how to wire an electric auxiliary fan to come on with vehicle ac

[Ron Sebastian]

Can't seem to find info on it anywhere. I would like to add an electric fan to cool the condenser in front of the radiator. I have about 10" to fit the fan between the oil cooler and outside edge of the condenser. I would like to have the fan turn on with the compressor, but not cycle on and off like the compressor does. Is there a way to wire it without a switch? Here's the best I could find. Any help is appreciated.

 

[Ricko1966]

It's easy ground one side of your fan. Buy a common 4 prong relay hook terminal 87 to the power side of your fan. Hook #30 to battery positive post with a suitable fuse. Ground terminal #86 Hook number 85 to the A/C compressor clutch positive. Turn on the A/C the fan comes on to cool the condenser. They do make relays with diodes to prevent voltage spikes I'd run what ever relay was easily available and add a diode if I had problems

 

[bucket]

Wire the fan relay to the supply side of the low pressure cutoff switch. As long as the AC is turned on, the fan will run.

 

[AuroraGirl]

Can't seem to find info on it anywhere. I would like to add an electric fan to cool the condenser in front of the radiator. I have about 10" to fit the fan between the oil cooler and outside edge of the condenser. I would like to have the fan turn on with the compressor, but not cycle on and off like the compressor does. Is there a way to wire it without a switch? Here's the best I could find. Any help is appreciated.

so you want the fan to come on when you choose ac, but you want the fan to run while its cycling, but you dont want to be tied to temperature controls unrelated to ac?

Interesting, but id assume the ac controls trigger the compressor with the clutch by grounding, presumably, and the cycling doesnt affect that, id think, so youd only need to have a ground controlled relay from the control head. i could be wrong on how it works but that is just based on general knowledge

 

[AuroraGirl]

Wire the fan relay to the supply side of the low pressure cutoff switch. As long as the AC is turned on, the fan will run.

thats a great idea

 

[Ron Sebastian]

Wire the fan relay to the supply side of the low pressure cutoff switch. As long as the AC is turned on, the fan will run.

Yes, Bucket always comes through, thanks!

 

[bucket]

Yes, Bucket always comes through, thanks!

There is one downside though, if you drive the truck in cold weather. You won't get up to temp very well with the defogger on, since it activates the AC compressor.

 

[AuroraGirl]

There is one downside though, if you drive the truck in cold weather. You won't get up to temp very well with the defogger on, since it activates the AC compressor.

maybe a manual switch inline the trigger to the relay is in order

 

[Ron Sebastian]

There is one downside though, if you drive the truck in cold weather. You won't get up to temp very well with the defogger on, since it activates the AC compressor.

In Florida here it doesn't ever seem to get cold, and if it does it's only a short time.

 

[Ron Sebastian]

maybe a manual switch inline the trigger to the relay is in order

I'll have to kick that thought around. Thanks!

 

[Ron Sebastian]

Wire the fan relay to the supply side of the low pressure cutoff switch. As long as the AC is turned on, the fan will run.

Now does it matter which wire I tap? Dark or light green?

 

[saltdog]

I did it the way @bucket said, but used that wire to trigger the relay. In winter you can always pull the relay

 

[CalSgt]

I went with the expensive option, a Derale PWM fan controller. It has a thermometer that is glued to the radiator tank and also hooks into the compressor wire.

It ramps the fans to at least 60% when it sees the A/C turned on regardless of engine temp.

And engine temp is adjusted to work with the thermostat, it will run the fans anywhere from 10% to 100% speed depending on the reading it gets from the thermometer. I think mine set to go full tilt at about *205 or so.

It also runs the fans for 30 seconds or so after shutting off the ignition.

 

[bucket]

Now does it matter which wire I tap? Dark or light green?

Yes it matters, but I don't remember which one. If you tap into the wrong one, it will cycle on and off with the compressor.

With the ignition on, truck not running, AC switched on and that connector undone, there should be 12v on just one of those wires. That's the one.

 

[Ron Sebastian]

Added some pics in case anyone needs some pointers in the future.

https://www.gmsquarebody.com/threads/retrofit-electric-engine-cooling-fans.18475/page-3#:~:text=Retrofit%20Electric%20Engine%20Cooling%20Fans