1991 Pontiac Grand Am Radiator Fan Problem

Hello:
I have a 1991 Pontiac Grand Am with a 2.5 liter 4 cylinder engine in my shop with a radiator fan control problem. If the air conditioning compressor is running, the radiator fan runs. If the air conditoner is
switched off, the radiator fan turns off. The radiator fan does not run, no matter how hot the coolant gets. I have already replaced the temperature sensor, located just below the thermostat housing with a Borg Warner WT 3081. When cold, the resitance across the sensor is almost 2000 ohms. When hot, the resistance across it is about 150 ohms. The sensor connects to inputs on the ECM module with a black and a yellow wire. If I start the engine with the sensor unplugged from the ECM module, the check engine light comes on. If I start the engine with the sensor plugged into the ECM module, the check engine light does NOT come on. The temperature gauge in the dash gives correct readings at all times. What could be wrong? Also, I have thought about installing a toggle switch to send power to the fan control relay to turn the fan on when the Air Conditioning is off, but I can't locate the fan control relay. Where is it? I have a full wiring diagram of the car in a Hanes
manual, but still haven't been able to track down the problem. I can send a scan of the wiring schematics to anyone who might need to see them to help me. Robert McPherson 502) 367-9324 or (502) 298-6997
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Is the car overheating? It sounds to me like it is working correctly. The fan doesn't run all the time. It only runs when the computer determines it is needed....

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Woody:
The fan NEVER runs if the air conditioning is switched off-No matter how hot the radiator water gets. The only way to get the fan to run is to leave the air conditioner on all the time, which is a problem, since the outdoor temperature is now in the 50s.
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It might be helpful to see that schematic. Can you put it on a website and post a link to it.. (http://www.tinypic.com/for example)
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