Interesting observation

[SlammaDingdong]

Using the Nelson harness, i'm still tapped into the A/C clutch compressor trigger for part of my e-fan control; soon to change i hope. Anyhow, on my '04 i recently noticed that the e-fans cycle when the mode switch is set to defrost, even w/o the A/C switch engaged. Has anyone else seen this?

And yes, I'm tapped into the right trigger wire, as the fans do work just fine when the A/C switch is engaged.

This should motivate me to test the "hot lead" on the A/C accumulator as source of a fan trigger. Having the e-fans cycling on a 10* morning while running the defroster is just

[RedHeartbeat]

Slamma, your a/c runs when you have the defroster turned on to help dry out the moisture/fogging of the windshield. Without the a/c running you're not going to have much success beating down a fogged up windshield. Since the PCM doesn't know if it's freezing cold or blistering hot outside it doesn't know if you want the e-fan turned on or not, it just does it to improve the efficiency of the a/c unit. I would suggest throwing a disable switch on the relay so as to prevent the fan from running during cold mornings for a faster warm-up. Since the relays are triggered by a ground wire it will be a very low current going through the disable switch and should be easy to wire into the loom.

[SlammaDingdong]

Should have figured that. I can always count on you to straighten me out RedHeartbeat So the compressor is cycled to help clear the moisture? My thinking was that by tapping the "A/C on" lead to the accumulator, it would avoid using the whole clutch trigger deal. If defrost was selected w/o the A/C switch engaged, would that lead still be hot?

[RedHeartbeat]

I believe the a/c compressor being hooked into the defrost cycle was originally engineered into systems to cycle the compressor in winter months to keep in it good condition year-round. If you live in a dry climate the a/c running won't be an issue. The load of the a/c tugging against the engine during warmup helps to heat the engine faster. I didn't realize the new GM trucks had a button just to engage the a/c compressor. Us OBS guys don't have one so I don't know if it makes a difference or not. Best thing to do is open the hood and see if it disengages. You can always add an idiot light to the circuit and hang it some place you can see while the hood is down.