Electrical Maintenance
79
To test the alternator under load, Thermo King
recommends the use of a clamp-on ammeter to
monitor output current, both on initial startup and
under full unit load conditions. For example, on
multi-temp units, all remote evaporators should be
turned on.
General Diagnostic and Warranty
Evaluation Procedure
Complete the following diagnostic procedures
before replacing an alternator or the voltage
regulator.
1. When testing an alternator, use accurate
equipment such as a Thermo King P/N
204-1079 digital multimeter and a Thermo
King P/N 204-947 amp clamp or an
equivalent.
2. Make sure the drive belts and pulleys of the
charging system are in good condition and are
adjusted properly before testing the alternator.
Be sure the pulleys are the correct size. Worn
belts, loose belts and worn or improperly sized
pulleys will lower the output of the alternator.
3. The battery must be charged and in good
condition, the battery cable connections must
be clean and tight, and the 2, 2A, 2Y (sense),
and EXC (excitation) circuits must be
connected properly. All charging circuit
connections must be clean and secure. If the
unit battery is questionable, a known good
jumper battery should be substituted for
alternator testing.
NOTE: If the unit battery is questionable, a
known good jumper battery should be
substituted for alternator testing.
NOTE: Do not perform this test with a
battery charger connected to the unit battery.
NOTE: All voltage readings should be taken
between the chassis ground on the alternator
and the terminals indicated, unless stated
otherwise.
4. Check that the resistor bypass fuse (F4) has
been removed. Units with Thermo King
alternators must have the resistor bypass fuse
(F4) removed.
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 2 and 2A
circuits, and fuse FS8.
7. Disconnect the alternator harness from the
voltage regulator. On Thermo King
alternators, carefully push on the spring clip to
release the plug lock.
8. Turn the unit on, enter the Output Test Mode,
and energize the Alternator Excite Output.
Refer to the appropriate Diagnostic Manual
for information about the Output Test Mode.
9. Check the voltage at the sense circuit (2Y
circuit). Battery voltage should be present. If
not, check the sense circuit (2Y circuit) in the
alternator harness and the main/unified
harness, fuse F20 on the base controller, and
the 2 circuit and fuse FS8 in the battery and
main/unified harnesses.
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 alternator harness and the
main harness.
NOTE: LED 26 lights up when the EXC
circuit is energized. The EXC circuit is
operated by a Smart FET so during normal
operation the unit must be running for the
EXC circuit to be energized.
11. Turn the unit off and reconnect the alternator
harness.
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.
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.
