The cooling fan isn't spinning or is spinning too slowly because the drive belt is broken, loose, or slipping. No fan = no airflow through the coolers. Overheating follows within minutes.

What you'll see

The engine overheats relatively quickly -- faster than you'd expect from a dirty cooler. The coolers themselves may look clean. If the belt is slipping, you'll hear a squealing noise from the engine area, especially under load or on startup. If the belt has snapped, there's no noise -- the engine runs normally but heats up fast because there's no cooling airflow. The alternator also stops charging (same belt usually drives both fan and alternator), so you may notice the charging light come on.
Before you assume this is the problem

If the fan is spinning properly and the belt looks OK, the overheating cause is elsewhere -- check coolant level, radiator condition, or engine load. See: Clogged Radiator, Low Coolant.

See all causes of overheating / high temperature shutdown →

How to diagnose

  1. Check if the fan is spinning

    With the engine running (carefully, from a safe distance), look through the ventilation openings to see if the cooling fan is turning. If it's not spinning, the belt is broken or has come off. On some compressors, the fan is directly driven from the engine -- if the fan isn't turning, something is mechanically broken.
    Result: Fan spinning = belt OK, look elsewhere. Fan not spinning = belt problem.

  2. Inspect the belt (engine off)

    Shut down and check the fan belt. Look for cracks, glazing (shiny smooth surface), fraying, or chunks missing. Check the tension -- you should be able to deflect the belt about 10-15mm in the middle of the longest span. A loose belt will slip under load, reducing fan speed. A glazed belt slips even when properly tensioned.
    Result: 10-15mm deflection = correct tension. More = too loose. Glazed/cracked = replace.

  3. Check pulleys for damage

    A worn or damaged pulley can chew through belts. Check the fan pulley, crankshaft pulley, and alternator pulley for wear, wobble, or damage. Run your finger inside the V-groove -- it should be smooth. Rough or polished spots indicate misalignment.
    Result: Smooth, aligned pulleys = OK. Worn/wobbling = replace.

How to fix it

  1. Replace the belt

    Replace with the correct size and type. Loosen the alternator or tensioner to slip the new belt on, then tension to spec. A new belt will stretch slightly in the first few hours -- re-check tension after 8 hours of running. Always carry a spare belt on site -- it's one of the most common field failures.

  2. Adjust belt tension

    If the belt is OK but too loose, adjust the tensioner. On most engines, the alternator pivots to provide tension. Loosen the alternator mounting bolts, lever the alternator away from the engine to tension the belt, then tighten the bolts. Check the manual for the correct deflection spec.

Common mistakes

Don't over-tighten the belt. An overtight belt wears out bearings on the water pump, alternator, and fan hub. The belt itself also wears faster. The correct tension allows 10-15mm deflection. Also: always carry a spare fan belt on site. A 10-dollar belt can shut down your entire operation for a day if you don't have a spare.

Parts & tools

Replacement fan belt (correct size -- carry a spare!). Wrenches for alternator pivot bolts. Pry bar for tensioning. Belt tension gauge (optional).

Review safety precautions before starting →

Safety

Keep hands, clothing, and tools away from the fan and belts while the engine is running. The fan can cause serious injury. Always shut down the engine before inspecting or replacing belts.

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