Accepted Answer

If both relays and fuse are good, there may be a problem with the ECM. It may be necessary to have the system tested with a class 2 scanner to run functions tests and locate the problem. I am sending a breif discription of the system from the GM manual. On and off is controled through the ECM grounding of the system.

Cooling Fan Control

The engine cooling fan system consists of one cooling fan and two relays. The relays are powered by the battery positive voltage circuit and controlled by a switched ground from the engine control module (ECM).

During operation, the ECM supplies the ground path for the cooling fan relays through the cooling fan relay control circuit. This energizes the cooling fan relay coil, closes the relay contacts, and supplies battery positive voltage from the cooling fan fuse through the cooling fan motor supply voltage circuit to the cooling fan. The cooling fan motor is grounded through its own ground circuit.

The ECM commands the fan on under the following conditions:

• Intake air temperature is above 4°C (40°F).
• Engine coolant temperature exceeds approximately 114°C (237°F).
• Throttle position goes below 89% for 15 seconds.
• A/C refrigerant pressure exceeds 2945 kPa (427 psi).
• A/C is requested and engine speed is below 4750 rpm.
• AF23-5 Transmission fluid temperature exceeds 130°C (266°F).
• VT25-E Transmission fluid temperature exceeds 140°C (284°F).
• When the engine coolant temperature exceeds 105°C (221°F) at key off, the fan will run for up to 300 seconds.

The ECM commands the fan off under the following conditions:

• A/C is requested and engine speed exceeds 6250 rpm.
• Engine coolant temperature drops below approximately 117°C (242°F).
• Throttle position is greater than 95%.
• Intake air temperature goes below 2°C (36°F).

Expert:	Bob
Pos. Feedback:	99.3 %
Accepts:
Answered:	2/3/2007

Auto Service Technician

40 year GM Tech. +30 Certified. ASE Master Tech.(expired, retired) Medium Duty, Heavy Line, Retired