why is ac compressor coming on for heat...

[lasvegas69charg]

why does my ac compressor comes on for def or heat...i thought it was for just ac and ac-max

[resq302]

The A/C system comes on for defrosting to act as a dehumidifier inside the car.  Humidity is the cause of the fogging on the windshield.  As for why it comes on with your heating system?  That I don't know.  On my cars it shouldn't activate the A/C compressor when the heat is on, only when the Defroster is on.

[A383Wing]

I does in the newer cars, but it didn't in my 66...I'm thinking the AC switch is wired wrong?

Bryan

[lasvegas69charg]

All I did was hook the compressor wire to the compressor.

[HPP]

I'd verify if it does indeed come on during heat or just defrost as the sensitivity on slides can sometimes blur that line.

As resq302 said, it does help remove humidity from the air which assists with removing fog, but a/c systems must also be run periodically to maintain proper lubrication. Having them come on during defrost ensure they will see some run time during the winter months to eleviate any issues come summer time.

[Cooter]

MAny newer cars do this in Defrost mode as well. We get at least one/week that can't understand why his/her car is getting less MPG during these winter months...We ask them one of two questions.
1. Do you let your car warm up in the mornings?
2. Do you forget and leave the Defrost button ON?

Either one will contribute to poor mileage.

[Nacho-RT74]

3rd gens doesn't come on on heater

but I know 2nd gens have some kind of solenoid, relay, vacuum, or device what works at some specifc moments... dunno exactly when.

the fact is that is somehow linked between Blower speed wire ( brown ) and Compressor wire ( Blue ) thought this device

[lasvegas69charg]

ok. I will replace the time delay and see if that fixes the prob.

[Nacho-RT74]

that is IF there is actually a problem