5. Alternator: checking the output

  1. The charge warning lamp should light when the ignition is switched on and should remain lit as the engine is started, but should go out as soon as the engine speed increases significantly above idle. If this is not the case, first check the bulb itself and the connections to the instrument panel. Note that the lamp is connected directly to the alternator via the smaller, blue, wire which appears at the alternator connector plug, from the D + terminal. Nota also that a faulty charge warning lamp operation is usually (but not always) caused by faulty brushes; a lot of time may be saved if these are checked first. See Section 7.
  2. If the fault persists, remove bath side panels to expose the battery terminals. Check that the battery and alternator connections are securely fastened and that the battery is fully charged.
  3. Accurate assessment of alternator output requires special equipment and a degree of skill. A rough idea of whether output is adequate can be gained by using a voltmeter (range O to 15 or O to 20 volts) as fellows.
  4. Connect the voltmeter across the battery terminals. Switch on the lights (UK models only) and note the voltage reading: it should be between 12 and 13 volts.
  5. Start the engine and run it at a tast idle (approx 1500 rpm). Read the voltmeter: it should indicate 13 to 14 volts.
  6. With the engine still running at a tast idle, switch on as many electrical consumers as possible (lights, stop lamp, turn signals and any accessories). The voltage at the battery should be maintained at 13 to 14 volts. Increase the engine speed slightly if necessary to keep the voltage up.
  7. If alternator output is low or zero, check the brushes, as described in Section 7. If the brushes are in good condition the alternator requires attention.
  8. Occasionally the condition may arise where the alternator output is excessive. Clues to this condition are constantly blowing bulbs; brightness of lights varying considerably with engine speed; overheating of alternator and battery, possibly with steam or fumes coming from the battery. This condition is almost certainly due to a detective voltage regulator, but expert advice should be sought.
  9. Note that the alternator voltage regulator can be renewed without removing the alternator from the machine. The procedure is part of brush renewal (Section 7).