To Test a Potentiometer (three pin connector)
Note: Manufacturers resistance readings will vary and must be checked for correct operation
- Electrically isolate boiler / system controls
- Remove potentiometer wires (at the p.c.b end) to prevent stray resistances from the p.c.b itself (Some potentiometers are built into the p.c.b and are sealed units, so not possible)
- Set multimeter on ohms scale for resistance readings
- Turn potentiometer to fully OFF position
- Connect multimeter leads to bottom connection and to wiper (middle) connection on three pin connector of potentiometer 1 ohm = ok
Switch to ON position but no more further 1 ohm = ok
Slowly turn potentiometer to FULL ON position where the resistance reading decreases proportionally to 0 ohms = ok
No variation when rotated = faulty
- Connect multimeter leads to top connection and to wiper (middle) connection on three pin connector of potentiometer
Turn potentiometer to fully OFF position 0 ohms = ok
Slowly turn potentiometer to FULL ON position where the resistance reading increases proportionally to 1 ohm = ok
Off position again should read 0 ohms = ok
To Test a Potentiometer (two pin connector)
- Electrically isolate boiler / system controls
- Remove potentiometer wires (at the p.c.b end) to prevent stray resistances from the p.c.b itself
(Some potentiometers are built into the p.c.b and are sealed units, so not possible)
- Set multimeter on continuity buzzer mode
- Turn potentiometer to fully OFF position
- Connect both multimeter leads to both connections on potentiometer
Turn potentiometer to ON position continuity = ok
No continuity = faulty
Alternative test (where manufacturers resistance readings are known – example):
Potentiometer off (fully ant-clockwise)
| Connections to test resistance | (K Ohms) |
|---|---|
| Top / bottom | 9.6 |
| Top / wiper | 0 |
| Bottom / wiper | 9.6 |
Potentiometer on (fully clockwise)
| Connections to test resistance | (K Ohms) |
|---|---|
| Top / bottom | 9.6 |
| Top / wiper | 9.6 |
| Bottom / wiper | 0 |
If the readings are comparable with the manufacturers readings, the potentiometer can be assumed as working. A non-functioning potentiometer will normally give no variation between connections when the shaft is rotated.
