing with the 1996 housing. The 1996 Heater Core Housing has the Blend Door Actuator on the bottom of the case providing a simplified access. I have heard that to move the Blend Door Actuator to the bottom of the housing, the wiring to the Blend Door Actuator must be changed. The wiring for the 1990 Corvette is
RED Red - +5V reference
BLU Blue - Mix value feedback
BLK Black - ground
YEL Yellow - Motor
TAN/BLK Tan/Black - Motor

Has anybody done this swap? What wiring needs to be changed?

tekheavy

Not to hijack your thread, but since you've already done this job once, I have a question for you. I have a 91 which has a bad blend door actuator. It also has the actuator on top of the heater case. Do I have to take the entire dash out to replace it, which I'm assuming, or is there any easier way to do this? Any help would be appreciated.

frosted

I replaced mine (on a '91) last year. I'd take the dashboard off, though someone with experience just might be able to remove the heater core housing without removing dash. There are a couple of screws which can only be accessed from top and of course the ducting needs to be disconnected and moved out of the way. Compared to the total time removing the dash isn't that much.. And it allows for a little better ergonimics and visibility

Carnivore

There is a video on youtube of someone who removed the heater core and did not take the dash pad off. Note that he had to cut the A/C heater side window air duct so he could detach it from the heater core case after the case was taken out. The heater core is on the passenger side and blocked by wiring, relays, etc.

My view is that removal of the dash pad is only a small part of what has to be done. I have attached a PDF file of the instructions from the 1990 Corvette FSM for heater core removal with references to the FSM. Hope this helps.

tekheavy

Yeah, I've watched that video and I do have the FSM. Just thought there may have been an easier way to get at the actuator, like from the engine compartment. I guess not. Do i physically have to remove the heater case to get the actuator off or can I remove it with the heater case still mounted in the car?

frosted

Heater case needs to be removed. I can't remember the actuator to be even visible even with the dash pad removed.

Carnivore

I just loosened the back part of the case (against the firewall) enough to get at the actuator. I would also recommend buying an endoscope. Some bolts are very difficult to see)

tekheavy

Good info. Thanks for your help. Good luck on your 96 heater case swap.

frosted

On mine the actuator was (probably) ok but the coupling was loose, allowing the blend door to move freely roughly 1/4 of the travel. I 3D printed a new coupling which much reduced the 'flappiness'. I was originally chasing a 'thump' which seemed to have something to do with AC/heater.

Carnivore

I purchased and installed a 1996 heater core case with the actuator on the bottom. It came with the 1996 pigtail.

So....

In 1990 heater core case pin 6 has a Tan wire and pin 10 has a Black wire (top Mount)

In 1996 heater core case pin 6 has a Black wire wire and pin 10 has a Tan wire (bottom Mount)
The wiring change switches which direction is cold to heat and heat to cold. That is, reverses the direction of the heater core box vent door.

The Air Mix Valve Motor controls the position of the Air Mix Valve. The Motor can be operated in either direction by the polarity if the voltage applied to it. When the solid state switch grounds terminal C8 in the Heater and A/C programmer, then the terminal 6 of the air Mix Valve is positive. If the condition of the solid state switches is reversed and C7 is grounded, then the terminal 5 of the Air Mix Actuator is positive and the Motor runs in the reverse direction. The position feedback Pot indicates the position of the Valve at any point of its travel. As the Valve moves between fully open and fully closed, he voltage at terminal D13 (9) on the Heater and A/C programmer will vary between zero and 5 Volts.