Tag Archives: alternator won’t charge

Do You Have A Faulty Alternator or Demagnetized One?

This Tech Tip has been prepared by Delco-Remy:

If you’ve installed an alternator and it won’t charge, don’t remove it just yet. Here’s why: most Delco Remy alternators have “Auto Start” capability that doesn’t require a separate wire to excite the alternator to charge. During the manufacturing or remanufacturing process—when each alternator is tested for performance—a residual magnetism is generated in the alternator, so it begins charging as soon as the vehicle starts.

On rare occasions, the alternator can become demagnetized when it’s been on the shelf for a prolonged period of time. When that happens, the voltmeter indicates battery voltage instead of the typical alternator charging voltage.

Before you remove the alternator, first try to determine if a loss of residual magnetism is causing the loss of voltage. You can do this by flashing the field:

  1. With the vehicle ignition in the “off ” position, remove the rubber boot from the alternator battery terminal and the rubber grommet from the small “R” terminal.
  2. Take a small piece of wire and touch one end of it to the battery alternator terminal. Then touch the other end to the small “R” terminal. (Note: This may create a small spark, which is normal.)
  3. Remove the wire and start the vehicle. The voltmeter should now increase its voltage reading to around 14 volts.
  4. If the alternator isn’t charging, then re-check all connections and belt tension before removing the alternator.
  5. Put the rubber boot and the rubber grommet back on.

Click here to watch a video about this Tech Tip.