Now i have a 1990 and i can confirm that mine will turn off and on randomly and will only change modes if i turn the car off, press the button for the mode i want then turn the car back on. and then it wont switch again. super annoying. I also have a 91 that worked great and i know it doesnt have the same HVAC because i tried swapping it to diag and found the connector in the back was different and did not plug in. And after a few trips the local junkyard and ripping some out i found that the 4 1990's i tried (some worked better then others but none worked 100% all of the time) only had the yellow connector with a wierd pig tail coming off of it and everything else had a black connector with no pig tail like my 91. See pictures at the end.
I obviously wasnt getting anywhere with trying to get a good used one, and buying a new was out of the question when i knew i could do some simple wiring and do it for almost free.
Step 1. Go to junk yard and pull a ''new'' style HVAC unit. There are 2 connectors, one for the blower motor control (this is not removable from the unit and swaps over no problem) it is an opaque whitish color. The other will be black. Once it is removed grab some wire cutters and cut the 16 pin black connector harness. try and get as much of the wires as possible. I was able to get about a foot ( i have a parts car so it was free but junkyard quoted me $15).
step 2. grab the yellow connector and the harness it is apart of from the 90. Cut the yellow connector off. it will not be used again but leave enough wires showing so the colors can be identified for later. In my pictures i have alread combined the harness, sorry i dont have any before pics.
step 3. Solder time. if you have the factory service manual, alldata, mitchel or haynes/chilton, completly disregard thier information as they do not have the correct schematic for the 90, but the diagrams they do have some of the wire colors are wrong. The connectors have different wire color and different pin layouts. on the yellow, 7 on top with 9 on the bottom, the black is 8 and 8. Out of the 16 wires, 10 wires share the same color... IE pin one on black is blue with a black stripe and pin one on the yellow is also blue with a black strip. solder all wires to thier corresponding color.
The ones that dont match are pin 2- red to red with black. pin 5- green with black to brown with black. pin 13 white with yellow to white. pin 14 white with green to white with red. pin 15 green with yellow to red with green. and pin 16 red with blue to red with black.
Thats it. Wrap it up and install your "new" fully functioning HVAC into your car and enjoy the sweet sweet smell of unused vent ducts! .... maybe roll your windows down and stand outside if it was as bad as mine!
The yellow connector would have been attached the the whitish colored connector at the bottom where all of the electrical tape is.
