GENERAL Assist Technical Reference
Thermistor Testing
Power-off resistance testing and power-on voltage testing
Use the MOBILE TECHNICIAN app to cross-check thermistor resistance and voltage values against temperature.
Download MOBILE TECHNICIAN from the Apple App Store or Google Play Store. The Thermistor Value Conversion function can be used to compare temperature, resistance and return-voltage values.
Safety: Isolate power before disconnecting thermistors or taking resistance measurements. Power-on voltage checks must only be completed by appropriately qualified personnel using safe electrical testing practices.
1. Thermistor testing with power OFF
With power isolated, the thermistor can be disconnected and its resistance measured directly. Thermistor resistance is measured in kΩ and must be compared with the correct thermistor data for the temperature being sensed.
| 1 | Isolate power to the air-conditioning system. |
| 2 | Unplug the thermistor from the PCB. |
| 3 | Measure resistance across the thermistor terminals using a multimeter. Record the reading in kΩ. |
| 4 | Measure or estimate the actual temperature at the thermistor bulb. |
| 5 | Use the relevant thermistor resistance data or MOBILE TECHNICIAN to confirm the resistance is reasonable for the measured temperature. |
| 6 | Inspect the thermistor bulb, wiring and connector for physical damage, corrosion, crushed cable or insulation damage. |
Important: A thermistor cable shorted to earth may damage the Main PCB or Controller PCB. Do not reconnect a visibly damaged thermistor until the cause has been rectified.
2. Thermistor testing with power ON
With the system energised, the thermistor circuit can be checked by measuring the DC supply voltage and the thermistor return voltage. The reference supply is approximately 5 V DC.
| Check | Expected result |
|---|---|
| Thermistor supply | Measure from DC ground to one side of the thermistor connector. Expected supply is approximately 5 V DC. |
| Thermistor return | Measure from DC ground to the other side of the thermistor. The return voltage should be below 5 V DC. |
| Temperature conversion | Compare the measured return voltage with the relevant thermistor voltage graph or MOBILE TECHNICIAN conversion value. |
Ground reference tip: If the fan motor is DC, the black fan-motor wire may be used as a common DC ground point. The black wire of the pressure transducer may also be used where available.
3. Reference values from training material
The following examples are taken from the supplied thermistor training material. Always use the correct service data for the specific model and thermistor being tested.
| Thermistor | 0°C | 20°C | 30°C |
|---|---|---|---|
| Pipe temperature thermistor | 176.0 kΩ 1.1 V | 62.9 kΩ 2.2 V | 39.6 kΩ 2.8 V |
| Room temperature thermistor | 33.6 kΩ 1.1 V | 12.5 kΩ 2.2 V | 8.0 kΩ 2.8 V |
Example MOBILE TECHNICIAN values shown in the training material:
Room temperature: 22.9°C / 11.000 kΩ / 2.381 V
Outdoor temperature: 21.8°C / 11.622 kΩ / 3.836 V
Room temperature: 22.9°C / 11.000 kΩ / 2.381 V
Outdoor temperature: 21.8°C / 11.622 kΩ / 3.836 V
4. Diagnostic interpretation
| Finding | What to investigate |
|---|---|
| Resistance does not match temperature | Incorrect thermistor, failed thermistor, poor thermal contact, damaged cable or inaccurate temperature comparison. |
| Open circuit / extremely high resistance | Broken thermistor element, disconnected plug, broken conductor or poor terminal connection. |
| Very low resistance | Shorted thermistor, damaged cable or thermistor exposed to a much higher temperature than expected. |
| No 5 V DC supply | PCB supply circuit, connector, wiring or possible PCB damage. |
| Return voltage does not correspond with resistance | Connector contact, wiring, PCB input circuit or incorrect ground reference. |
Do not condemn the PCB from a thermistor fault code alone. Confirm the thermistor resistance, cable condition, 5 V DC supply and return voltage before determining whether the thermistor or PCB is faulty.