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Discussion Starter #1 (Edited)
Has anyone done this?

A little background. I get better (slightly better) fuel economy with our 4.7L Commander if I turn off the AC compressor (turn off the switch on the dash controls) going up hills or when accelerating from a stop. That makes sense. The compressor load for these vehicles is easily 5 to 10hp, and could be as much as 20hp. I haven't actually measured it, but they have a big compressor (to run two evaporators and keep a big cabin cool).

I'm looking to automate this process somewhat.

Back in the day (1980's, maybe late 1970's), when cars were all horribly underpowered and people wanted maximum fuel economy, there was a device called the "Pass Master." It was basically a vacuum operated switch that interrupted (either directly, or with a relay) the power to the AC Compressor Clutch when intake manifold vacuum fell below a certain level. So when you were accelerating hard, or passing someone, or climbing a steep hill, the compressor would cut out, allowing all engine power for driving the wheels and accelerating the car.

I've got a vacuum switch (an adjustable one). I'm wondering where is the best place to splice this in (perhaps with an "activation switch"). I'm thinking there are three possible locations to wire this thing in.

  • At the compressor itself. The down side is, I think the current load there is higher than the rating of the switch, so I'd have to use a relay with the switch to handle the high current. Still, that's absolutely the simplest, and the easiest wires to find. Possibly harder for an "activation switch" setup, too.
  • At or near the relay panel, spliced into the wires for the control coil on the AC Compressor relay. Simpler because I wouldn't need an additional relay.
  • At or near the AC switch in the dash HVAC console controls. Here it might even activate/deactivate the light, showing when it's working. But I'm not certain that the AC switch is a push on/push off switch. It might be a momentary switch which sends a signal to some computer which actually controls the compressor relay.
Does anyone have a wiring diagram for the AC Compressor clutch circuit on the 4.7L Commander (MY 2006 would be ideal). Which wires am I looking for in each of the three locations?
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