Cooling Fans

  • 29 POSTS
  • 1999 GMC SIERRA
  • V8
  • 4WD
  • 250,000 MILES

The fan in the behind the radiator is not turning, would that be the cause of my engine creating alot of heat and entering the cab, is it supposed to turn a all times or periodically, because I marked the top of the fan and drove the truck for 15 minutes and the blade I marked was at the same position. And is there another fan somewhere. Would that create the loss of antifreeze from the surge tank. I need a description of the fan locations for both.

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Thursday, August 25th, 2011 AT 3:36 AM

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  • 4,228 POSTS

If I am correct, there should be a clutch driven fan mounted to the water pump pulley, driven by the belt. This fan turns at all times when the enginje is running, and if the clutch is functioning correct, locks up in higher engine temperatures. These(clutch) can fail, either never locking up, or locking up at all times. Not locking up is the most common. It shows that you also have an electric auxiliary cooling fan, which sounds like the one you are referring to. The description of it's operation is as follows.

Engine Cooling Fan Description - Electric
General Description

The auxiliary electric cooling fan consists of (1) radiator support (2) wiring harness (3) bolt/scew (4) auxiliary cooling fan assembly and provides additonal cooling for low speed vehicle operations, extended idle, stop and go conditions, and running the air conditioning systerm.


The auxiliary cooling fan is controlled by the VCM. The VCM controls the ground path for the cooling fan relay. The relay consists of (1) underhood fuse block (2) auxiliary cooling fan relay (3) wiring harness and is used to control the high current flow to run the cooling fan motor. The VCM determines operation of the independent fans based on various sensor inputs.

The cooling fan is controlled by the VCM based on the following inputs:

The A/C system

The Engine Coolant Temperature (ECT) sensor

The Vehicle Speed Sensor (VSS).

The VCM will turn the auxiliary cooling fan ON when any of the following conditions exist:

Certain VCM Diagnostic Trouble Codes (DTCs) are set.

ECT above 107 C (225 F).

A/C head pressure above 300 psi.

Once the auxiliary cooling fan has been turned ON by the Engine Coolant Temperature (ECT) sensor, the VCM will turn the fan OFF when that temperature has dropped about 4 C (7 F). If the auxiliary cooling fan has been turned ON by high A/C head pressure, the VCM will turn the fan OFF when the pressure has dropped to 250 psi. The minimum ON time for the auxiliary cooling fan is 30 seconds.

You would only see a loss of coolant due to fans malfunction, at the same time you would be overheating and boiling over.

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Thursday, August 25th, 2011 AT 4:25 AM

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