Overview
The System Signal Test verifies serial communication between indoor and outdoor units in a split or multi-system configuration. This diagnostic ensures that both units are correctly exchanging control signals through the communication line.
Signal testing can be performed from either the indoor or outdoor terminal block, allowing technicians to isolate faults in the communication wiring or unit control boards.
Safety Warning
⚠️ Live DC and AC Circuits
Always use insulated probes and avoid shorting terminals.
Testing is performed with power ON—take necessary precautions.
Testing Conditions
Ensure power supply is active and both units are energized.
Use a digital multimeter set to DC voltage.
Identify the following terminals on both indoor and outdoor PCBs:
Terminal 1 – L (Live)
Terminal 2 – N (Neutral)
Terminal 3 – Signal
Procedure
1. Indoor Unit Test
Locate the indoor terminal block.
Disconnect the signal wire (Terminal 3) leading to the outdoor unit.
Measure voltage between Terminal 2 (Neutral) and the disconnected signal wire.
This measures the outdoor unit signal returning to the indoor connection.
Expected DC Voltage Range:
Depending on model type, expected readings vary (see table below).
| Model Type | Indoor Signal |
|---|---|
| AST & ART/C | 60 – 120V DC |
| ASTA & ASTB | 40 – 90V DC |
| ASTG | Approx. 200V DC (Static) |
| ARTG & 3 Phase | Approx. 295V DC (Static) |
| ARTH | 50 – 250V DC |
2. Outdoor Unit Test
Move to the outdoor unit’s terminal block.
Disconnect the same signal cable (Terminal 3) connected to the indoor unit.
Measure voltage between Terminal 2 (Neutral) and the disconnected signal wire.
This tests for the indoor unit signal reaching the outdoor terminal.
Expected DC Voltage Range:
| Model Type | Outdoor Signal |
|---|---|
| AOTA | 60 – 120V DC |
| AOTG | 3 – 110V DC |
| AOTH | 3 – 110V DC |
3. Cross-Check Communication
If the indoor test shows normal voltage but outdoor test does not (or vice versa), the interconnecting signal cable may be damaged or open circuit.
If no signal is detected on both ends, suspect a fault on the indoor or outdoor control PCB.
Diagnosis and Interpretation
| Observation | Diagnosis | Recommended Action |
|---|---|---|
| Signal voltage within range on both units | Communication normal | No fault detected |
| Signal on one side only | Broken or disconnected interconnect cable | Check and repair wiring |
| No signal from either unit | PCB fault (indoor or outdoor) | Replace control board |
| Unstable or fluctuating signal | Poor connection or noise interference | Check terminations and grounding |
Technical Notes
The communication line (Terminal 3) carries a DC serial data signal used for system control between indoor and outdoor units.
Typical voltage is measured as an average of pulsed DC—values may fluctuate during operation.
Always ensure terminal 2 (Neutral) is used as the reference when measuring.
For multi-split systems, test each indoor-to-outdoor pair individually.
