A/C Control with Standalone on 99
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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
![](http://img32.imageshack.us/img32/8770/ac1w.jpg)
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?
![](http://img193.imageshack.us/img193/6382/64583604.jpg)
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.
I'd like to reopen this topic and come to consensus.
Here is the A/C system wiring diagram for a 99
![](http://img32.imageshack.us/img32/8770/ac1w.jpg)
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?
![](http://img193.imageshack.us/img193/6382/64583604.jpg)
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.
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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.
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.
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