Accessory Drive Belt Diagnosis

GENERAL INFORMATION
Accessory drive belt noise may be described as a thumping noise heard at, or just above idle speed, as a result of belt pilling. In a single-belt accessory drive, belt pilling occurs when small particles of belt material separate from the belt grooves, and form small balls or pills in the belt grooves. While some belt pilling is normal, excessive pilling can cause a thump or knocking as the pills pass over various pulleys. This causes the belt to repeatedly jump off of and slap onto the pulley.

Accessory drive belt noise is most noticeable at idle. Various other drive system components (tensioner, A/C compressor, generator) may appear to be the cause of the noise which is the result of the belt. For vehicles that exhibit this condition.

Before replacing the belt to resolve a noise condition, inspect all of the pulleys in the accessory drive system for misalignment. This can be accomplished by observing the various pulleys with the engine running. Runout or wobble will identify pulleys requiring attention. Observe particularly the crankshaft and coolant pump pulley for runout that may be induced by loose pulley fasteners.

Once alignment of the belt drive system is confirmed, the service belt should be installed and the noise re-evaluated even if replacement of other components appears to be required. Frequently, a belt tensioner which appears excessively "jumpy" can be fixed by belt replacement only. This is also true for belt noise which appears only with the A/C compressor engaged.

PROCEDURE
The thump or knocking noise easily misdiagnosed as an engine noise. In order to diagnose belt pilling, adhere to the following procedure:
1. Remove the drive belt.
2. Run the engine for no more than 30 to 40 seconds, in order to confirm that the engine noise disappears.
3. Inspect the power steering pulley for proper alignment.
4. Inspect all other pulleys for excess paint, debris or pills in the grooves.
5. Clean the pulleys with a wire brush, as necessary.