System-4 tripping traced to one indoor PCB fault
Aircon case in Queenstown, Singapore: electrical/control traced to faulty indoor PCB in one bedroom unit drawing abnormal current and sending fault signal upstream, triggering outdoor protection trip after targeted diagnosis checks.
Case details
What client reported
The customer said the system had been shutting down two or three times a day for a week. It only happened when one specific bedroom unit was on. If that unit was left off, the other three rooms ran fine. That one unit also had a fault light that the others did not.
What we found
The customer's own observation about which unit caused the trips was the key clue — we went straight to isolating that indoor unit
- System run test with all four indoor units on — system tripped within two minutes of the suspect bedroom unit being switched on
- System run test with suspect bedroom unit disconnected from the control wiring — the other three zones ran for 20 minutes without shutdown
- Suspect indoor unit PCB inspected — visible burn marks on one component on the board, consistent with an electrical failure
- Current measured at the PCB supply during start command — reading was above the normal range for this model
The indoor PCB in the affected bedroom unit had a component failure. This caused it to draw too much current during operation. The abnormal current and corrupted control signal were sent to the outdoor unit. The outdoor unit read this as a protection event and shut down all zones. The outdoor unit, compressor, and the other three indoor units were all normal.
What we did
The faulty indoor PCB was replaced with a matching board for the Midea system-4. After fitting, all four indoor units were switched on together. The system ran through a full cycle without tripping. Current draw at the replaced unit was within the normal range for the model.
The system has run without fault since the PCB was replaced. The other three indoor units were not affected at any point. Their PCBs, refrigerant circuits, and motors were all normal. The fix was limited to one board in one indoor unit.
Timeline
Day 1
System tripping when one bedroom unit was switched on — fault light visible on that unit
Day 1
Isolated the suspect indoor unit by disconnecting it from the system — the other three zones ran stably. This confirmed the fault was in that unit's PCB.
Day 1
Faulty indoor PCB isolated and confirmed, replacement board fitted, all four zones running stably
What we learned
Why one faulty indoor unit can trip a whole multi-split system.
- In a multi-split system, the outdoor unit monitors control signals from all indoor units. This is part of the system's protection logic.
- A faulty indoor PCB can send abnormal current or corrupted signals. The outdoor unit reads these as a system-level fault and shuts everything down to protect itself.
- Replacing the outdoor unit would not have fixed this fault. The outdoor unit was working correctly — it was reacting to a bad signal from the indoor side.
- The first step is to find which indoor unit causes the shutdown. Test each one on its own. If the system runs stably with one unit disconnected, that unit is the cause.
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