Hi


When you start a 2005 blazer should the engine fan not be spinning so engine heats up to operating temperature?


It seems mine spins as soon as it starts up. Is the clutch on the fan gone then?

 

it will fully engage on cold start

 

I hate em, get an electric fan

 

Thanks

 

Its completely normal. But after a minute or so and the fan does not disengage (stays engaged at all times) the clutch needs to be replaced.

 

With a cold engine, there should be significant resistance when turning the fan by hand. Immediately after shutting down the engine at operating temperature, the fan should turn fairly easily by hand, but not free wheel, (it should never free wheel). To accurately test the clutch, reflective tape is applied to the water pump shaft and a laser or infra red non contact tachometer is used to measure water pump rpm.

The clutch has two fluid chambers and a thermostatic coil. On a cold start up the fluid is in the first chamber and the fan should be fully engaged, (100% of water pump rpm). As the fluid warms, it gradually transfers to the second chamber and the fan disengages, (~20% of water pump rpm). During warm up, the fan should disengage above ~2500 engine rpm. When the coolant in the radiator reaches operating temperature, the warm air crossing the thermostatic coil keeps the fan disengaged. If there is a major loss of coolant, ambient temperature air being drawn across the clutch is not warm enough to keep the fan disengaged. The fluid transfers back to the first chamber, and the fan engages.

 

For those that are wondering, Here is the clutch fan operation as quoted from a Hayden brochure.

Thermal Fan Clutch
Varies the fan speed with temperature of the air behind the radiator.
Engaged (high speed) operation provides maximum cooling
Disengaged (low speed) operation provides fuel savings and noise reduction.
Greater life expectancy than a non-thermal clutch
Briefly engaged at cold start-up.
Engages at about 170°radiator air temperature (about 30°lower than coolant temperature).
The air temperature coming through the radiator is sensed by the bi-metal thermal
spring on the front of the thermal fan clutch. It expands and contracts with the change
in air temperature operating a valve inside of the clutch. When cold, the silicone
drive fluid is pumped from the working area to the reservoir. When hot, the valve opens
allowing fluid from the reservoir to be transferred to the working area thereby increasing
the fan speed. The clutch disengages as the air temperature decreases, closing the valve
and allowing the silicone fluid to be pumped back into the reservoir.
A thermal fan clutch is engaged on a cold startup because the fluid drains into the
working area when the engine is shut off. The fan clutch will slow down shortly after
startup as a result of a pumping action produced by a difference
in speed between the shaft and the body of the clutch

 

Thanks for the answers, I will try turning it cold and then try turning it after it has warmed up to 100c. See if I can feel a difference.