Remember, for most sensors, resistance goes up as temperature goes down. A
temperature increase results in a resistance decrease. If the ohmmeter shows a
direct short (zero ohms), or an open circuit (infinite ohms), check the sensor wiring
as well as the sensor itself.
If the sensor resistance goes up with temperature increases, it is not a "standard"
sensor. If all system sensors are the same type, are matched to the control, and the
sensors and control are operating properly, there is no need to replace them. If one
sensor or the control stops working properly, the entire control system should be
replaced, rather than mixing control and sensor types.
Check sensor wiring for staples shorting out the wires, loose or corroded
connections, and outright breaks. Ohmeter readings will indicate these conditions.
Make sure sensor wiring is more than one foot away from 120V line voltage wiring
and motors, and more than two feet from 240V wiring and motors. It is very easy for
a tiny electrical current to be induced in the low voltage sensor wires. This current
will make normal control operation impossible.
Controls With Digital Displays
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.
4.3 TROUBLESHOOTING OPERATIONS