Electrical Maintenance (Rev 1/99)
33
Faulty Cab Control Box
Before replacing the cab control box make sure that another
problem, such as a loose wire connection or a bad ground, is
not causing the cab control box to malfunction.
Thermometer Calibration Test
1. Place the sensor in an ice-water bath for a few minutes
and allow the sensor temperature to stabilize.
2. Check the thermometer reading. It should be 32 ± 2 F
(0 ± 1 C).
3. If the thermometer reading is out of calibration, replace
the sensor and repeat the test.
4. If the thermometer reading is still out of calibration, the
cab control box is defective.
Thermostat Switch Sequence Test
This test should be performed during scheduled preventive
maintenance operations. Make sure that the thermometer is
calibrated before performing this test.
NOTE: Press the Setpoint key to display the thermostat
setpoint.
Engine Operation Test
1. Start the unit on engine operation and set the thermostat
at least 6.3 F (3.5 C) below the thermometer reading.
The Cool Indicator and the Running Indicator should
be on, the Heat Indicator should be off, the power relay
should be energized, the heat relay should be de-ener-
gized, 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 ther-
mometer reading, the Cool Indicator and the Running
Indicator should both go off, the power relay 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
seconds. When the thermostat setpoint is approxi-
mately 3.6 F (2 C) above the thermometer reading, the
Heat Indicator and the Running Indicator should both
come on, the heat relay should energize, and the unit
should shift to heat.
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 relay should de-energize,
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
Indicator and the Running Indicator should both come
on, the power relay 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 power relay, the heat relay, 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 power relay by energizing
pin 6 in the connector on the back of the cab control
box.
2. The thermostat energizes the heat relay by grounding
pin 10 in the connector on the back of the cab control
box.
Therefore, the cab control box is not defective if it energizes
pin 6 and grounds pin 10 properly.
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