Electrical Maintenance
80
14. Turn the unit on and allow it to start. Using the
clamp-on ammeter, check the current flow in
the 2A wire.
A positive reading indicates the alternator is
charging. On unit startup, the current flow
should momentarily increase to allow for
battery current used during preheat and
cranking. Within a short time the current
should fall to normal unit load plus charge
current to the unit battery (typically 5-10
amps).
A reading on the clamp-on ammeter at or near
0 amps indicates the alternator is not charging.
Checking the unit ammeter will show a
discharge condition. The alternator is
defective if there are no problems in the
wiring. Recheck the wiring before assuming
the alternator is defective.
15. Check the voltage at the B+ terminal. The
voltage should increase until it reaches the
anticipated voltage regulator setting as shown
in the table below. Record the voltage.
The voltage regulator setting varies inversely
with the temperature as shown below.
Regulator voltage can vary from
approximately 15.2 Vdc at -40 F (-40 C) to
approximately 13.2 Vdc at 176 F (80 C).
If the voltage does not increase to the
anticipated voltage regulator setting, the
alternator is defective if there are no problems
in the wiring. Recheck the wiring before
replacing the alternator.
16. If the voltage does increase until it reaches the
anticipated voltage regulator setting, compare
the voltage at the B+ terminal to the voltage
between the battery terminals. The voltage at
the B+ terminal should be no more than 1.0
Vdc higher than the voltage between the
battery terminals.
If the voltage at the B+ terminal is no more
than 1.0 Vdc higher than the voltage between
the battery terminals, continue with Step 17.
If the voltage at the B+ (POS) terminal is
more than 1.0 Vdc higher than the voltage
between the battery terminals, clean and check
the wires and connections in the 2A and 2
circuits and repeat this check.
17. Increase the charging system load as much as
possible by running the unit in high speed
heat.
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.
Temperature
Anticipated Regulator Voltage
-40 F (-40 C)
From 15.2 Vdc to 14.0 Vdc
77 F (25 C)
From 14.4 Vdc to 13.6 Vdc
176 F (80 C)
From 14.2 Vdc to 13.2 Vdc
