Thermo King MD-MT, Operator'S Manual

Page: 63 / 192

Electrical Maintenance
63
Unloading Timer (EUT)
The unloading timer energizes the unloading relay
(EUR) for approximately 20 seconds when the
engine or the electric motor is first started to
reduce the load. The unloading relay opens the
7K-7KU circuit, which supplies power to the
EXC terminal on the alternator.
This de-energizes the field coil, which keeps the
alternator from charging and allows the alternator
to turn freely. Disabling the alternator also
prevents the INDL circuit from energizing the fan
enable relays (FER and RFER). This keeps the
evaporator fan motors de-energized to reduce the
current draw on the battery while the alternator is
not charging. If the EUT is defective, it may be
difficult to start the engine or the electric motor.
Testing the Unloading Timer
1. Remove the CYCLE-SENTRY module from
the option board to prevent the Auto Start
system from preheating and cranking the
engine.
2. Disconnect the 20 wire from the engine reset
switch to prevent the engine reset switch from
tripping.
3. Place an On/Off switch in the Off position,
place the CYCLE-SENTRY switch in the
Cont Run position, and place the
Diesel/Electric switch in the Diesel position.
4. Disconnect the unloading timer wires (BLU
and ORG) from the main wire harness (ER
and 7KB) at the two pin connector.
5. Check the resistance between the ER wire in
the main wire harness and ground. If the
resistance is more than 90 ohms, check the ER
wire to the unloading relay, the unloading
relay coil and the CH circuit for bad
connections or opens.
6. Place the On/Off switches in the On position
and press the
O
N
key.
7. Check for battery voltage at the 7KB wire in
the main wire harness. If battery voltage is not
present or if it is low, check the 7KB circuit
and the other circuits and components back to
the battery for bad connections or opens.
8. Press the
O
FF
key.
9. Connect the unloading timer wires to the main
wire harness at the two pin connector. Attach
one lead of a voltmeter to the ORG wire and
attach the other lead to the BLU wire.
10. Press the
O
N
key and check the voltage
between the ORG and BLU wires. No voltage
or very low voltage should be present when
the
O
N
key is first pressed. Approximately 20
seconds after the
O
N
key is pressed, battery
voltage should appear between the ORG and
BLU wires. If battery voltage is present when
the
O
N
key is first pressed, or if battery voltage
does not appear approximately 20 seconds
after the
O
N
key is pressed, the unloading
timer is defective and must be replaced.
11. Press the
O
FF
key, replace the
CYCLE-SENTRY module, and connect the
20 wire to the engine reset switch after
completing the test.
Control Circuit Fuse
The control circuit fuse protects the electrical
system if a circuit breaker fails to open. If the fuse
burns out, check for a grounded 2 wire or for any
of the conditions that would cause one of the
circuit breakers to open. Replace the defective
circuit breaker and replace the fuse. The fuse is
located between the battery and the unit control
circuits (typically stored inside the battery box).