Thermo King 914 724, Manual

Page: 46 / 111

Electrical Maintenance (Rev. 1/99)
36
NOTE: Press the Setpoint switch to display the thermostat
setpoint.
1. Start the unit and set the thermostat setpoint 8 to 10 F
(4.4 to 5.6 C) below the thermometer reading. The cool
relay should be energized, the heat relay should be de-
energized, and the unit should operate in cool.
2. Slowly turn the thermostat dial up to raise the thermo-
stat setpoint about 2 F (1.1 C) every 5 seconds. When
the thermostat setpoint reaches the thermometer read-
ing ± 1 F (± 0.6 C), the unit should shift to null. The
cool relay and the heat relay should both be de-ener-
gized.
3. Continue to slowly turn the thermostat dial up and raise
the thermostat setpoint about 20 F (1.1 C) every 5
seconds. When the thermostat setpoint reaches 2 ± 1 F
(1.1 ± 0.6 C) above the thermometer reading, the unit
should shift to heat (optional). The heat relay should be
energized and the cool relay should be de-energized.
4. Slowly turn the thermostat dial down to lower the ther-
mostat setpoint about 20 F (1.1 C) every 5 seconds.
When the thermostat setpoint reaches the thermometer
reading ± 1.0 F (± 0.6 C), the unit should shift to null.
The cool relay and the heat relay should both be
de-energized.
5. Continue to slowly turn the thermostat dial down and
lower the thermostat setpoint about 2 F (1.1 C) every
5 seconds. When the thermostat setpoint reaches 2 ±
1 F (1.1 ± 0.6 C) below the thermometer reading, the
unit should shift to cool. The cool relay should be
energized and the heat relay should be de-energized.
If the unit does not shift operating modes properly, make
sure that the cool relay, the heat relay, the associated wiring,
and the wire connections are not defective before assuming
that the TCM is faulty. Specifically:
1. Battery voltage must be present on wire CR1 (pin 5)
and wire HR1 (pin 7) in the 14 pin connector on the cab
control box wire harness when the unit is turned on.
2. The thermostat energizes the cool relay, which shifts
the unit to cool, by closing internal contacts that con-
nect the CR1 circuit to the CR circuit.
3. The thermostat energizes the heat relay, which shifts
the unit to heat (optional), by closing internal contacts
that connect the HR1 circuit to the 11R circuit.
Therefore, the TCM is not defective if it energizes the CR
and HR circuits properly.
Thermostat Calibration Procedure
1. Set the thermostat at 32 F (0 C).
2. Start the unit and let it run until it shifts from cool to
null.
3. Use a known good thermometer (or a 32 F [0 C] ice-
water bath) to check the switch points. The unit should
shift from cool to null at 32 F (0 C).
4. If the units shifts modes properly and is out of calibra-
tion by less than 5 F (2.7 C), recalibrate the thermostat
by loosening the set screw and repositioning the ther-
mostat dial.
If the thermostat shifts modes erratically or is out of calibra-
tion by more than 5 F (2.7 C), at least one of the thermostat
components is faulty. Test the thermostat components and
make the needed repairs before calibrating the thermostat.
DEFROST SYSTEM
Engine Operation
A defrost cycle can be initiated by pressing the manual
defrost switch or by the defrost timer when the defrost ter-
mination switch is closed. Starting a defrost cycle energizes
the defrost relay and the 26 circuit, and de-energizes the fan
relay, which stops the evaporator and condenser fans.
Energizing the 26 circuit diverts hot gas into the evaporator
coil, melting the ice. The defrost termination switch de-
energizes the defrost relay when the evaporator temperature
rises above 52 F (11.1 C).