Create 12+ When Ground Is Lost

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=143465
Printed Date: May 12, 2025 at 1:56 PM

Topic: Create 12+ When Ground Is Lost

Posted By: alan.c
Subject: Create 12+ When Ground Is Lost
Date Posted: August 15, 2017 at 5:09 PM

Hi guy's.

This is my first post, I'm somewhat of a newbie when it comes to car electrics..

OK so basically I'm trying to wire up a radiator cooling fan. The fan thermo switch is a single wire and is grounded when the engine is cold, when the engine gets to up to temperature the ground is lost..

I need to wire up a relay that will allow the cooling fan to come on when the engine is up to temperature.

Is there a simple way to wire this up and with using just one relay?

Thanks is advance
Alan.

Replies:

Posted By: the12volt
Date Posted: August 15, 2017 at 5:15 PM

Using an SPDT relay, wire as follows:

30 and 85 to ignition 12V+
86 to ground from thermo switch
87a 12V output to cooling fan
87 not used

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Posted By: alan.c
Date Posted: August 15, 2017 at 5:24 PM

Perfect I'll try this tomorrow and let u know how I get on.

Thanks
Alan.

Posted By: i am an idiot
Date Posted: August 15, 2017 at 5:53 PM

Are you powering the fan with the relay? Or is the relay providing a low current signal to some type of controller?

Posted By: alan.c
Date Posted: August 16, 2017 at 1:09 AM

Powering the fan from the relay

Posted By: i am an idiot
Date Posted: August 16, 2017 at 10:53 AM

Cooling fans tend to pull high amounts of current at startup. You may want to use a second relay to power it directly off the battery. Do not connect the single relay to an ignition wire of your vehicle.

Posted By: alan.c
Date Posted: August 16, 2017 at 12:14 PM

At the moment I have it wired directly to the battery via a 15amp fuse and single relay as mentioned above.

I'm not to sure how I'd go about wiring up a second relay

Posted By: i am an idiot
Date Posted: August 16, 2017 at 12:27 PM

As long as you have it directly to the battery you are fine. I just did not want you pulling tons of current through an ignition wire.