Controls: Tester Method
A more accurate method of testing differential thermostats is with a special tester
designed for that purpose. (Figure 6-10) Normally, these units have four leads or
terminals. These are connected to the collector and storage sensor terminals of the
control. The tester supplies a calibrated resistance to the two sensor inputs.
The dials on most testers are not very accurate. If the control has a digital display,
use that instead of the numbers on the tester dial.
To check suspicious sensors, disconnect the wires from their terminal at the control.
Leave the connections intact at the sensor for the time being. Using an ohmmeter
and Table 3-3 or 3-4, determine if the resistance of the sensor is appropriate for the
temperature it should be measuring.
If the control has a digital display, leave the sensor wires connected and use the
control display to check the sensors. If the displayed temperatures do not appear
appropriate, disconnect the sensor wires from the control and use an ohmmeter as
Many digital displays indicate short or open circuits by flashing the digits on display.
Again, an ohmmeter should be used on disconnected sensor wires.
6.1 MAINTENANCE PROCEDURES