relays to run electric fan

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=1412
Printed Date: July 05, 2025 at 2:35 AM

Topic: relays to run electric fan

Posted By: datsun2adz
Subject: relays to run electric fan
Date Posted: June 20, 2002 at 2:42 PM

I have a car that I have put an electric fan on instead of the factory viscous fan, it came with it's own tempurature sensor and wiring. I have just installed A/C on the car and I was wanting to have the fan kick on when the air was turned on and I got it to do that but now when the A/C is off and the fan kicks on because of temperature so does the A/C compressor. How should this be wired?

Replies:

Posted By: CTMobileMedia
Date Posted: June 20, 2002 at 4:19 PM

Put a diode on the fan wire at the junction with the A/C wire, so that voltage on the A/C wire powers the fan but the fan wire won't put voltage on the A/C wire. Simple fix.

Dave

CT Mobile Media

Posted By: HotRod53F100
Date Posted: June 23, 2002 at 6:21 PM

I agree, a diode will do the trick. Just be sure that the diode is rated for the maximum current of the fan motor and then some. You're just getting feedback into the A/C clutch because it's not isolated. You're other option is to drive a relay with the A/C and another relay with the temp control and just hook the contacts driving the fan in parallel. You're DC voltage will be on the other side of the relay from the coil and will isolsate the two. The diode will be far easier if you size it right.

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HotRod