Thermo King UT-1200, Manual

Page: 80 / 169

Electrical Maintenance
80
17. Increase the charging system load as much as
possible by running the unit in high speed or
defrost.
18. Monitor the alternator output voltage.
With the increased load, the alternator output
voltage should decrease no more than 0.5 Vdc.
The voltage may increase as much as 1.0 Vdc.
If the alternator output voltage decreases no
more than 0.5 Vdc the alternator is good.
If the alternator output voltage decreases more
than 0.5 Vdc, the alternator is defective if
there are no problems in the wiring. Recheck
the wiring before replacing the alternator.
Alternator Diode Quick Check:
This check confirms proper diode function.
19. With the unit still running, set the digital
multi-meter connected from the alternator B+
output to chassis ground for AC volts. No
more than 1.0 Vac should be present. A
reading of more than 1.0 Vac indicates
damaged alternator diodes.
20. Turn the unit off.
Field Current Test
Use this test to determine if the alternator can be
repaired. Perform this test with the unit turned off.
1. Attach a clamp-on ammeter to the 2A wire
near the B+ terminal on the alternator.
2. Energize the field on the Thermo King
alternator by connecting a jumper wire
between the F2 terminal and the B+ terminal.
Do not connect the F2 terminal to ground or
the alternator will be damaged.
3. Note the ammeter reading. The ammeter
reading indicates the field current, which
should be 2.0 to 6.0 amps at 12 volts.
• No field current or a low field current
indicates an open circuit or excessive
resistance in the field circuit. Remove the
voltage regulator and brush assembly and
inspect the slip rings. If the slip rings and
are acceptable, install a new voltage
regulator and brush assembly and repeat
the test. If the brushes are not the problem,
replace the alternator.
• High field current indicates a short in the
field circuit. Replace the rotor or the
alternator.
Battery
NOTE: The Microprocessor Power switch must
be placed in the Off position before connecting
or disconnecting the battery terminals. The
Microprocessor Power switch is located on the
control box side of the unit.
Inspect/clean the battery terminals and check the
electrolyte level during scheduled maintenance
inspections. A dead or low battery can be the
cause of an ammeter indicating discharge due to
lack of initial excitation of the alternator even
after the unit has been boosted for starting. The
minimum specific gravity should be 1.235. Add
distilled water as necessary to maintain the proper
water level.