Painless fan relay problem.

[shatar4]

I have electric fans on my Challenger but they will not kick on when the temp gets hot. I thought it was the ground so I grounded the sending unit to the block. Still nothing. I then took my test light and put the ground to the battery and the test lead on the positive side of the thermo switch. They kick on when I do this. Any idea's? Thanks.

[dodj]

Is the temp sensor the only control device or is there a relay as well?

[shatar4]

There is a 30 amp relay also.

[dodj]

Sounds like you are completing the sensor part of the cct with the meter. Effectively, the meter test leads are 'jumpering' the temperature sensor, sending the 12vdc signal to the relay coil which picks up and starts the fans. At this point, it seems to me that the sensor is bad or the sensor is not getting a correct reading of the temperature. ie, not a good temp contact with the rad tank.

[GoodysGotaCuda]

Sounds like you are completing the sensor part of the cct with the meter. Effectively, the meter test leads are 'jumpering' the temperature sensor, sending the 12vdc signal to the relay coil which picks up and starts the fans. At this point, it seems to me that the sensor is bad or the sensor is not getting a correct reading of the temperature. ie, not a good temp contact with the rad tank.

x2 on the test light completing the circuit, that is why they are no longer used on new vehicles for diagnosis. They can let the smoke out of modern electronics pretty quickly!

An ohm meter may show an open circuit within the sensor "positive" and ground. However the sensor should not be receiving "power", rather it is completing a ground circuit for the relay with the engine block ground upon getting to a certain temperature. Be sure the temperature rating of the sensor is what you are expecting, you may try heating it in water and checking the resistance (carefully!)

[dodj]

They can let the smoke out of modern electronics pretty quickly!

All modern electronics run on smoke.... If you let the factory smoke out, it will never work again!