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Charging system diagnostics: battery, alternator, and related systems

by Bob O'Neill and Bob Lincoln

Charging system inspection and maintenance

Dirt and electrolytes between the battery terminals can cause a drain on the battery. To reduce or eliminate this condition, be sure that the battery is clean. Include the battery terminal posts and the harness clamps; as the clamps age, they oxidize and the acids in the battery eat away at the metal.

Check the voltage between the battery post and the harness clamps. If you notice a difference the post and clamp should be cleaned with a wire brush (special brushes are sold at parts stores for under $5).

Examine the cables where they enter the clamps, as the insulation can split and battery acid can corrode the wire, causing a poor connection. Also consider battery terminal covers to reduce exposure to elements and to maintain good connection.

If you notice that the clamps are really distorted or 'worn out' they should be replaced. Improper connections at the battery (the battery post and clamps) can cause issues related to the charging system.

Diagnosis steps

When your charging system is having issues test it with these steps to isolate the problem.

  1. Check the battery. Ensure that the connections are very sound and that the clamps and terminals are clean to insure a good connection.
  2. With the engine running and headlights and accessories off, measure the voltage across the battery terminals. If the charging system is performing properly, the voltage should be between 13.3 volts (DC) and 14.7 volts. If you measure this voltage within this range the charging system is should be working properly.
  3. A shorted diode in the alternator can still allow a charging voltage of up to 13.3 volts, and can be detected by the battery going dead within several hours. If this happens, charge the battery out of the car to confirm it is still good. If you are still experiencing charging system issues, then the battery is suspect.
  4. If the voltage measured in step one is outside the range, check the voltage across the alternator output terminals. On the alternator, the top large terminal is the positive and the lower large terminal is negative. The voltage across these terminals should be between 0 and 0.4 volts higher than the voltage across the battery. If the measurement is outside this range check the 'overcharging and undercharging' section below.<a name="terminal"></a>
<a name="charging"></a>Overcharging and Undercharging

To understanding testing for overcharging or undercharging it is important to understand what generates the "charge."

Inside the alternator there is a field coil, which generates the charging current. If more current passes through the field coil, the alternator generates more current. The current that flows through the field coil is regulated by the power module, which is controlled by a signal from the logic module. [history of the alternator]

The logic module has an input which monitors the charging system voltage. One side of the coil is switched to ground by the power module's control circuit. The other side of the coil is connected to the 'on' terminal (the 12-volt, or "hot" terminal) of the ignition key switch. The logic module switches or 'cycles' the ground side at a fixed frequency which varies the time that the terminal is grounded. This is called the duty cycle.

When the system is at 0% duty cycle, the terminal is open and no current is produced by the alternator. When the system is at 100% duty cycle, the terminal is always grounded which allows the alternator to produce a maximum current output. As the demand on the electrical system changes the logic module adjusts the duty cycle to properly charge the battery based on the battery temperature. As accessories (especially headlights) create a greater demand on the system, the duty cycle is adjusted accordingly.

To test for proper charging regulation, turn off all accessories and start the engine. Then check the voltage across the two small terminals at the alternator. You're testing the field coil terminals for a voltage near 5 volts DC. These terminals are the two small ones below the large positive terminal. Then while still connected, turn on all accessories and the voltage should be near 10 volts.

If the voltages are both high (accessories off and on), the alternator may be bad. The regulator is at fault if both the charging voltage and the field coil voltage are both either high or low.

Check the wiring diagram for your model year and locate the sense line to the logic module. Check the voltage sense line to the logic module. It should have the same voltage as at the alternator.

Check the voltage at the switching control circuit. A code 41 will be triggered if this circuit is shorted or open. If the circuit is shorted the alternator will run full field and will overcharge, if it's open the alternator will undercharge because it will run with no field.

LINKS: Alternators used by Chrysler | Alternators vs generators | Other electrical repairs
| Code 41

Sensors, Switches, and Other Systems | Main Repairs Page | EEKs

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