Thermo King UT-1200, Manual

Page: 84 / 169

Electrical Maintenance
84
NOTE: The cylinder head cover must be
removed to access the glow plugs.
A shorted glow plug will show excessive current
draw (more than 13 amps) during preheat, and
may cause fuse F5 (40 A) to blow. Check each
glow plug individually.
Unit Wiring
Inspect the unit wiring and the wire harnesses
during scheduled maintenance inspections for
loose, chafed or broken wires to protect against
unit malfunctions due to open or short circuits.
AC Components
Electrical Contactors
Test the contact points by checking the voltage
drop across each set of points when the contactor
is energized and the system is operating. If the
voltage drop across a set of points is more than
0.25 Vac, replace the contactor.
Test the contactor coil as follows:
1. Check the voltage to the coil. It should be at
battery (source) voltage. If not, check for an
open circuit.
2. Check the voltage after the coil. It should be 0
volts. If not, check for an open or high
resistance in the circuit to ground. If the
voltage is 0 but the contactor does not pull-in,
the coil is probably open. Ohm check to
verify.
Condenser Fan Rotation
The condenser fan is belt driven. On electric
standby operation, check for correct fan rotation
by placing a small cloth or sheet of paper against
the condenser fan grille on the front of the unit.
Correct rotation will hold the cloth or paper
against the grille. Improper rotation will blow the
cloth or paper away from the grille. To correct
incorrect rotation, check the motor and motor
contactor wiring per the unit wiring diagram.
Phase Select Module for Truck Unit
Model 50 Applications
The phase select module is designed to monitor
both single and 3 phase nominal input voltages
between 200 and 480 volts AC.
Operation
The phase select module will detect missing
phases, phase orientation and low voltage on three
phase or single phase power. The Brown, Gray,
and Black wires are used to sample the power at
L1, L2 and L3 respectively. Operating power from
12 to 24 volts is supplied to the module via the 8
wire and CH wire.
When the voltage sensed rises above 165 volts
AC and all three phases are present, the module
ER wire will output 12 volts DC after the
conditions exist for 2 to 4 seconds. This signal
informs the microprocessor that electric standby
operation is possible. If the voltage drops below
160 volts AC or a phase is lost, the ER output is
turned off after the condition exists for 3 seconds.
The module continues to monitor the power and
the module ER wire will again output 12 volts DC
within 2 to 4 seconds after the voltage rises above
165 volts AC and all three phases are present.
CAUTION: Model 50 units use high
voltage ac for electric standby operation.
Lethal voltage potentials can exist on
connections in the high voltage box. Take
appropriate precautions and use extreme
care when testing the unit.
1. MCA–Motor Contactor
2. MCB–Motor Contactor
4. Overload Relay
5. Phase Select Module
Figure 152: High Voltage Box with Cover Removed
ARA1941
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