Bottom Line
Maintenance Q&A
Foiling fan problems

By Paul Abelson
senior technical editor


Q: I have a 2006 Freightliner Columbia day cab with a Detroit engine. It has only 480,000 miles. I bought it a year ago for hauling dirt on local road building and construction projects. Lately it got quite noisy and my mileage dropped to below 4 mpg. My dealer said the Horton fan clutch is shot and needs to be replaced. Is there any way I can have it fixed without buying a new one?

A: When fan clutches fail, it’s usually in the full-on position. That’s done to protect the engine from overheating. If it overheats, the control module will de-rate the engine so all you can do is limp home.

Today’s high-power engines with exhaust gas recirculation (EGR) run significantly hotter than pre-2002 models. To help cool them, large-diameter nine- and eleven-blade fans can draw 100 horsepower or more. That explains the severe drop in fuel mileage. It also is why many drivers use the override switch to engage the fan for extra braking when going downhill. If left to freewheel, the fan absorbs virtually no power. When engaged, air through the radiator exerts a retarding force to augment your engine brake.

There are three basic families of fans. Electro-magnetic clutches engage using magnets to actuate the clutch. Electro-pneumatic clutches engage using air pressure from your truck’s air system. Electro-viscous clutches vary driving force by changing the viscosity of the fluid driving the fan. The thicker the fluid, the more force will be delivered to drive the fan.

Your fan clutch is a Horton electro-pneumatic model, the most common type.

The first step is to carefully inspect the unit. Most problems arise from either corrosion or heat damage. De-icing salts carried by spray land on aluminum hubs and connectors. Hubs have been corroded to the point that outer shells are coated with powder and have no structural strength at all. Spray gets into plugs and sockets, destroying electrical contacts. That damage not only weakens hubs and causes clutch failure, but also prevents them from being repaired or rebuilt.

Under-hood heat, especially with pre-2007 EGR engines, can destroy fan clutches as well as other under-hood components, including power steering pumps, air conditioners and alternators. High temperatures “cook” greases, destroying bearings.

If you determine that corrosion or temperature didn’t cause your failure, check your pressure switch. Trace the air line from the fan clutch back to the switch and disconnect it. If the fan stops turning, the switch is the culprit. Replace it.

If the fan cycles on and off rapidly, you have an electrical control issue. That’s usually a problem in the engine control module. Check the sensors and the ECM.

If the hub can be repaired, have it rebuilt by a technician experienced in working on fan drives. Use only OEM quality parts. Will-fit parts could be cheaper, but may not stand up to the heat and corrosive salts. You may be better off with a factory remanufactured fan clutch. You’ll get a quality product with a good warranty. LL