# ecm negative wire for ac relay



## lloydt (Jul 1, 2017)

I am trying to make sure it's not the ecm before I go buy one so here we go. I have voltage at the ac relay but do not get the negative the ecm is supposed to provide, therefore it will not pull in the relay. I have a good wire all the way to the ecm but connected car running I get not resistance to ground on the negative side. Everything else works, am I missing something? any help would be greatly appreciated.


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## Roader (Nov 9, 2014)

The ECM needs 12v from a series circuit that includes the thermo control amplifier and the dual-pressure switch. You should have 12v at ECM harness pin 21. You should have 12v at pin 2 of the thermo control amp connector. Thermo control amps are notorious for failing and would be a lot cheaper than an ECM.


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## lloydt (Jul 1, 2017)

Thank you.


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## Cusser (Apr 16, 2004)

Agree - do NOT run out and buy an expensive ECM !!!


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## smj999smj (Jan 1, 2006)

I would send the ECM to Circuit Board Medics and have them check/repair the ECM before buying a new one...if the ECM "is" the problem.


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## lloydt (Jul 1, 2017)

thanks to all for your help. After troubleshooting to the thermo control amp, I decided to do a bit of board level troubleshooting to see what was wrong with the board, WA-LA the capacitor on the board had failed. After replacing the capacitor it all works fine. If I didn't have to buy 20 capacitors for $7.00 it would have been $.43cents. Don't get me wrong the time I spent doing all this was not worth the $40.00 I could have paid for a new one but it was fun to fix and hopefully this will help the next person. It was the none polarized capacitor on the thermal board, there was only 1. Good luck to all those with the same problem and thanks to those who helped, and by the way it's 4.75 volts at the ecm not 12.


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## Roader (Nov 9, 2014)

Good info. What value is the cap? And as I recall the thermo amp is encased in a chunk of potting material. How did you get to the cap?


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