Is there a way to make it so the A/C compressor does not come on for the defrost setting on my 99? I read in the owners manual that the A/C is on for every heater setting except for 2. I always use my defrost but don't want to have the A/C on, so I can conserve some gas.

I'm thinking maybe a tuner can get rid of it but doubt a handheld is gonna be able to do that.

 

its a safety feature...let it run...besides that the little bit of extra fuel used is next to nothing...it only take a very small amount of power to run the compressor...if you really want to do this what you could do is install a compressor on-off switch in the ac request circuit that requests the PCM to turn the ac compressor on.

 

The compressor comes on to activate the dryer unit for defrost. Without it, there would be no point to running defrost as the dryer unit removes moisture from the air allowing defrost to work.

It's not that the a/c unit comes on to chill your air, it doesn't actually run any freon through the condenser unit thus you're not really adding much more drag on the accessory belt and the mileage loss is barely noticable.

 

If the AC system is on then the R-134a, a non chlorine based solution, (not Freon, a chlorine based) has to pass through each component. It is a single path refridgeration cycle. Think of it as a series circuit. There is no way to skip from one part of the liquid-vapor compression cycle to another part.

I believe most compressors will take about 5 HP or less while normally operating the vehicle at any point of time. I would say that it is costing you at most a couple pennies a mile.

One time I turned the AC on and off to see how the load calculation changed with Autotap... It was less than a percent. So just for the hell of it I converted that amount of horsepower into the energy of gasoline with 15% efficiency and then applied the standard cost of about 2.50 per gallon on it. I was bored one night and ran out of beer....

Anyways, the whole point of posting this was to say I don't think it would be worth your time to modify the wiring harness if you looked at the fuel saved. Now, If you just really want to do that.... I think that the switch on the HVAC panel or the PCM would be the culprit.

You could just make a switch to disable the singal going out to the relay for the AC on the driver's side of the car. (Under the hood)

That would make it stop.