Thermo King T-500R Maintenance Manual page 124

CAUTION: Energizing the circuit with the resistor bypass fuse installed will damage Thermo King alternators. Be
sure the resistor bypass fuse is removed for Thermo King alternators.
5. Check and note the battery voltage at the battery with the unit turned off.
6. With the unit off, check the voltage at the B+ terminal on the alternator. Battery voltage must be present. If not, check the 2A
circuit.
7. Disconnect the main wire harness from the voltage regulator. On Thermo King alternators, carefully push on the spring clip
to release the plug lock.
8. Use the Mechanics/Premium HMI Control Panel to turn the unit on and enter Non-Running Service Test Mode. Place the unit
in High Speed Cool. Refer to the appropriate Microprocessor Diagnostic Manual for information about the Service Test
Mode.
9. Check the voltage at the sense circuit (SENSE circuit). Battery voltage should be present. If not, check the sense circuit
(SENSE circuit) in the main wire harness.
10. Check the voltage at the excitation circuit (EXC circuit). 10 Vdc or more should be present. If not, check the excitation circuit
(EXC circuit) in the main wire harness.
11. Turn the unit off and reconnect the main wire harness to the voltage regulator.
12. Attach a clamp-on ammeter around the 2A wire connected to the B+ terminal on the alternator. All wires connected to the B+
terminal must pass through the clamp-on ammeter.
13. Connect a digital multi-meter between the B+ terminal at the alternator and chassis ground.
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 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.
Temperature Anticipated Regulator Voltage
-40 C (-40 F) From 15.2 Vdc to 14.0 Vdc
25 C (77 F) From 14.4 Vdc to 13.6 Vdc
80 C (176 F) From 14.2 Vdc to 13.2 Vdc
Electrical Maintenance 8-4