provide ground when powered

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=96667
Printed Date: May 12, 2025 at 1:20 AM

Topic: provide ground when powered

Posted By: ewingr
Subject: provide ground when powered
Date Posted: August 26, 2007 at 2:28 PM

I have an older car with a newer engine, and I want to have my fan on my engine turn on when I turn on the A/C. I was thinking I could run a wire to a relay from my compressor power wire that would provide 12v when the compressor comes on. I would like that to trigger a ground at a relay, which I figured I would wire into the ground on my fan relay. Sound possible?

I've looked at the diagrams on relays here, but don't see a situation for what I want, and don't know enough about wiring to figure that out. Hoping for some direction.

So, hot wire from A/C (when A/c turned on) to what pole on the relay?

Ground wire from my fan relay to what pole on the relay?

I am thinking power to the relay to 30.

Ground to the relay to 85?

What type of relay? SPDP?

Many thanks!

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Thanks
Roger

Replies:

Posted By: the12volt
Date Posted: August 26, 2007 at 3:40 PM

Using an SPDT relay, connect terminal 85 to power from A/C. Connect ground to terminals 86 and 30. Terminal 87 will be ground out from relay to fan relay.

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Posted By: hotwaterwizard
Date Posted: August 26, 2007 at 10:17 PM

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

Posted By: ewingr
Date Posted: August 29, 2007 at 9:12 PM

Thanks! I'll give it a try this weekend. I appreciate your response.

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Thanks
Roger

Posted By: hotwaterwizard
Date Posted: August 29, 2007 at 9:56 PM

For switching the negitive look at this.

posted_image

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

Posted By: ewingr
Date Posted: August 31, 2007 at 10:50 PM

Thanks, The 12Volt Administrator. This worked perfect, and providing me with specific wires to specific terminals was very very helpful.

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Thanks
Roger