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

[EDS_5_OH]

Did you ever get this solved? Our 1990 head unit is flaking out on us again and I can get 1994 units all day long for cheap. The 5 button version is rare and harder to find for replacement.

 

[DerekTheGreat]

Nope, I pussed out haha. What I found before I gave up/got distracted was the early trucks like mine are fed something like +8V to the controller and all the servo motors whereas the later ones use +5V. This is why that usually the later style controller will freak out and start flashing when you try to plug one of those in an older truck and vice versa. Everything worked though, so if a dude[tte] were to take some time and wire in some resistors to cut the voltage down it is plausible that the later controller should work. The one I had in place worked for about a week before giving me the fits, so it probably ended up blowing some stuff on the inside of it.

 

[OLD BLUE 88]

After a total rebuild of my 88 HVAC system, I was able to get the max/recirc working by grounding the second wire on the pigtail coming off the compressor. As described, it only activates when the fan is set to med, high, and full cold.

My new challenge is the HVAC head unit is falling apart and there are none to find in the junk yards, and I'm not having much luck in the aftermarket.

Any help is much appreciated.

 

[DerekTheGreat]

I think I have a couple I could part with, don't think they were from an '88 specifically, but the case will be the same so you could swap the guts and be on your way.

 

[Cigarhead]

1989 1990 use a 5 button head unit without MAX or RECIRCULAT
When you use later units they will short out. It may be immediately or in a year.
They cause flashing and glitches in the unit itself and can cause other issues including complete failure.

If you need to prove this to yourself go to Doorman and look for a 91 1500 hvac control unit. Look at the buttons.
Then look for one for a 90 and it is out of stock.
I called then to verify and they confirmed it as well.

I did a full week of research looking at 100s of photos, looking at the buttons. This confirmed it.

You can swap them they plug right in... no matter which year you have.... but they won't work properly.

 

[DerekTheGreat]

Yah, that's because the early stuff uses 8v to control the motors and such whereas the later stuff ('91 & up) use 5v. I mentioned that in an earlier post. If you can step that voltage down on an earlier truck, you can use the later controller and net yourself a recirc button.