Electrical Maintenance (Rev 1/99)
34
Electric Standby Operation Test
1. Start the unit on standby operation and set the thermo-
stat at least 6.3 F (3.5 C) below the thermometer read-
ing. The Cool Indicator and the Running Indicator
should both be on, the Heat Indicator should be off, the
electric relays should be energized, the heat contactor
should be de-energized, and the unit should be running
in cool.
2. Slowly turn the thermostat dial up to raise the thermo-
stat setpoint about 2 F (1 C) every 5 seconds. When
the thermostat setpoint is approximately equal to the
thermometer reading, the Cool Indicator and the Run-
ning Indicator should both go off, the electric relays
should de-energize, and the unit should shift to null.
3. Continue to slowly turn the thermostat dial up to raise
the thermostat setpoint about 2 F (1 C) every 5 sec-
onds. When the thermostat setpoint is approximately
3.6 F (2 C) above the thermometer reading, the Heat
Indicator and the Running Indicator should both come
on, the heat contactor should energize, and the unit
should shift to heat (optional).
4. Slowly turn the thermostat dial down to lower the ther-
mostat setpoint about 2 F (1 C) every 5 seconds. When
the thermostat setpoint is equal to the thermometer
reading, the Heat Indicator and the Running Indicator
should both go off, the heat contactor should de-ener-
gize, and the unit should shift to null.
5. Continue to slowly turn the thermostat dial down to
lower the thermostat setpoint about 2 F (1 C) every 5
seconds. When the thermostat setpoint is approximately
3.6 F (2 C) below the thermometer reading, the Cool
Indictor and the Running Indicator should both come
on, the electric relays should energize, and the unit
should shift to cool.
If the indicator lights do not come on and go off properly,
and if the unit does not shift operating modes properly, the
cab control box is probably defective.
If the indicator lights come on and go off properly, but the
unit does not shift operating modes properly, make sure that
the electric relays, the heat contactor, the associated wiring
and the wire connections are not defective before assuming
that the cab control box is defective. Specifically:
1. The thermostat energizes the electric relays by energiz-
ing pin 1 in the connector on the back of the cab control
box.
2. The thermostat energizes the heat contactor by ground-
ing pin 10 in the connector on the back of the cab con-
trol box.
Therefore, the cab control box is not defective if it energizes
the pin 1 and grounds pin 10 properly.
DEFROST SYSTEM
Engine Operation
A defrost cycle can be initiated by pressing the manual
defrost key or by the defrost timer when the defrost termina-
tion switch is closed. Starting a defrost cycle energizes the
defrost relay which energizes the hot gas solenoid and de-
energizes the fan relays. Energizing the hot gas solenoid
diverts hot gas into the evaporator coil to melt the frost and
ice. De-energizing the fan relay stops the evaporator and
condenser fans.
The defrost termination switch de-energizes the defrost
relay when the evaporator temperature rises above 52 F
(11.1 C).
To check the defrost cycle, run the unit in cool to drop the
evaporator coil to a temperature below 42 F (5.6 C). Press
the manual defrost key. The unit should shift from the cool
to defrost. If the unit continues to cool, double check the
evaporator coil temperature, and refer to Testing the Defrost
System.
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