I just did a motor swap in a lightning that I bought. Truck had no engine so I do not know the history of what did and didn't work. The motor I swapped in is built and I used the ac compressor from the donor truck, which worked fine. Here's the problem, had ac filled up to factory specs, ac blows ice cold like it should. With the fan control set to OFF, the compressor still runs... Doesn't matter what position it's in, it runs. I swapped the hvac control unit with my other truck thinking I had a bad switch. No fix. I checked the compressor relay, it's good. Fuse is good. I pulled all the relays, fuses and the diode, still spins! Unplugged both pressure sensors, still spins! Unplugged the compressor plug, it doesn't spin.... Whats goin on here? How can it be engaged with all the relays fuses and pressure sensors unplugged? Any help would be greatly appreciated.

 

Did you use the original harness? One way can i see it running all the time is if it is getting it's current directly from the battery through the ignition switch

 

your clutch wire shorted to ground? I can't remember if the clutch is turned on with switched power or ground?

 

What year is this vehicle? The 98 in your profile or something else? If the 98, there's no relay as the coil is supplied by the series circuit of the selector switch, the HPCO switch, and the cycling switch. That would mean you have a short to battery power in that circuit.

Clutch coil is switched on by supplying power. One side is already grounded.

 

This is a 2001 lightning. I used the motor and harness out of my 98'.

 

If anyone could give me a pin diagram of the harness for my f150 and the lightning so I can compare. The compressor has power with the key on... So somewhere something isn't correct. I used to have access to ondemand5 but I don't anymore

 

never mind that, i compared both using a diagram and they are 100% identical. so my problem is somewhere else...

 

Bump...

 

You need to start chasing wires at this point Joey. The ground comes from the PCM and its got a steady powerom relay/fuse.

 

That's what I'm not sure of. It always has power and just gets ground when the switch is on? Because I traced the power wire from switch to compressor. Every inch

 

Then maybe I said it wrong, grounded through the case, and power from switch, I can look it up to be sure tomorrow

 

Well I'm not sure because I traced the hot wire and all seems normal. So maybe it is ground triggered.

 

The compressor control circuit for the 2001 is NOT identical to that of a 98. The 01 uses the WOT relay, controlled by the PCM, to control the compressor clutch. The 98 uses the circuit I previously described.

The PCM supplies a ground to the low side of the WOT relay coil to energize it; The relay, when energized, switches an Always Hot feed to the compressor clutch coil to energize the compressor.