Thermo King 914 724, Manual

Page: 40 / 111

Electrical Maintenance (Rev. 1/99)
30
Check Sensor and Sensor Circuits
1. Turn the unit ON (both On-Off switches must be turned
ON).
2. Check the power to the thermometer (see Check
Power).
3. Disconnect the thermometer sensor harness from the
thermometer sensor wires (3 pin connector).
4. Check the voltage between the positive SN3 circuit (red
wire - pin C) and the negative SN1 circuit (black wire -
pin A) in the 3 pin connector on the thermometer sensor
wires. The voltage should be 5.0 ± 0.5 volts.
5. If the voltage is not correct, the thermometer is defec-
tive and must be replaced.
6. If the voltage is correct, check the wiring and connec-
tors in the SN1, SN2, and SN3 circuits to the sensor.
If there are no faults in the SN1, SN2, and SN3 circuits,
reconnect all sensor circuit connectors and recheck the
thermometer display. The black, white and red wires in
the thermometer lead must match the black, white, and
red wires in the sensor lead.
If the thermometer display has not changed, attach a
new sensor and check the thermometer display.
a. If the thermometer display is now normal the old
sensor is defective.
b. If the thermometer display is not normal, the ther-
mometer is defective and must be replaced.
Faulty Thermometer
Before replacing the thermometer, make sure that another
problem, such as a loose wire connection or a bad ground, is
not causing the thermometer 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. Stir the
ice-water bath occasionally to keep the temperature sta-
ble.
2. Check the thermometer reading. It should be 32 ± 1.8 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
thermometer is defective.
Testing the Thermostat
Check Power
1. Make sure the master On-Off switch is ON.
2. Open the remote control box cover.
3. Turn the remote control box On-Off switch ON. The
thermostat’s On LED should light up.
4. Check the voltage between the 8X circuit (T12 termi-
nal) and the CH circuit (T13 terminal) on the front of
the thermostat. Battery voltage (12 or 24 volts) should
be present.
5. If battery voltage is not present, check:
a. The wiring, connectors, and components in the
power circuit (8X, 8, 2B, 2A, and 2) to the battery).
b. The wiring, connectors, and ground connections in
the ground circuit (CH) to the battery.
6. If battery voltage is present and the thermostat’s On
LED is not lit, the thermostat is defective.
Check the Sensor Output
1. Turn the unit ON (both On-Off switches must be turned
ON).