dual activation relay

I'm installing dual electric fans in my air conditioned 1974 Chevrolet Impala and I need a wiring diagram for a relay to operate off of a thermostatic switch and when my A/C is on. I would like for the cooling fans to come on when the engine reaches the preset temperature of the thermostatic switch mounted in the intake manifold or when the A/C is turned on.  Please help and thanks in advance.

If both are ground switching circuits, merely connect them in parallel to the (ground) of the fan relay.

Otherwise, or if you want only one fan on for one situation and both for another, you'll need to add diodes.
But that circuit depends on what switching polarity you have (ie, +12V or ground).
It also depends on if those switches close for the "on" state. IE - most temp switches etc are NO (Normally Open) and close when the "fan on" temperature is reached. But others are NC and turn off instead (eg - older oil-pressure switches turn off with pressure - though that means "on" for an alarm... ain't Logic fun?!).

Thanks maybe I can refer to one of the relay diagrams for an alarm and use the information you have provided to  wire it up correctly.  You wouldnt happen to know which relay diagram I should be looking at to get a basic idea of what wire goes where would you?

You'll need to figure out what polarity is being switched unless you are setting them up...


The simplest is probably (fused power) +12V to both fans, and each fan with its relay to ground (ie, fan- to #30; #87 to ground).


The switch side (relay coil) also depends on polarity.
If the relay coils can be hot (ie, +12V - probably from the above fused +12V, else from the switched IGN +12V if you want them off with IGN-off, else from the switched ACC +12V if you want them on with Acc-on and IGN-on but off during cranking) with hence <whatever> +12V to #86....
Then each of the two "on" switches can be between ground and #85.
You then insert a diode (1N4004 etc) between the two #85s with the diode line ends toward the #85/switch which is to turn on BOTH fans.
(A 3rd relay could be used to eliminate the diode.)

The relay coil can also be cold (ground to #85) with each switch providing +12V to each #86. But...
Then TWO diodes of which both non-line ends go to the "single fan" relay. The other non-lined ends go to the respective +12V switches.
[ Hence the single fan +12V switch goes thru its diode to supply +12V to its relay coil, but it won't feed thru to the other switch and relay because of the crossover diode.
Likewise the two fan +12V switch supplies +12V to both relays but that +12V won't back-feed to the single fan switch because of its diode. ]


A combination of +12V & ground switching is also possible.
So to a combination of hot & cold fan - though that gets messy.