My '73 F100, XLT, 390, C6, Factory AC, when first bought from orig owner, it had no high speed fan, either low or off. The switch has 3-4 positions. When we rebuilt the interior a few years back, we found a burned connector at the fan switch and repair it (properly) and everything worked fine, except, if the fan ran on high with AC on, after 5-10 mins, the whole system would go dead. Everything else still working fine and after another 5-10 mins, it would come back on.

When I say dead, I mean completely dead, no fan, no compressor engagement, no nothing that uses electricity in the HVAC. I always thought a resetting breaker somewhere in the the system was kicking in and cycling. If I ran on medium or low fan, it never happened. I figure after 40 years, the fan maybe pulling too much current hence the breaker cycling and I would not use high. No problem.

Well, last summer, leaving work one hot as hell day, I did not pay attention and the system went dead. The problem is that it has never come back!

I used my trusty vent windows and got through the summer. Now its starting to get cold and while I can feel the heat wafting through the vents when I slide the temp lever over, I want to get my fan working. I've check everything.

No master breaker, fuse or fusable link as had been expected was found and have studied the schematics and all to no avail. While fixing a bad wiper switch today, I pulled the dash panel and found a 3 wire heavy duty connector going to the back of the HVAC's temp control panel. 12v on the brown/purple wire, none on a lime green (ground?) and none on a red/black striped wire.

Any experience or ideas? I have pics of the harness and connector. My next step would be to try and trace these 3 wires and/or replace the temp control panel piece that this 3 wire connector mates to. I've seen them in catalogs, not too bad ($). BTW, the fan switch repair looks all good, no heat-up problem there anymore. If the fan worked but not the AC compressor, or vice versa, I would have something specific to chase. But here, the entire thing is just plain dead????

 

Jump power and ground to the fan and see if it'll kick on at any speed

 

I pulled the dash out again today and buzzed through it with my multi meter and figured out what each wire connected to the climate panel did. Since all system power was out (fan and AC compressor clutch), I starting with the power wire and surprisingly found it hot (with the ignition on) at the Valve-AC Vacuum Selector (LMC's 43-0752) in the panel. However, no power was coming back out to the other 2 leads in that same connector, no matter the panel settings.

It was pretty easy to isolate from there. One of those out wires is for the AC Compressor to energize from and the other energizes both (splits upstream) the fan motor directly and the fan speed selector switch in the panel. Turns out the fan switch on the panel only speeds the fan up 3 speed faster than its default low. To test, I jumped the circuits bypassing the Selector switch and fan and AC compressors kick on just great!

The bad news, LMC wants $79.95 for that Selector so I'm now off shopping. One to learn on and share. In short, when all power is out to the entire HVAC system, and there is no obvious fuse or breaker problems, check the selector switch at the dash panel for power where it all gets routed from and take action from there.