Electrical Maintenance 8-5
NOTE: An alternator can easily exceed its rated output. An alternator MUST at least reach its rated output when full fielded.
An alternator that has a defective rectifier diode may reach 75% of its rated output with a full field.
13. Stop the unit.
14. Use the readings obtained previously to determine the problem by referring to the Diagnosis Chart.
NOTE: This assumes that the alternator did not charge properly prior to the full field test.
Field Current Test
(Checks the field windings, brushes and slip rings)
Perform this test with the On-Off switch in the OFF position.
1. Attach a clamp-on ammeter to the 2A wire near the B+ terminal on the alternator.
2. Connect a jumper wire between the F2 terminal on the alternator and a chassis ground, and note the ammeter reading.
3. The ammeter reading indicates field current. The following chart shows the field current for each alternator with 12 volts
applied to the field:
a. No field current or a low field current indicates an open circuit or excessive resistance in the field circuit. Replace the
voltage regulator and brush assembly, inspect the slip rings and repeat the test. If the brushes are not the problem, replace
the rotor or the alternator.
b.High field current indicates a short in the field circuit. Repair or replace the alternator.
Alternator Rating Field Current 12 Volts
23 Amp
37 Amp
65 Amp
120 Amp
1.0 to 3.0 Amps
3.5 to 4.5 Amps
4.0 to 5.0 Amps
5.0 to 6.0 Amps
Removing Screws and Bolts
1. Through Bolts
2. Screws surrounding rotor shaft
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