Thermo King 002008, Manual

Page: 45 / 155

Electrical Maintenance
45
14. Stop the unit.
15. Use the readings obtained previously to
determine the problem by referring to the
Diagnosis Chart below.
NOTE: This assumes that the alternator did
not charge properly prior to the full field test.
Field Current Test
This test checks the field windings, brushes, and
slip rings. Perform this test with the Zone 1/Host
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
a 120 amp 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
120 Amp 2.0 to 5.0 Amps
Diagnosis Chart
Amperage in 2A Voltage at B+ Problem/Solution
At or above rated output
At or above battery voltage and
increasing Voltage regulator defective / Replace
voltage regulator and brush assembly
Approximately 60 percent
of rated output Approximately equal to battery voltage
and does not change, or rises slightly Receiver diode defective / Repair or replace
alternator
Low or no output
Less than or equal to battery voltage
and decreasing
Stator windings, field windings, brush or
diode defective / Perform Field Current Test
to check brushes and field coil, or replace
alternator