I have a 1967 420 in which the a/c has stopped working. Since neither the blower or compressor will come on, I suspected a power or ground issue. None of the wires going to the unit have any power when tested with a multi-meter, so I'm leaning toward a power supply issue. I have cleaned all of the contacts going to the control switch, with no results. I also cleaned all of the fuses in the fuse panel and the connections at the a/c resistors on the firewall, again with negative results. My next step now would be to run a new hot wire to the correct terminal on the control unit to check it's operation. The problem is that I don't know which of the wire(s) going to the control unit are supposed to be hot, so I don't know which terminal to connect it to. My service manual does not have a/c wiring information on the wiring diagram, and the separate a/c manual for the car is unavailable. If anybody can provide some help with this, it would be appreciated.

 

Red wire is always hot. Any wire with purple is switched hot. Black is ground. Anything with brown is switched grounded.