(Fujitsu) General Australia and New Zealand

AJY126LALH - Transmission Error and Short Circuit on X1/X2

This article summarises the quality assurance investigation into an AJY126LALH where a transmission error occurred during commissioning and the PCB was found to measure short circuit across the transmission terminals.

Problem Description

During commissioning, a transmission error occurred. The attending technician checked the outdoor unit PCB and found that the X1-X2 communication terminals were measuring short circuit.

PCB Analysis

1. No obvious or conspicuous physical damage was identified on the PCB.
2. Regardless of the terminal setting being ON or OFF, a short circuit condition was measured not only between X1-X2, but also between X1-G and X2-G.
3. The ground terminals 1, 2 and 3 should normally be common / homopolar at approximately 0 Ω.
4. The ground line associated with G1 was found open circuit (O.L.).
5. Further investigation identified an abnormal condition at L111, a filter component used for noise reduction.

L111 Component Findings

The factory found that pin 4 and the coil circuit of L111 were open circuit. Other pin combinations showed continuity.

An X-ray inspection did not clearly identify the failure mechanism, so the component was sent to the L111 manufacturer for further analysis.

Component Manufacturer Assessment

The L111 manufacturer advised that every component is electrically tested before shipment.

• The manufacturer did not believe the component had been shipped in the failed condition.
• The manufacturer also did not consider the damage likely to have occurred during its manufacturing process.
• The manufacturer considered that the failure most likely occurred after shipment.
• The suspected mechanism was over-current of approximately 100 mA or more flowing through the component.

Product Traceability - AJY126LALH R001173

The factory reviewed the production test history for the affected unit.

• The PCB had been tested at the FGCA factory.
• During the factory test, the PCB was connected to a PC through the X1-X2 terminals.
• The factory test record for serial R001173 showed that testing completed correctly.
• Based on this record, the transmission circuit was functioning correctly at the time of factory testing.

Factory Cause Assessment

FGL concluded that the fault was not present when the product was shipped from the FGCA factory and was most likely created after shipment.

Because the normal communication current is significantly below the level suspected of damaging L111, FGL did not identify a design issue with the transmission circuit.

Investigation Conclusion

• The X1-X2 transmission circuit was shorted when the unit was commissioned.
• L111 was found internally damaged, with melted wiring and shorted coil windings.
• Factory records confirmed that the X1-X2 circuit operated correctly during production testing.
• The component manufacturer considered the damage likely to have been caused by an external over-current event after shipment.
• No systemic design or manufacturing defect was identified.
• FGL advised that the market should continue to be monitored for similar cases.

Service Guidance

If a similar transmission fault is found on an AJY-series system:

1. Isolate power before carrying out resistance or continuity testing.
2. Check resistance between X1-X2, X1-G and X2-G.
3. Inspect the communication wiring for accidental connection to mains voltage or another external voltage source.
4. Check for damaged, pinched or crossed transmission wiring.
5. Inspect PCB connectors and the transmission circuit for signs of overheating or electrical damage.
6. If L111 or the transmission filter circuit is suspected, record all resistance measurements before replacing the PCB.
7. Where possible, confirm whether the fault existed before or after site wiring was connected.
8. Escalate repeat cases with wiring details, measured voltages, resistance values and installation information.