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The viscous fan clutch is one of the most common causes of overheating in traffic on an R107. It is easy to test and inexpensive compared with the damage an overheated engine can suffer.
What it does
The mechanical fan is driven by the water pump pulley through a viscous clutch filled with silicone fluid. When the air coming through the radiator is cold, the clutch slips so the fan turns slowly and quietly. When the air gets hot, a bimetal coil on the front engages the clutch and the fan spins close to pump speed, pulling a lot of air through the radiator.
Symptoms
- Temperature rises in traffic or at idle, drops at highway speed
- No loud roar from the fan for a few seconds after a hot start
- Oily residue on the clutch from leaking fluid
- Fan wobble, or a fan that spins very freely by hand when the engine is hot
Cold and hot spin test
- Cold, engine off: spin the fan by hand. It should turn with some resistance, usually less than one or two turns before stopping.
- Rock test: grab the tip of a blade and move it back and forth. More than a few millimeters of play points to a worn bearing.
- Hot, engine off: after the engine is fully warm and has been running at idle, stop it and carefully try to turn the fan (use a rolled newspaper or a stick, never your fingers near a running fan). It should feel noticeably stiffer than when cold.
- Listening test: with a hot engine, the fan should roar briefly on startup and you should hear a clear increase in airflow when the clutch engages.
When to replace
Replace it if the fan turns freely when hot, if it is leaking, if it wobbles, or if the car overheats in traffic with a good radiator and thermostat. A clutch of unknown age on a car that runs warm is worth replacing.
Which part
Use a quality clutch matched to your engine, from a known supplier (Behr is a common original supplier). Clutches differ in engagement temperature and torque, so do not fit one from a different engine just because it bolts on.
Quick answers
- Can I replace the fan clutch with an electric fan? It is possible, but the original system works very well when healthy.
- Should the fan be loud on startup? A brief roar on a cold start is normal; it should quiet down within a minute.
