Maintenance and Troubleshooting | 6-9
Shorted/failed thermocouple
Check that the thermocouple leads are not shorted to chassis ground. To
do this, use an ohm meter to measure between terminal C on the sensor
board and chassis ground, and between terminal D on the sensor board
and chassis ground. If shorted, replace the thermocouple.
To check the thermocouple you will need a digital volt meter and a Type K
test meter.
First display the thermocouple millivolts on the front panel display.
The display should measure a positive voltage.
If the display measures a negative voltage, reverse the wires leading to
pins C and D on the sensor board.
To check the continuity of the thermocouple, measure resistance
across pins C and D on the sensor board. The resistance should be less
than 5 ohms.
If this doesn’t solve the problem, remove the thermocouple from the
sensor and connect a Type K test meter.
- Using a thermometer, check the temperature of an environment.
Place the thermocouple in this same environment and check that
the reading of the test meter measures the same temperature as
the thermometer. If the thermocouple measures the temperature
incorrectly (greater than 1% of expected temperature), replace the
thermocouple.
Reversed thermocouple wires
To check if the thermocouple wires are reversed, use the controller to
display the thermocouple millivolts so that they appear on the front
panel display. If the millivolts are negative, the thermocouple wires are
reversed.
Thermocouple compensation circuit check
Using a volt meter, measure across terminals 4 (-) and 12 (+) on the
sensor board. If approximately 15 volts is not present, check that inter-
connecting wiring is correct.
If interconnecting wiring is correct, check across 15V Sup and 15V
Com on the control unit for approximately 15 volts.
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