12v trigger to disable neg output
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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=99309
Printed Date: June 23, 2025 at 2:05 PM
Topic: 12v trigger to disable neg output
Posted By: prolsc
Subject: 12v trigger to disable neg output
Date Posted: November 24, 2007 at 4:28 PM
i have a fan that im trying to wireup in my car. The fan has to have the neg. disabled for it to work. I would like to use the 12 volt power source, from the ecm to disable the neg. side of the fan. Is this possible with a relay? The fan has a constant 12v , and a neg. ground, and has a neg. trigger.
Replies:
Posted By: Velocity Motors
Date Posted: November 24, 2007 at 10:09 PM
So you want the ECM switched ignition to turn off the fan ? So that means that when the ECM is off the fan will be on all the time ? Sorry, I don't understand what your trying to do here.
-------------
Jeff
Velocity Custom Home Theater
Mobile Audio/Video Specialist
Morden, Manitoba CANADA
Posted By: prolsc
Date Posted: November 25, 2007 at 9:21 AM
Velocity Motors wrote:
So you want the ECM switched ignition to turn off the fan ? So that means that when the ECM is off the fan will be on all the time ? Sorry, I don't understand what your trying to do here.
How the fan works, that im putting in the car, has full time power going to it., off a 12volt source. It has2 individual grounds , one from the battery and one from the ecm. In order for the fan to come on, the one from the ecm must be disabled. The fan is out of a toyoto. Its a dual fan set up. The ecm which is in a mustang uses a 12v to trigger. the fan., the stock fan. I need the wire from the 12v out of the ecm to hold a ground until it reaches its temp. , then it activates the fans.
Posted By: tvcomputerwiz
Date Posted: November 29, 2007 at 8:03 AM
ok here you go... 
Posted By: dualsport
Date Posted: December 04, 2007 at 11:32 PM
If you measure the amount of current needed for the fan control line to turn off, what kind of value do you get?
Possibly you could just use a diode in series between the fan control and the mustang ecm output, if the output pulls down to ground when it's off.
When it switches on and goes to 12V, the diode would be reverse biased, and effectively opens up the ground to the fan, turning it on.
If your ecm output doesn't pull to ground when off, but instead switches between an open circuit and +12V, you might add a pull down load resistance to do it. Without details about the amount of power the two signals use, it's just a guess at this point.
Posted By: hotwaterwizard
Date Posted: December 05, 2007 at 5:50 PM
You just confused me real bad !  Do you need to utalize the Positive 12v output from the ECM to produce a Negitive output for the new fan? If so a relay can be used as an interface. 
------------- John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
Posted By: dualsport
Date Posted: December 05, 2007 at 7:01 PM
From what I could make of it, when his fan control line is grounded, the fan remains off, and when the ground is removed, the fan then turns on. I had a Toyota that did that, where it was done kinda backwards from what you'd normally expect. The fan switch would be grounded when the temp was low, and open circuit when it got hot. So, if you were to remove the connector for the fan switch, it would turn the fan on. I think the sense of the signal is the same, where high (or open circuit in this case) = fan on, and low (or ground) = fan off.
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