Questions for electronics gurus

[Probe Dude]

Jay,
Using a relay itself will isolate the +12V AC signal from the now switched
ground signal to the other relays.  You won't need a diode in there, there
is nothing to backfeed.

Take a spdt or spst relay, tie one end of the coil to ground and the other
end to your +12V signal from the A/C.
On the contacts, wire the common terminal to your coolant temp switch and
the normally open terminal to chassis ground.

As a separate issue sometimes you will see a reverse biased diode across a
relay's coil to suppress inductive kickback spikes, but car electronics are
well designed to handle spikes so you don't need it.

Dave

>I am looking to *also* activate the fans with a single
>wire that sees 12V when the A/C compressor is turned
>on.  I know I will have to wire in a relay to invert
>the signal, but here is my question: