HVAC control head swap, "Max A/C" Style - Wiring Diagram Needed.

Hey everyone.

Recently I swapped the factory HVAC head in my '89 for one from a '93 or '94 truck to gain the "Max A/C" button. Things were fine for a few days but now I'm having problems like everyone else does with the swap. Long story short, I was wondering if anyone has the diagram for the head unit so I can compare & contrast with the diagram found in my shop manual.

What I did notice is that the donor HVAC head's harness has completely different wire colors than my '89's. Surprisingly, everything worked yet I'm wondering if some wires are in the wrong spot or the "feed back" signal wires for the servo motors are in the wrong spots. I'm also wondering if the later style head looks for different voltage values. I did not have any issues with the factory head unit..

Thank you for your input and reading this jibber-jabber!

94 C1500 as I recall:




Sent from my Commodore 64 using 300 Baud Modem.

Cool, thanks Macfluke!

Any chance you have any literature which describes how the thing operates? Like what the expected voltage ranges are? Looks like ~0-5 volts which if I remember right is what my factory controller also specified although I think it gave ranges to go along with bi-level positions. Can't wait to go home and compare though, this might be the break I need.

We with the older 88's and 89's really need a solution to this problem. The replacement heads on Amazon are only about $150 but they work only till it gets hot. The original head with out max A/C seems to call for Max A/C automatically under certain temps and conditions....very hot weather. When the new 93/94 head runs into these conditions it seems to go crazy. The 93/94 head did work all winter with no problems. My solution was to take the 89 head apart and spray it liberally with electric contact cleaner. The old head is now back working but I am not sure for how long. Any help here would be appreciated by many of us!

According to my GM service manual the system will automatically engage the recirc/max setting only if the temp indicator is at full cold, blower motor at medium or hi position and air diverted only to the vents. I'm going to compare the diagrams & such but if all else fails, I'll just go back to using the factory piece and utilizing those steps to keep it at max. With your truck though, you could actually have a problem with your wiring & those servo motors.

DerekTheGreat said:

Cool, thanks Macfluke!

Any chance you have any literature which describes how the thing operates? Like what the expected voltage ranges are? Looks like ~0-5 volts which if I remember right is what my factory controller also specified although I think it gave ranges to go along with bi-level positions. Can't wait to go home and compare though, this might be the break I need.

Click to expand...

I never found anything on exactly how it operates... and the wiring is somewhat different, like the medium blower relay which the 89 doesn't have... if you figure this thing out keep us posted...

DerekTheGreat said:

According to my GM service manual the system will automatically engage the recirc/max setting only if the temp indicator is at full cold, blower motor at medium or hi position and air diverted only to the vents. I'm going to compare the diagrams & such but if all else fails, I'll just go back to using the factory piece and utilizing those steps to keep it at max. With your truck though, you could actually have a problem with your wiring & those servo motors.

Click to expand...

mine works that way having to be on full/high for it to engage the recirc... also you need to keep it functional with both outside/recirc working because if its always on recirc eventually you will get a funky smell from the ductwork...

Yeah I'll keep you posted haha. I want it to work and it's just going full retard now so it's time to educate myself and then figure it out, if possible.

For sure on the musty smell. I'd do recirc pretty much only in city traffic so I can avoid cigar & cig smell at stoplights, not to mention exhaust stank too.

I’ll be interested to see where you go with this. Mine in my 88’ is having all sorts of issues. I haven’t taken it apart yet but I’m hoping a good cleaning will help.

Oh man, I'm trying to upload pics from my service manual but it's weird with a computer. I'll try and do it with my phone shortly but I think I see what the problem is: My '89 is using 8V to control the motors where as the '94 uses 5V. Hard to tell if the Module uses an internal resistor to step the voltage down or if it's done by "resistor 8" on the schematic provided by Macfluke.. If it's done by the Module I wouldn't expect it to be a problem. From what I've seen, the motors are the same regardless of year so I assume the wouldn't care about what voltage they see.

Next issue is the harness description from my '88 service manual does not match what I'm seeing for '94, so I can't tell if the wires are in the same spots or not. There are more discrepancies too: My diagram shows nothing for illumination or blower motor control so perhaps that's in a different section of my manual. More hunting needed..