Thermo King SB-210, Manual

Page: 67 / 159

Electrical Maintenance
67
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.
14. Stop the unit.
15. 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)
Press the
O
FF
key to turn the unit off before
performing this test.
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
37 Amp 3.5 to 4.5 Amps
65 Amp 4.0 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% of
rated output Approximately equal to battery voltage
and does not change, or rises slightly Rectifier 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