Alternator - testing

Note: To carry out the complete test procedure use only the following test equipment - a 0 to 20 volt moving coil voltmeter, a 0 to 100 amp moving coil ammeter, and a rheostat rated at 30 amps.

1 Check that the battery is at least 70% charged by using a hydrometer.

2 Check the drivebelt tension.

3 Check the security of the battery leads, alternator multi-plug, and interconnecting wire.

Cable continuity check
4 Pull the multi-plug from the alternator and switch on the ignition, being careful not to crank the engine. Connect the voltmeter between a good earth and each of the terminals in the multi-plug in turn. If battery voltage is not indicated, there is an open circuit in the wiring which may be due to a blown ignition warning light bulb if on the small terminal.

Alternator output check
5 Connect the voltmeter, ammeter and rheostat as shown (see illustration).

6.5 Alternator output test circuit
6.5 Alternator output test circuit

Run the engine at 3000 rpm and switch on the headlamps, heater blower and, where fitted, the heated rear window. Vary the resistance to increase the current and check that the alternator rated output is reached without the voltage dropping below 13 volts.

Charging circuit positive side check
6 Connect the voltmeter as shown (see illustration).

6.6 Alternator positive check circuit
6.6 Alternator positive check circuit

Start the engine and switch on the headlamps. Run the engine at 3000 rpm and check that the indicated voltage drop does not exceed 0.5 volt. A higher reading indicates a high resistance such as a dirty connection on the positive side of the charging circuit.

Charging circuit negative side check
7 Connect the voltmeter as shown (see illustration).

6.7 Alternator negative check circuit
6.7 Alternator negative check circuit

Start the engine and switch on the headlamps. Run the engine at 3000 rpm and check that the indicated voltage drop does not exceed 0.25 volt. A higher reading indicates a high resistance such as a dirty connection on the negative side of the charging circuit.

Voltage regulator check
8 Connect the voltmeter and ammeter as shown (see illustration).

6.8 Alternator voltage regulator test circuit
6.8 Alternator voltage regulator test circuit

Run the engine at
3000 rpm and when the ammeter records a current of 3 to 5 amps check that the voltmeter records 13.7 to 14.15 volts. If the result is outside the limits the regulator is faulty.

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