Diagnosing Your Charging System

Description and Diagnosis

If you are having difficulties with your charging system, use this test to narrow down the problem:

  1. Start the engine (jump it if you have to).  If the engine will not run without the jumper cables on, then shut off the engine on the car you are jumping from.
  2. Measure the voltage across the battery terminals with the jumper cables disconnected.  The charging voltage should be between 12.8V and 14.7V.  If the voltage is OK, then your charging system is currently functioning and it is likely that you have a bad battery or battery terminals.
  3. Check the voltage across the alternator output terminals (the top, large terminal is the positive and the lower, large terminal is the negative).  This voltage should also be between 12.8V and 14.7V.  If the voltage is low, then you are undercharging and if the voltage is high, then you are overcharging.  If the voltage across the alternator is good but the voltage at the battery is bad, then check the connections and battery terminals.

Battery Terminal Problem

To check the condition of your battery terminals, turn accessories on until the system voltage drops (lights get dim, etc.).  Measure the voltage between the battery post and the terminal itself for both posts.

If there is a significant voltage here (more than a tenth or so), your battery terminals need to be cleaned.  Remove the terminal, clean off the post and the inside of the terminal with a wire brush or a battery terminal cleaning tool.  Reinstall the terminals and spray them with terminal grease (or you may use any type of grease to protect the terminals.  Battery terminal covers are also helpful as they will keep the grease from coming off.

If the voltage is very low or zero (make sure you are getting a good connection with the meter), check the voltage between the battery posts and each of the wires.  Some wire is usually exposed near the terminal or look for a connector to tap off of.  If this voltage measures above a tenth or so, then the terminals need to be replaced.

Undercharging and Overcharging

Check to see if the regulator is functioning by starting the engine and turning all accessories off.  The voltage across the alternator field coil terminals (two small terminals, below the positive output terminal) should be less than about 5V (roughly).  Turn all accessories on (headlights, brights, fan, rear defrost, radio, etc.)  and measure the field coil voltage again.  It should be above about 10V (roughly).

If the field coil voltage is always high, but charging voltage is always low, then the alternator a probably bad.  If the field coil voltage and the charging voltage are both either high or low, then the voltage regulator is not functioning properly.  Check the voltage sense line to the logic module (make sure it is getting the same voltage seen at the alternator) and switching control circuit between the logic and power modules.  If this circuit is shorted, the alternator will run full-field (overcharge) and if it is open, the alternator will run with no field (undercharge).  This condition would be accompanied by a fault code 41.

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Updated 10/31/2003.

Copyright © 1996-2003 Russ W. Knize.