A belt that's cracked, frayed, or glazed (smooth and shiny instead of slightly rough) has lost its grip. A glazed belt slips even at the correct tension. Belts have a service life -- they're a wear item that needs periodic replacement, not a set-and-forget part.

What you'll see

The belt squeals despite being at correct tension. Looking at the belt, you see: a shiny, glassy surface on the contact sides (glazing from heat and slip), cracks across the belt perpendicular to the length (age/heat damage), fraying or chunks missing from the edges, or a generally worn-out, thin appearance. The belt may also be hardened and inflexible -- bend it and it feels stiff instead of supple.
Before you assume this is the problem

If the belt is new and in good condition but still squeals, the problem is tension, alignment, or contamination -- not the belt itself. See: Belt Too Loose, Pulley Misalignment, Oil Contamination.

See all causes of belt squeal / screaming sound →

How to diagnose

  1. Inspect the belt visually and by touch

    Check the belt surfaces that contact the pulleys (the angled sides of a V-belt). They should be slightly rough or textured. If they're shiny and smooth (glazed), the belt has lost its friction coefficient and will slip. Also check for cracks, fraying, hardening, or any physical damage. If the belt is more than 3-5 years old and shows any of these signs, it's time for a replacement.

    Result: Slightly rough texture = OK. Glazed, cracked, frayed, or hard = replace.

How to fix it

  1. Replace the belt

    Match the belt number stamped on the old belt (e.g., A48, 4L520, BX52). This is the belt profile and length. Buy the exact same belt. Loosen the motor to remove the old belt, install the new one, and tension to 10-15mm deflection. Don't pry the belt over the pulley -- you can damage the belt cords. Loosen the motor enough to slip the belt on easily.

  2. Replace both belts on dual-belt drives

    If the compressor uses two belts, always replace both at the same time. A new belt is slightly shorter than a worn one. If you mix old and new, the new belt carries almost all the load while the old one rides loosely. This overloads the new belt and it wears out fast. Always use a matched pair.

Common mistakes

Using belt dressing spray to revive a glazed belt. Belt dressing is a temporary grip enhancer that softens the rubber. It works for a few hours then makes the glazing worse as the softened rubber gets polished again. Replace the belt instead. Also: don't store spare belts in direct sunlight or near heat sources. UV and heat degrade rubber belts even in storage. Keep spares in a cool, dark location.

Parts & tools

Replacement belt (match the belt number exactly). Wrenches for motor mounting. Keep a spare belt on hand.

Review safety precautions before starting →

Safety

Shut down and disconnect power before inspecting or replacing belts. A fraying belt can catch on the pulley and pull your hand in.

This issue can also cause

  • Unusual Noise / Knocking New or unusual sounds from the compressor: often caused by loose parts, worn bearings, or valve problems.