Charging system troubleshooting

Charging system trouble is indicated by an illuminated alternator warning light on the instrument panel, or by an under-or overcharged battery.

The alternator generates electrical current by electrical induction. That is, a magnetic field is placed in motion to induce a current in a stationary coil. When the engine is running and the alternator is spinning, part of the current it produces energizes its electromagnetic field. When starting, some other current must be provided to initially energize the field and begin the current generating process. This current is provided by the battery through the alternator warning light in the instrument panel.

On some models of Porsche cars a burned out instrument cluster alternator warning light will prevent the alternator from charging.

As soon as the alternator output equals battery voltage, the warning light goes out. Normally, the warning light should be off when the ignition is off and the engine is stopped. The light should only come on when the ignition is turned on (current to the alternator rotor) and go out again when the engine is started and the battery is being charged.

Table f lists symptoms of trouble indicated by the alternator warning light, their probable causes, and suggested corrective actions, most described later in this group.

An undercharged battery is usually associated with starting trouble. Again, make sure that the battery is in good condition and capable of accepting a full charge before blaming the charging system.

Causes of an undercharged battery are:

• The simultaneous use of many electrical accessories for long periods of time.

• Leaving accessories or lights in operation with the engine stopped.

• Frequent long periods of starter usage.

• Frequent short-trip driving.

• Improper alternator V-belt condition or tension.

• Broken or frayed charging system wiring.

• Corroded connectors at the D+ and B+ terminals of the alternator.

• Worn, corroded, or loose battery cable connections at the chassis.

Simplified charging system circuit























from Ignition switch (terminal 15)


from Ignition switch (terminal 15)


i Alternator | warning light



1984 Porsche 911 Wiring Diagram

from fuse box


Alternator to engine compartment heater fan


from fuse box


i Alternator | warning light


Anti-theft control j module

Alternator to engine compartment heater fan


Table f. Warning Light Troubleshooting


Test and probable cause

Corrective action

Ignition off, engine off, warning light on

Test: Disconnect blue wire (D+) from alternator.

a. Light goes out: Faulty alternator diodes.

b. Light does not go out: short to ground in wiring harness or wiring connector.

a. Repair or replace alternator.

b. Repair or replace faulty wiring.

Ignition on, engine off, warning light off

a. Battery fully discharged.

b. Bulb burned out.

Test: Disconnect blue wire (D+) from alternator. With battery connected and ignition on, touch blue wire to ground.

c. Light does not come on: Faulty bulb socket, open circuit between socket and terminal 19 of ignition switch, or open circuit between blue wire (D+) on alternator and instrument cluster.

d. Light comes on: Loose connection between regulator and alternator or loose connection between brushes and regulator.

e. Light comes on, no faults with regulator: Internal alternator faults or faulty regulator.

a. Charge battery.

b. Remove and test bulb. Replace faulty bulb.

c. Replace instrument cluster bulb holder. Repair wiring to alternator.

d. Inspect brushes. Correct loose connections.

e. Repair or replace alternator or voltage regulator.

Engine running at any speed, warning light stays on

a. Loose or broken alternator V-belt.

Test: Disconnect red wire (B+) from alternator. Do not short wire. Using a voltmeter, test between red wire and ground with battery terminal connected.

b. No voltage to alternator: Open circuit between red wire on alternator and starter or between starter and positive (+) pole.

c. Exciter diodes burned out.

d. Faulty regulator or faulty alternator windings.

e. High voltage drop between red alternator positive (+) wire and starter due to broken, loose, or corroded wires.

a. Replace or adjust V-belt.

b. Repair wire or connections between alternator and starter.

c. Repair or replace alternator.

d. Test charging system and replace faulty components as needed.

e. Repair wires or connectors.

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