800.MACTOOLS
49
Connect:
•
Black test probe to the positive (+) battery post.
•
Red test probe to the positive (+) alternator cable.
Start the engine and run it at 2,000 RPMs. The current reading should be from 3A to 7A.
NOTE:
Low battery voltage produces a higher current.
Alternator Diode Test
The diode pack in an alternator consists of two diodes in series. To test the alternator diodes, remove the diode
pack from the alternator. Then touch one test probe to one side of the alternator diode pack and touch the other
test probe to the other side. Record the reading; then reverse the test probes and repeat the test. For one of the
diode tests, the meter should display the voltage drop across the series diodes at typically about 0.8V; testing in
the other direction should display OFL (OverFLow). If the reading is about 0.4V, one diode is shorted. A reading
below 0.2V indicates two shorted diodes.
NOTE:
Shorted diodes in the alternator can cause a low current output and run the battery dead overnight.
Red
Black
Anode
Cathode
Diode
Pack
Meter setup to measure voltage drop of the alternator diode pack:
•
Set the rotary switch to diode test (
) setting.
•
Insert the black lead in the “
COM
” jack.
•
Insert the red lead in the “
” jack.
Disconnect the battery cable from the alternator output terminal.
Dismantle the alternator and remove the diode pack from the alternator.
