Aftermarket Electric Fan A/C Circuit?

I've got an aftermarket electric fan on a vehicle that originally had a mechanical fan. The fan works great at keeping the engine at operating temperature, comes on at 200° and shuts off at 185° F with a thermal switch in the coolant, triggering a relay.

The A/C however is mediocre when the fan is off, when the fan cycles on though, boy howdy, you get icy cold air.

So I want to add an additional relay to trigger the fan to be on when the A/C is on.

I initially thought I'd tap into the A/C compressor clutch and have the fan on when it's on, but the clutch cycles on and off, so there are times that the clutch wouldn't be energized, and the thermal switch wouldn't trigger the fan.

Anyone done this before? Have any suggestions on what would be the ideal circuit to get a signal to trigger the fan on with the A/C?

The vehicle didn't come with an auxiliary fan for the A/C? Usually vehicles with mechanical fans have a small secondary electric fan that is triggered with the A/C activation.

You would want to activate the fan relay with the A/C switch, not the clutch wiring.

Running the fan only with the A/C clutch should be fine. If the compressor is off, the condenser doesn't need cooling.

An even better way is a pressure switch in the high side of the A/C system. This lets the fan run when the A/C needs the cooling, but doesn't keep the fan on constantly when you're on the highway with the A/C on (it'll usually come on as you come to a stop or get to a low enough speed and then shut off once you're moving fast enough).

I always just tap in to the A/C clutch circuit. That's the way a lot of OEs did it in the 80s. Yeah, the fan kicks on when on the highway, but the alternative is hacking in a high pressure switch if you don't have one already.

If you tie in on the relay coil side, you need to add a diode to the system. There are two ways to do it with adding a second relay - either use the second relay to control the coil side of the fan relay, or use the second relay to control the fan directly.

Yeah, I'd run the second relay off the clutch circuit. If you're in the Fay area, I'll give you a relay, socket and fuse. I bought 10 ea on amazon for the RAV's intercooler, so I have like 9 sets left over.

if you need to, a manual switch would work too, just make sure it loses power when the ignition is cut

Well, the compressor clutch was my first thought, but I second guessed myself. That should be simple enough. Thanks everyone for setting me straight.

In reply to Dr. Hess:

I'm set on relays and harnesses thanks. I've got a few in a box from a similar Amazon order for some other project. I think I bought a 10 pack because they weren't that much more than just the one I needed.

I'll actually be in Fayetteville for work tomorrow, quick trip to the University water lab and back. I'm hoping to talk my coworkers into Thai Diner for lunch, but one guy is campaigning pretty hard for Flying Burrito.

That's how I got a 10 pack too. I figgered that with all my automotive projects, they would eventually get used.

I'm in B'ville tomorrow during the day. For lunch, I suggest La Huerta at Crossover and Mission. That's my favorite restaurant in Fay, and the best TexMex around.

Well, I think that I would like to have the fan on all the time when the A/C is running. If you think about it, that is what the mechanical fan was actually doing but hey,that's just me.