Vacuum operated A/C switch

[Rower4VT]

I know many vehicles will turn the a/c compressor off under WOT conditions to allow for max power, however, I'm wondering if anyone has rigged one up that shuts off the compressor at a more FE-enabling vacuum of say 15 in-Hg or so. Has anyone done this? Any idea on where to get an adjustable vacuum-operated relay/switch?

[soda_pop503]

Originally Posted by Rower4VT

I know many vehicles will turn the a/c compressor off under WOT conditions to allow for max power, however, I'm wondering if anyone has rigged one up that shuts off the compressor at a more FE-enabling vacuum of say 15 in-Hg or so. Has anyone done this? Any idea on where to get an adjustable vacuum-operated relay/switch?

Thank you for posting this. I was looking for this very device. since I have not found one, I think i am going to crack open the good old Haynes manual and find a place to tap into the vacuum sensor to figure out a voltage range. then, using a relay that connects at the right voltage hook that up to the relay that controls the compressor. Hopefully i can incorporate a potentiometer to make it adjustable.

if it works well I can probably replicate it and sell them real cheap. like 5 or 10 bucks, depending on what parts cost. PM me if interested, and ill try to keep you updated.

[Jay2TheRescue]

Wasn't the system that Chrysler put on the K cars operated on vacuum?

[GasSavers_RoadWarrior]

I dunno about the early ones but the SMEC/SBEC (ECU) controls it on the later ones. They have a MAP sensor that is 0-5V output, 5V or close to it being atmospheric, think it's a 12V supply. Real easy way to do it would just be to use a 74 series logic chip, off a MAP sensor driving a relay and a pot to trim the switching voltage up or down a bit

[GasSavers_bobski]

An op-amp set up as a comparator is about as basic as it gets electronics-wise. You would just need to pick an op-amp IC that runs on a single voltage source (as opposed to the typical +/- split-source chips).
The most basic design would be to feed the MAP sensor signal to the op-amp's non-inverting input and your target voltage (from a potentiometer) to the inverting input. The output would flip states when the MAP voltage exceeds or drops below the target voltage.
The trouble with that is if the MAP signal is hovering right around the target voltage... There's always some noise present in electrical signals, so you could end up with a fluttering on/off signal situation. You need a circuit with some hysteresis.
There's probably a far more elegant solution, but what first comes to mind for me is a design using two op-amps, an R/S flip-flop and an inverter. Set one op-amp to trigger slightly above the target voltage, the other slightly below. Use the inverter on one amp output so you get an active signal from the first amp when the MAP signal is high (relative to the target) and an active signal from the second when it's low. Feed these signals to the set and reset inputs on the flip flop, and use the output to control your A/C system (via a transistor and relay or whatever).

[GasSavers_RoadWarrior]

Could probably "debounce" it with a capacitor, could probably do it with a single transistor, capacitor, relay and a few resistors. That would be the low device count way, circa 1970... but this is 2009, what you really need is a PIC or Basic stamp development kit... (Sarcasm)