Outdoor Communication Fault - Isolating PCB vs Connected Components
This article provides a structured method for diagnosing an outdoor communication fault and determining whether the cause is the communication circuit, controller PCB power supply, inverter stage, wiring harness, or a connected component pulling down one of the DC supply rails.
Possible Causes
- Signal wiring or terminal fault
- Controller PCB supply instability or repeated resets
- A connected component dragging down a 5 V, 12 V, 15 V or high-voltage DC rail
- Inverter / IPM fault affecting DC bus stability
1. Confirm the Communication Circuit First
Signal Voltage Test - Indoor to Outdoor
Measure between Terminal 2 (Neutral) and Terminal 3 (Signal).
Indoor Signal - Expected Values
| Platform | Expected Signal Voltage |
|---|---|
| AST & ART/C | 60-120 V |
| ASTA & ASTB | 40-90 V |
| ASTG | ~200 V static |
| ARTG & 3-phase | ~295 V static |
| ARTH | 50-250 V |
Outdoor Signal - Expected Values
| Platform | Expected Signal Voltage |
|---|---|
| AOTA | 60-120 V |
| AOTG | 3-110 V |
| AOTH | 3-110 V |
Indoor signal present but outdoor signal absent - investigate the outdoor communication circuit, outdoor PCB power stability, or a connected load pulling down a supply rail.
No signal at either end - check supply, wiring, polarity and terminal connections first.
2. Verify the Controller PCB Power Environment
Main DC Bus
Confirm a stable 330-370 V DC supply feeding the main PCB / inverter system.
An unstable DC bus may cause:
- Repeated PCB resets
- Intermittent communication
- Random or changing fault codes
Expected Supply Rails
| Circuit / Load | Expected Supply |
|---|---|
| Outdoor fan HV supply | 330-370 V DC Red-Black |
| Outdoor fan control supply | 15 V DC White-Black |
| Thermistors | 5 V DC |
| Pressure transducer | 5 V DC |
| EEV coil | 12 V DC |
| Reversing valve solenoid | 240 V AC when energised in heating |
3. Component Integrity Checks
3.1 Thermistors
Power ON:
- Reference DC ground, typically fan motor Black / DC negative.
- One side should show approximately 5 V DC supply.
- The return side should show a value below 5 V.
Power OFF:
- Check resistance is plausible and not open or short circuit.
- Check each sensor circuit for a short to earth.
3.2 EEV Coil - 12 V DC
5-wire EEV:
- Red to each other wire: 45-50 Ω
- Each wire to earth: O/L
6-wire EEV:
- Red to White / Orange: 45-50 Ω
- Brown to Yellow / Blue: 45-50 Ω
- All wires to earth: O/L
Also check for corrosion beneath the EEV coil and confirm the coil is correctly seated and clipped onto the valve body.
3.3 Pressure Transducer - 5 V DC
Power ON:
- Red to Black: 5 V DC supply
- Black to White: varying return voltage proportional to pressure
- Example reference: 2.8 V ≈ 2875 kPa
With power OFF, use the specified diode-test pattern for the applicable sensor and confirm there is no short to earth.
3.4 Fan Motors
DC BLDC fan - Power ON:
- Red-Black: 330-380 V DC
- White-Black: 15 V DC
DC BLDC fan - Typical diode test with power OFF:
| Meter Connection | Typical Reading |
|---|---|
| Red lead → Black lead | O/L |
| Red lead → White lead | 0.5-1.8 V |
| Black lead → Red lead | 0.8-1.1 V |
| Black lead → White lead | 0.4-0.6 V |
For an AC fan motor, check winding resistance for open/short conditions and verify insulation to earth is high.
3.5 Compressor - DC Inverter
Winding balance - Power OFF:
- U-V, V-W and U-W should be reasonably equal.
- Typical total winding resistance is approximately 0.3-2.0 Ω depending on unit and temperature.
Insulation to earth:
- Each winding to earth should be >1 MΩ.
An imbalanced or leaking compressor can destabilise the inverter stage and indirectly cause communication faults.
4. Power Electronics Checks - Power OFF
4.1 IPM / Inverter Module
| Test | Typical Reading |
|---|---|
| P to U / V / W | ~0.4 V |
| U / V / W to N | ~0.4 V |
| P to N | 0.7-0.9 V |
| N to P | O/L |
A failed IPM can destabilise the DC bus and cause communication dropouts or repeated PCB resets.
4.2 Diode Bridge / Rectifier
| Test | Typical Reading |
|---|---|
| + to ~ | ~0.4 V |
| ~ to - | ~0.4 V |
| + to - | 0.7-0.9 V |
| - to + | O/L |
5. Staged Reconnect Test
The staged reconnect method is one of the quickest ways to prove whether a connected component is pulling down the PCB supply or DC bus.
Step 1 - Start with High-Risk Loads Disconnected
With power isolated, disconnect:
- Compressor U/V/W
- Fan motor(s)
- EEV coil
Power the unit and confirm:
- DC bus remains stable at 330-370 V DC
- 5 V sensor rail behaves normally
- Thermistor and pressure-transducer readings are plausible
- Communication signal between Neutral and Signal is present and within the expected range
Step 2 - Reconnect One Circuit at a Time
- Thermistors and pressure transducer
- EEV coil
- Fan motor 1
- Fan motor 2
- Compressor - reconnect last
6. Quick Component Mapping
| Component | Key Checks |
|---|---|
| DC fan motor | 330-380 V DC HV supply, 15 V DC control supply, diode pattern, no hot-plugging |
| DC compressor | Balanced winding resistance, >1 MΩ to earth, then IPM diode test |
| EEV | 45-50 Ω coil legs and O/L to earth |
| Reversing valve solenoid | Confirm 240 V AC switching in heating and check coil for open/short condition |
Recommended Diagnostic Order
- Confirm incoming supply and communication wiring.
- Check indoor and outdoor signal voltages.
- Confirm the main DC bus is stable.
- Check 5 V, 12 V and 15 V rails.
- Test thermistors, pressure transducer and EEV.
- Test fan motors.
- Test compressor windings and insulation.
- Check IPM and rectifier diode patterns.
- Use the staged reconnect method to identify the circuit that causes communication or rail collapse.
- Only condemn the main PCB after the connected loads and DC supply environment have been proven.