7 Clever Ways on How to Tell if Alternator Bearing Is Bad

Reviewed by Dr. Deepakkumar Jani

To diagnose a bad alternator bearing, listen for a growling or grinding noise from the alternator, indicating bearing failure. Measure alternator output; decreased voltage or fluctuating readings can signal bearing issues affecting performance. Check for excessive play or roughness in the alternator pulley by hand, indicating worn bearings. Overheating around the alternator area can also suggest bearing degradation. These symptoms require immediate attention to prevent alternator failure and potential damage to the electrical system.

Troubleshooting for Alternator Bearing Failure

Challenge/IssueSymptomsTroubleshooting StepsSolution
Bearing Wear or Failure1. Check the electrical load
2. Inspect the battery and charging system
1. Perform auditory inspection
2. Test alternator output
3. Manual inspection for play or roughness
Replace alternator or bearing assembly
Electrical System Overload1. Check the electrical load
2. Inspect the battery and charging system
1. Check the alignment of the alternator and pulleys
2. Inspect belt tension and condition
Reduce load; ensure battery and alternator are functioning properly
MisalignmentAlign alternator and pulleys; replace the belt if necessary1. Check the alignment of the alternator and pulleys
2. Inspect belt tension and condition
1. Check the alignment of the alternator and pulleys
2. Inspect belt tension and condition
Contamination1. Inspect for dust, dirt, and fluid leaks around the alternator
2. Check seals and gaskets
1. Inspect for dust, dirt, and fluid leaks around the alternator
2. Check seals and gaskets
Clean alternator area; replace seals and gaskets as needed

This article provides DIY alternator users with a detailed look at how to tell if an alternator’s bearing is going bad or has completely failed.

Listen for Bearing Noise

The most obvious symptom of a failing alternator bearing will be noise coming from the alternator itself. As the bearing wears out, you’ll start to hear a high-pitched whining or screeching sound that changes pitch alongside engine RPMs. This noise indicates the bearing is no longer smoothly spinning and beginning to fail. When you first hear this bearing noise, it may come and go or only happen at certain engine speeds. But it will progressively get louder over time as more wear occurs.

How to Tell if Alternator Bearing Is Bad

 Image Credits: Alternator Ball Bearing by 4volvos is licensed under (CC 0 1.0)

Conduct Bearing Side-to-Side Play Test

You can confirm a bad bearing and check its looseness using a simple side-to-side playtest. With the engine off, locate the alternator and grasp the alternator pulley with both hands. Apply light pressure to aggressively rock the pulley from side to side, feeling for any play or clunking of a loose bearing. A healthy bearing will have little to no discernible play as you rock it. But a worn bearing will have noticeable end play, rocking loosely within the alternator. This test is most convenient on belt-driven alternators allowing easy pulley access.

Examine Pulley for Play and Rust

Closely inspect where the pulley meets the alternator shaft for signs of lateral movement or rust trails. Grab the pulley firmly and move it laterally to detect any play between it and the shaft. Rotate the pulley 360 degrees to inspect from all sides. As a bearing fails, you may see rust trails forming on the shaft from tiny bits of worn bearing material making their way out. The pulley may exhibit minor lateral play as the bearing loses its tension against the shaft. Combined with noise, pulley play, and rust confirm without a doubt the bearing has degraded past its service life.

Watch for Battery Light

Along with noise, a bad alternator bearing hinders optimal alternator operation and can cause charging issues. Evidence of charging issues comes from the battery/charging system warning light. Keep an eye on your battery/charging system light as you increase RPMs during driving, especially with additional accessories putting a load on the electrical system. If the light flickers on as RPMs rise, it could indicate the alternator isn’t spinning fast enough to keep up with charge demands because of increased drag from the failing bearing.

Use a Multimeter to Test the Alternator Output

You can definitively diagnose a bad bearing based on substandard alternator charging performance on testing. Using a multimeter, conduct an alternator voltage output test with engine RPMs between 2000-3000. Compare results to factory specifications. If bearing wear prevents the alternator from reaching the specified charging output, it will fail the charging test. Compare charging voltage readings before and after replacing the bearing to confirm repair restored normal operation.

When to Replace an Alternator Bearing

Don’t wait until total bearing failure or seizure leads to catastrophic damage. Address that first screeching hint of wear right away. The bearing is only around $15-30, saves the alternator, and prevents getting stranded with a dead charging system. Prioritize replacement any time the bearing shows detectable play, exhibits coarse noise, surfaces rust on the shaft, or causes poor voltage output. Replacing it early on makes the job much easier too, before extensive wear escalates disassembly difficulty.

DIY Alternator Bearing Replacement Procedure

With the confident diagnosis of a bad bearing, here is the process to replace it yourself and restore the charging function for around $40 in parts.

  • Gather a bearing kit, new brushes, electrical contact lubricant, and high-temperature grease.
  • Begin by disconnecting cables and wiring to fully remove the alternator.
  • Remove external components to access the brush end frame, saving small parts for reinstallation.
  • Take care not to contact the diode trio, or desolder, or cut all windings to enable prying the frames open.
  • Remove the rotor assembly.
  • Press out both old bearings and thoroughly clean housings.
  • Press in the new bearings until fully seated. Pack new high-temperature grease around the bearing races.
  • Reassemble the components in reverse order.
  • Try to salvage and reuse hardware and small clips when possible or obtain replacements.
  • Make sure windings are properly reconnected without touching each other.
  • Once fully reassembled, reconnect pulley components, wires, and battery cable and remount it on the vehicle.
  • Clear any battery/charging lights, then start the engine and retest the operation.