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I have a 2000 Chevy impala 3800.
The Radiator fans only run when the Air Conditioning is on.
The fans never seem to come on if the Air is off and the Temperature will climb to about 3/4.
If you turn on the A/C both fans run at full speed and the engine cools down to just between 1/4 and 1/2 of the gauge.
I checked the Sensor under the thermostat housing and it's resistance decreases as the engine gets hotter. and increases as it cools down. I checked the voltage from the plug by unplugging it and it reads 4.98v so all seems normal electrically. If I unplug the sensor the fans run continuously and the A/C will not cool and the gauge goes to zero. but with the sensor plugged in the fans never activate without the A/C being on.......
I am at a loss other than a failure in the ECM.

also I have checked all relevant relays and fuses and I do not have a check engine light coming on either..
I looked in the schematic section of the haynes manual and it shows a wire from the ECU to both fan relays to activate them. It's almost as if the computer is ignoring the sensor!
 
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