Joe's Great A/C Thread leaves a lot of speculation, and the links to the NB diagrams are broken.

I'd like to reopen this topic and come to consensus.

Here is the A/C system wiring diagram for a 99


If I am reading correctly, it appears that I can jumper 1P to 1I and 1S. If I do so, it looks like the a/c fan and a/c comp clutch relay coils will take a ground path through the pressure switch, the a/c amplifier, the a/c dash pushbutton, and the fan switch, finally getting to a chassis ground behind the dash.

This may work fine. Or it may not. I don't know.

I was considering trying something else. Can/Should I use a couple transistors in this manner?


Bottom line is I want the a/c to function, and I don't want to use any I/O on the MS itself.

Comments, opinion, wild speculation all appreciated.

 

You need to use at least one output to switch off the compressor when rpms are high.

Jim

 

I considered and dismissed the thought. I don't have the I/O for it. My right pointer finger will work just as well.

 

I spoke with Jerry of DIY Autotune fame earlier. He advised me to not simply jump grounds. Here's the skinny.
The ground provided by the a/c pushbutton is very high resistance. It is not sufficient to reliably trigger the relay coil grounds. He says that it can and will cause the a/c relay(s) to fail.

So don't just jump them. Use a transistor. This applies to all years.