BYD Song EV Air Conditioning Troubleshooting and Solutions

Air Conditioning System Diagnosis: BYD Song EV300

This document outlines the diagnostic process for an intermittent air conditioning (AC) failure in a BYD Song EV300, which also triggered an "EV Function Limited" warning. The goal is to present the information clearly for an international audience.

Air Conditioning System Diagnosis

▍ Initial Complaint and Preliminary Analysis
A BYD Song EV300 electric vehicle, with approximately 15,000 km on the odometer, presented with an intermittent air conditioning (AC) issue where it would occasionally blow warm air. Furthermore, after running the AC for some time, the instrument cluster would display an "EV Function Limited" warning, and the "OK" (ready-to-drive) light would turn off.

Initial analysis suggested the problem could be related to multiple areas:

1. Intermittent AC Failure: This could stem from a fault within the AC system itself, such as a refrigerant leak or a malfunctioning AC control module.

2. "EV Function Limited" Warning: This warning, appearing after the AC was used, pointed towards a potential issue with the vehicle's energy management system or a high-voltage electrical fault.

▍ Diagnostic Process and Findings
Step 1: Initial Checks and System Pressure

• During testing, the vehicle started with the AC blowing ambient air.

• After a short period of operation, the "EV Function Limited" warning appeared on the dashboard.

Step 2: Reading Fault Codes

• Scanning the electric compressor revealed two diagnostic trouble codes (DTCs):

  • B2AB573: Start Failure

  • B2AB64B: Internal Temperature Abnormality

• Because the AC fault seemed to trigger the EV warning, the Battery Management System (BMS) and leakage sensor were checked using the VDS diagnostic tool. No faults were found in them, but the insulation resistance data stream read near zero, indicating a potential high-voltage leak.

Step 3: Compressor Replacement

• A direct measurement of the electric compressor's insulation resistance to ground showed 0.00 MΩ (megohms), confirming a severe insulation fault and a high-voltage leak path.

• The refrigerant was recovered. The faulty electric compressor was replaced. The system was then evacuated (vacuumed) and recharged with refrigerant.

• After this repair, the "EV Function Limited" warning no longer appeared. However, the AC cooling performance was still very poor.

Step 4: Electronic Expansion Valve (EXV) Diagnosis

• Data from the AC controller and the new electric compressor was analyzed. The readings showed abnormal pressure: high-side pressure was too high, while low-side pressure was too low.

• This pressure profile suggested the Electronic Expansion Valve (EXV) might be stuck closed. However, a continuity test on the EXV's wiring showed it was intact.

• As a next step, the refrigerant was recovered again, and the EXV was replaced. After recharging the system, the AC worked correctly, blowing cold air. The issue appeared to be resolved.

Step 5: Recurrence and Further Analysis

• Unfortunately, the customer returned two days later with the same complaint: the AC was not cooling again.

• Reading the electric compressor's data revealed new fault codes:

  • B2AB41D: Internal Current Too High

  • B2AB64B: Internal Temperature Abnormality

  • B2AB774: Speed Abnormality

• Live data from the compressor showed its speed was constantly stuck at approximately 5000 rpm, refusing to modulate down. Simultaneously, data from the AC controller showed the EXV opening percentage continuously rising until it reached 100%.

Step 6: Root Cause Identification

• The diagnosis concluded that the EXV opening was too large. This prevented the proper pressure differential needed for effective cooling. To compensate for the poor cooling caused by the fully open EXV, the compressor was forced to run continuously at its maximum speed. This sustained high-speed operation led to the observed faults: excessive current draw, abnormal internal temperature, and speed regulation failure.

• Since the EXV is controlled by the AC controller, and its wiring had already been verified as good, the AC controller itself became the prime suspect.

• To confirm the diagnosis, the suspected faulty AC controller was swapped with a known-good one from a test vehicle. The exact same problem (compressor running constantly at high speed, poor cooling) immediately appeared on the test vehicle. This successfully confirmed that the original AC controller was indeed defective.

▍ Final Conclusion and Key Takeaways

Final Conclusion: The root cause of the problem was a faulty Air Conditioning (AC) Controller. Its malfunction caused the electronic expansion valve to open excessively. This, in turn, forced the electric compressor to run continuously at high speed in an attempt to meet cooling demands, ultimately leading to the compressor's premature failure and triggering the "EV Function Limited" warning due to electrical overload.

Key Takeaways for Diagnosis:

1. The intermittent nature of the initial failure made it challenging to diagnose early on.

2. When repairing modern automatic AC systems, it is crucial to have a thorough understanding of the system's operating principles.

3. Closely monitoring live data from all relevant components—especially the AC controller and the electric compressor—is essential for accurately and efficiently pinpointing the root cause of a complex, intermittent fault.