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I'm showing three fan relays in the wiring diagram. The fans are powered by two 30amp maxi fuses. The relays and fan motors are grounded thru the engine computer. The computer takes input from the coolant sensor and the ac control to decide when fans are needed. You would need to know if the computer is grounding the relays, and if not, why not. You may need a scanner to monitor computer output.
The only other things would be the coolant temp sensor or the engine computer. Alot of the new GM's have the on and off function of the cooling fans controlled by the computer. If the coolant temp sensor is ok, then most likely the problem is in the engine computer.
V6 2.8L top right rear of engine. Fan also turns on through A/C pressure switch and coolant temperature sensor, but these should operate independent of the coolant temp switch.
V8 5.0L and 5.7L (Heavy Duty) lower right rear of engine above starter.
Base model V8 uses the coolant sensor (top right of engine) and A/C pressure switches (lower right corner of eng compartment on A/C line to turn on the cooling fan. Fan is energized on all applications through the cooling fan relay.
The Cooling fan is Wired in in such a way so as to Automaticly Turn on Cooling Fan If the A/C is Switched ON. If "WITHOUT the A/C on" The Cooling Fan Doen NOT come ON Untill Engine. is @ 210-230 Degrees
There IS a Problem with the Engine COOLING SYSTEM, A Faulty Coolant Temp Sensor Will Cause this Problem. The Cooling fan is Also A Candidate For Testing.
Answering your Quwstion YES The Engine Coolant Temperture Sensor Tells the ECM What the TEMP Is and the ECM Grounds the Cooling FAN Relay to Turn ON Cooling FAN. Temp Sensor is the Usual Culpret, BUT Relay Comes in a CLOSE Second. Hope this Helps !!!
since you have already tested fan and relay and both work, the problem may be the engine temp sensor. There are two sensors. One goes to instument cluster. This is engine coolant temp sensor, the other is engine temp sensor which is usually attached to engine block, sometimes they are connected to thermostat housing with the coolant sensor. You can pully the engine temp sensor and test that sensor same way as relay. (with mulitimeter)