General Assist Technical Reference
Multi Head Systems Not Cooling
AOTG36LBTA4 / AOTG30LBTA4 EEV control and outdoor PCB diagnosis
Field diagnostic guidance for uneven cooling, non-cooling indoor units and flooded indoor coils
Some multi-head systems may present with one or more indoor units not cooling while other indoor coils are overfed or flooded. Where refrigerant charge, piping, wiring and EEV coils test correctly, the outdoor control PCB and its individual EEV driver outputs should be considered.
Qualified refrigeration technicians only: Isolate power before disconnecting EEV plugs, wiring harnesses or PCB connections. Do not condemn an outdoor PCB based on sound alone. Complete electrical, mechanical and refrigerant checks first.
1. Reported symptoms
- One or more indoor units do not cool.
- Other indoor units appear overfed, with unusually cold or flooded coils.
- Cooling distribution is inconsistent across connected indoor units.
- System pressures or pipe temperatures vary significantly between branches.
- One EEV sounds normal during initialisation while other valves sound faint, irregular or silent.
- Refrigerant charge and EEV coil resistance may test within specification.
2. Preliminary checks
Before suspecting the outdoor control PCB, verify the following:
| Check | Required verification |
|---|---|
| Refrigerant charge | Confirm total charge by recovery and weighing where necessary. Compare against the model installation and service data. |
| Piping arrangement | Confirm each indoor unit is connected to the correct refrigerant branch and within permitted pipe-length limits. |
| Indoor/outdoor wiring | Check that each indoor circuit is matched to the corresponding outdoor connection and that no cables are crossed. |
| EEV coil and harness | Measure each coil against the service-manual specification and inspect plugs, pins and harnesses for damage or poor contact. |
| EEV body | Confirm there is no mechanical restriction, contamination or seized valve body. |
Important: Do not replace all EEV bodies or disturb the refrigerant charge until piping, wiring, EEV resistance and valve initialisation behaviour have been compared across all circuits.
3. EEV initialisation sound check
- Switch the system off and allow it to stop fully.
- Position yourself where all outdoor EEVs can be heard safely.
- Restart or re-energise the system in accordance with the service procedure.
- Listen to each EEV during the initialisation sequence.
- Compare the sound and duration of each valve against the others.
| Observation | Possible indication |
|---|---|
| All valves sound similar and initialise consistently | EEV drive operation appears consistent. Continue with refrigerant, airflow and branch diagnostics. |
| One or more valves are faint or silent | Possible harness, connector, EEV coil or PCB driver-output fault. |
| Sound varies randomly between starts | Possible intermittent connection, damaged harness or degrading PCB driver circuit. |
4. Outdoor PCB diagnosis
Where the refrigerant circuit, EEV bodies, coils and harnesses have been verified, compare the outdoor PCB outputs controlling each EEV.
- Confirm every EEV plug is connected to the correct PCB socket.
- Inspect the main PCB for corrosion, heat damage, cracked solder joints or damaged connector pins.
- Compare the drive signal or stepper output between channels using the procedure and test equipment specified in the service manual.
- Do not use a standard DC-voltage reading alone to assess a stepper-motor output. EEV drive signals are pulsed and should be checked using the approved diagnostic method.
- Where one channel is noticeably weaker or inconsistent while the EEV coil and harness are known good, suspect the corresponding PCB driver circuit.
Field finding: In one AOTG30LBTA4 case, valve A produced a normal initialisation sound while valves B, C and D sounded faint. Refrigerant charge, piping, wiring and all four EEV assemblies were checked, but the fault remained. Replacing the outdoor control PCB restored consistent valve initialisation and normal cooling across all heads.
5. Corrective action
- Record the model, serial number, PCB part number and symptoms before replacement.
- Isolate the outdoor unit and confirm stored electrical energy has discharged.
- Transfer all connectors one at a time to the replacement board.
- Match all DIP switches, rotary switches, jumpers and addressing settings to the original board.
- Confirm the replacement PCB part number and revision are approved for the unit.
- Restore power and allow the EEV initialisation sequence to complete.
- Confirm all valves now sound consistent.
- Run all connected indoor units in cooling and verify even coil temperatures and stable operation.
6. Recommended diagnostic sequence
| Step | Action | Expected result |
|---|---|---|
| 1 | Compare indoor-unit cooling performance | Identify underfeeding and overfeeding branches |
| 2 | Verify piping and wiring allocation | Each indoor unit matches the correct outdoor circuit |
| 3 | Check EEV coil resistance and harness continuity | All readings comply with model service data |
| 4 | Listen during EEV initialisation | All valves initialise with similar sound and duration |
| 5 | Compare PCB EEV driver channels | Consistent drive output on all channels |
| 6 | Replace and configure PCB if confirmed faulty | Even cooling across all connected indoor units |
7. Key field advice
Listen before replacing: Comparing all EEVs during initialisation is a quick and valuable early test. Uneven sound does not prove the PCB is faulty, but it can identify which circuit requires further electrical testing before refrigerant is recovered or valve bodies are replaced.
Technical content should be used with the applicable installation manual, service manual and wiring diagram for the exact model and serial number.