Outdoor communication fault — proven method to isolate PCB vs. connected components

Objective

A communication fault can be caused by:

  • Signal wiring/terminal issues
  • Controller PCB power instability (resets)
  • A connected component dragging down a DC rail (5 V / 12 V / 15 V or the HV DC bus)
  • Inverter/IPM failure affecting DC bus stability

The method below proves each section logically and safely.

1) Confirm the communication circuit first

1.1 Signal voltage test (Indoor ↔ Outdoor)

Measure between Terminal 2 (Neutral) and Terminal 3 (Signal).

Use these expected signal voltage ranges (platform-dependent):

Indoor signal expected:

  • 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:

  • AOTA: 60–120 V
  • AOTG: 3–110 V
  • AOTH: 3–110 V

Interpretation

  • Indoor signal present but outdoor absent → outdoor comm circuit / outdoor PCB supply stability / load pulling rails down.
  • Neither end shows signal → wiring/polarity/termination/supply issue first.

2) Verify controller PCB power environment (before condemning it)

2.1 Main DC power input to PCB

Confirm stable DC 330–370 V present as the DC power supply feeding the main PCB/inverter system.

Instability here often presents as:

  • Repeated resets
  • Intermittent comms
  • Random fault codes

2.2 Expected supply rails by load (critical for “PCB being dragged down”)

Use these expected rail values:

  • Outdoor fan HV supply (DC motors): 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 coil: 240 V AC (energised on heat mode)

3) Component integrity checks (power OFF unless specified)

3.1 Thermistors

Power ON (best for proving the PCB 5 V rail)

Reference DC ground (typically fan motor Black / DC negative).

  • One pin will show 5 V DC supply
  • The other pin will show a return voltage less than 5 V

Power OFF

  • Check resistance is plausible (not open circuit / not short)
  • Also check for short to earth (any sensor short to earth can damage or destabilise the controller)

3.2 EEV coil (DC)

5-wire EEV coil (12 V) — Resistance power OFF

  • Red to each other wire: 45–50 Ω
  • Each wire to earth: O/L

6-wire EEV coil (12 V) — Resistance power OFF

  • Red to White/Orange: 45–50 Ω
  • Brown to Yellow/Blue: 45–50 Ω
  • All wires to earth: O/L

Practical note: check for corrosion under the coil and ensure it is correctly seated/clipped.

3.3 Pressure transducer (5 V DC)

Power ON

  • Red to Black: 5 V DC supply
  • Black to White:varying return voltage used to calculate pressure
    • Rule of thumb example: 2.8 V ≈ 2875 kPa

Power OFF (diode test approach)

  • Use the specified diode patterns for the sensor (and confirm earth = O/L). Any short to earth or abnormal diode pattern points to sensor or harness fault.

3.4 Fan motors (AC or DC)

If DC BLDC fan (multi-wire: red/black/white + signals)

Key expected values (power ON):

  • Red–Black: 330–380 V DC
  • White–Black: 15 V DC

Power OFF diode test (typical pattern):

  • Meter Red lead → Black lead: O/L
  • Meter Red lead → White lead: 0.5–1.8 V
  • Meter Black lead → Red lead: 0.8–1.1 V
  • Meter Black lead → White lead: 0.4–0.6 V

Important: Do not disconnect or reconnect fan plugs with power applied.

If AC fan motor

  • Check winding resistance isn’t open/short
  • Insulation to earth should be very high (megger if permitted)

3.5 Compressor (DC inverter)

Winding balance (power OFF)

  • U–V, V–W, U–W should be equal-ish
  • Typical total winding resistance is ~0.3–2.0 Ω depending on unit and temperature (10–25°C)

Insulation to earth

  • Each winding to earth must be > 1 MΩ

Any imbalance or earth leakage can crash the inverter stage and indirectly cause comms faults.

4) Power electronics checks (power OFF)

4.1 IPM / Inverter module diode checks (typical expected readings)

  • 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 cause DC bus instability and comm dropouts.

4.2 Diode bridge (rectifier) diode checks

  • + to ~: 0.4 V
  • ~ to –: 0.4 V
  • + to –: 0.7–0.9 V
  • – to +: O/L

5) The staged reconnect test (fastest way to find the culprit)

This is the most reliable “all components must be checked” approach.

Step 1 — Start with high-risk loads unplugged

Power up with these disconnected:

  • Compressor (U/V/W)
  • Fan motors
  • EEV coil

Confirm:

  • DC bus stable 330–370 V
  • 5 V sensor rail behaves normally (thermistors + transducer plausible)
  • Signal voltage between N and Signal is present and within expected band

Step 2 — Reconnect in this order

  1. Thermistors + pressure transducer
  2. EEV coil
  3. Fan motor 1
  4. Fan motor 2
  5. Compressor (last)

If comms drop immediately after a particular reconnection, that circuit/load/harness is pulling down a rail or introducing instability.

6) Quick mapping to your component types

  • Fans: If multi-wire, treat as DC BLDC and confirm 330–380 V DC supply + 15 V DC control rail.
  • Compressor (DC): confirm balanced low-ohm windings and >1 MΩ insulation; then IPM diode test.
  • EEV (DC): confirm 45–50 Ω coil legs and O/L to earth.
  • Solenoid coil (AC): confirm correct 240 V AC switching in heat mode and that coil isn’t open/short.