Rick's Free Auto Repair Advice

Fan clutch issues Rainier Trailblazer S10 Ascender Bravada 9-7X S15 Envoy

Fix Fan clutch issues Rainier Trailblazer S10 Ascender Bravada 9-7X S15 Envoy

If you encounter Electronic Fan clutch issues on Rainier, Trailblazer, S10, Isuzu Ascender, Oldmobile Bravada, Saab 9-7X, and S15 GMC Envoy, read this.

The electronic fan clutch in the above vehicles experiences almost a 100% failure rate. The purpose of the fan is to reduce drag on the engine at highway speeds where “ram-air” is enough to cool the radiator. At highway speeds, a fully engaged clutch actually spins slower than the air coming through the radiator. So it reduces gas mileage. Thermostatically controlled viscous fan clutches have been around for decades to solve the problem. Those clutches are made from two halves with concentric circles that mesh. The gaps are filled with a silicone fluid. As the engine temperature rises, a thermostatically controlled valve closes off the flow of fluid and the concentric circles lock together under the pressure, transmitting the full torque to the fan blade. However, at higher speeds, the rush of aid past the thermostatic coil opens the valve, allowing the circles to disengage. At that point the fan blade “free-wheels” with the ram air.
Starting in 2002 GM replace the thermostatic coil with an electronically controlled “duty cycle” solenoid and an RPM indicator. The powertrain control module monitors engine and road speed and calculates how fast the fast should turn to properly cool the engine. To adjust fluid pressure in the clutch, the PCM pulses the solenoid on and off in short burst. This is called “duty-cycle.” For example, GM may divide each second into to pulses. If the PCM commands the solenoid on five times every other 1/10 of a second, the fan has a 50% duty cycle. The see how well things are working, the PCM compares the actual fan RPM to the RPM it expected when it calculated duty cycle. If the two speeds don’t match, the PCM makes further adjustments.
When the electronic fan clutches fail, they can lock on full time or free-wheel full time. Free-wheeling causes overheating and lock up reduces gas mileage and causes a howling noise at highway speeds.
To replace the fan clutch you’ll have to remove the fan shroud and fan blade. Mark the orientation of the fan blade before unbolting it completely. It must go back on in the same way. Then unbolt the old electronic fan clutch and swap it out.
The list price on a new fan clutch from the dealer is around $234. But Dorman Products makes an aftermarket replacement for $130. This is a new (not rebuilt part).

 

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat



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