Description
The pressure transducer sensor is a critical input component used to monitor the refrigerant system pressure in both heating and cooling modes. It converts the mechanical refrigerant pressure into a proportional DC voltage signal that the outdoor control PCB interprets to optimize compressor frequency, expansion valve operation, and system safety.
This procedure ensures the transducer’s voltage response and internal circuit integrity meet manufacturer specifications.
1. Function Overview
The sensor operates using a 5V DC reference supplied from the outdoor control board.
It provides a variable voltage output (0.5V–4.5V DC) proportional to system pressure.
The transducer consists of a semiconductor diaphragm with a strain gauge that changes resistance under pressure.
| Terminal Colour | Function |
|---|---|
| Red | +5V DC Supply (from PCB) |
| Black | Ground (0V) |
| White | Signal Return (Pressure Output) |
2. Power-On Test (Live Voltage Test)
Objective:
Confirm correct supply voltage and pressure signal output.
Procedure:
Power on the outdoor unit and allow the system to operate for a few minutes.
Set multimeter to DC voltage mode.
Place multimeter probes as follows:
Red lead on Red wire
Black lead on Black wire
Confirm 5.0 V DC supply.
Then measure the signal output voltage:
Red lead on White wire
Black lead on Black wire
Note the reading (should vary with pressure).
Compare the voltage against the corresponding pressure value using the chart or M Technician App.
Example:
| Measured Voltage (V) | Approx. Pressure (kPa) |
|---|---|
| 0.5 | 0 (Vacuum / Low pressure) |
| 1.0 | 700 |
| 1.5 | 1400 |
| 2.0 | 2100 |
| 2.5 | 2700 |
| 2.8 | 2875 |
| 3.0 | 3000 |
| 4.0 | 4200 |
| 4.5 | 5000 (High Pressure Cut) |
Example Reading:
Measured 2.8V DC = approximately 2875 kPa, which is within normal discharge operating range.
Tip:
Use the black ground wire of the transducer as the reference point for all voltage measurements to ensure accuracy.
3. Power-Off Test (Diode / Resistance Check)
Objective:
Verify sensor circuit integrity and rule out short or open circuit conditions.
Procedure:
Turn off system power and ensure capacitors are fully discharged.
Unplug the transducer connector from the PCB.
Set the multimeter to diode mode.
Measure between the following pin combinations:
| Multimeter Leads | Expected Reading (V) |
|---|---|
| Black → White | 1.0V – 1.1V |
| Black → Red | 1.2V – 1.3V |
| White → Red | 1.6V – 1.7V |
| Red → Black | 0.6V – 0.7V |
| White → Black | 0.5V – 0.6V |
| Red → White | 0.6V – 0.7V |
| Earth → Any lead | O/L (Open Circuit) |
Interpretation:
All readings within range → Sensor OK.
O/L between signal and supply → Open circuit fault.
Very low readings (<0.3V) → Internal short or moisture intrusion.
Continuity to earth → Ground fault; replace sensor.
4. Fault Diagnosis and Corrective Action
| Symptom | Possible Cause | Service Action |
|---|---|---|
| No voltage output | Open circuit or disconnected plug | Check harness continuity |
| Fixed voltage (e.g., 0.5V or 5V) | Sensor internal failure | Replace sensor |
| Unstable or fluctuating voltage | Loose pin contact / moisture | Clean, dry, and reseat connector |
| Incorrect voltage-pressure relationship | Calibration drift / sensor wear | Replace pressure sensor |
| Error code (Discharge / Suction Sensor Error) | Short or open detected by PCB | Verify with diode test and replace as required |
5. Service Notes
Normal operating range: 0.5V–4.5V DC.
Voltage <0.5V indicates vacuum / no pressure, >4.5V may indicate overpressure condition.
Always test under stable running conditions — transient readings during start-up or defrost are not valid.
Avoid megger testing the transducer — it can damage the sensing element.
6. Example Reference
Live Reading:
Voltage (White–Black): 2.8V DC
Calculated Pressure: ≈2875 kPa
Diode Check (Unplugged):
Black → White: 1.05V
Black → Red: 1.27V
White → Red: 1.67V
→ Sensor condition: Normal
7. Summary
Supply voltage: 5.0 V DC between Red and Black.
Signal output: 0.5 – 4.5 V DC proportional to refrigerant pressure.
Diode test: Confirms sensor integrity when unplugged.
Always compare voltage to pressure using the M Technician App for accurate diagnosis.
Example: 2.8V DC return corresponds to approximately 2875 kPa.
Power Off Test:
Measure voltage resistance as per table below
