Alternator - testing
1 The following in-vehicle alternator tests can be made irrespective of which type of alternator is fitted provided a 10 to 20 volt voltmeter, an ammeter (70 amp+) and a load rheostat are available. Alternatively a proprietary multimeter can be used.
2 Prior to undertaking any of the following tests, first check that the drivebelt tension is correct and that the battery is well charged.
Wiring continuity check
3 Detach the battery earth lead, then disconnect the wiring multiplug connector from the alternator.
4 Reconnect the earth lead, switch the ignition on and connect a voltmeter to a good earth point. Now check the voltage reading on each of the multi-plug terminals. A zero reading indicates an open circuit in the wiring whilst a battery voltage reading proves the wiring to be in good condition.
Alternator charging circuit continuity check
A Wiring multi-plug
B Voltmeter
Alternator output check
5 Connect up the voltmeter, ammeter and rheostat, as shown.
Alternator output check
A Ammeter
V Voltmeter
R Rheostat (30 amps rating resistor)
6 Switch the headlights on, also the heater blower motor and heated rear window (where fitted). Start the engine and keep it running at 3000 rpm whilst varying the resistance to increase the current loading. The rated output should be achieved without the voltage dropping below 13 volts.
7 Complete the check by disconnecting the test instruments and switching off the ignition, headlights, blower motor and heated rear window.
Positive side voltage check
8 Connect up the voltmeter as shown.
Alternator charge circuit voltage drop check - positive side
Switch on the headlamps then start the engine and note the voltage drop. Now run the engine at 3000 rpm. If the voltage shown is above 0.5 volt it is indicative of a high resistance in the positive side of the charge circuit, and this will need to be located and rectified. Switch the ignition and headlights off to complete.
Negative side voltage check
9 Proceed as described in paragraph 8 but connect the voltmeter as shown.
Alternator charge circuit voltage drop check - negative side
A voltmeter reading in excess of 0.25 volts is indicative of a high resistance fault in the negative side wiring.
Regulator control voltage check
10 Connect up the voltmeter and ammeter as shown
Alternator regulator control voltage check
then start the engine and check the voltage reading.
11 Increase the engine speed to 3000 rpm and note the ammeter reading. This should fall to between 3 and 5 amps at which point check the voltmeter which should read between 13.7 and 14.5 volts. Any readings given which are not within these limits indicate a fault in the voltage control regulator and this must be renewed.
12 Switch the ignition off and detach the test equipment. Disconnect the battery earth lead and reconnect the alternator multi-plug. Reconnect the battery earth lead to complete.