Testing the Generator

By Norman Nock

Originally published in the Austin-Healey Magazine, February 1983

If your generator has been in operation for many thousands of miles, it should be tested and inspected.

First Test

The generator leads must be removed from the terminals on the generator.

A voltmeter should be connected to the “D” terminal (largest) with the engine running at 1200 – 1500 RPM when a reading of between 2 and 3 volts should register on the voltmeter.

If the reading is low, check the condition of the commutator, brushes, residual magnetism in the field pole-pieces, and the armature.

Second Test

The next step is to test the field coils. Connect an ammeter between ‘D’ and ‘F’ of the generator, leaving the voltmeter connected as in the previous test. Increase the engine speed slowly until the reading on the voltmeter is at 12 volts; the ammeter should read no more than 2 amps. If the ammeter reads 3 or 4 amps instead of 2 amps, the field coils could have an internal short.

Remove the field coils and inspect for shorts.

Third Test

Connect ammeter in series between ‘D’ terminal and the ‘D’ wire that was connected to the terminal. Rejoin ‘F’ wire to its terminal. Run the engine at 2000 RPM. The output is governed by the specific gravity of the battery.

Approximate Generator Output






5 or less





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