AU
B2AB774

DTC B2AB774 indicates abnormal speed feedback from the electric A/C compressor — Seal U

Thermal Management System

DTC B2AB774 indicates abnormal speed feedback from the electric A/C compressor.

Specifically, the compressor control module (MCU) detects a deviation between the actual compressor motor speed and the target commanded speed exceeding the threshold (typically >15%), a lost speed signal, or excessive speed fluctuation (jitter).

This fault affects a core actuator in the thermal management system.

It can reduce A/C cooling/heating performance, cause thermal management failure, and subsequently trigger motor or battery over-temperature protection.

In pure electric models like the Qin EV, high voltage (typically 320V-750V) directly drives the electric compressor.

The system uses sensorless vector control (FOC) or Hall sensor feedback for speed control.

This fault essentially indicates instability in the closed-loop speed control.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Compressor mechanical fault: Scroll wear and binding, motor bearing seizure, or refrigerant oil degradation increases frictional torque, preventing actual speed from tracking the target value.
  • 2Compressor controller fault: Damaged IPM power module, MCU chip program runaway, or bus capacitor aging causing drive waveform distortion, resulting in speed fluctuation or loss of synchronization.
  • 3Abnormal speed feedback signal: Damaged Hall sensor, poor signal wiring harness shielding allowing EMC interference, oxidized connector pins causing speed pulse signal loss.
  • 4Thermal management system load abnormal: Severe refrigerant leakage (precursor to low-pressure protection), system ice blockage, or overcharging causes sudden compressor load changes, resulting in unstable compressor speed.
  • 5Abnormal high-voltage power supply: sudden drop in traction battery pack voltage (cell fault), burnt high-voltage distribution box contactor, compressor bus voltage sampling circuit drift triggering undervoltage/overvoltage protection.
What To Do
  • 1
    Diagnostic scan: Use VDS2000 or Launch PAD5 to read complete DTCs. Check the freeze frame data (compressor speed command value, actual feedback value, DC bus voltage, phase current, and IGBT temperature at the time of the fault) to confirm if the fault is "speed too high", "speed too low", or "abnormal speed signal".
  • 2
    Visual and wiring harness inspection: Inspect the compressor high-voltage wiring harness (orange) insulation for damage. Check the low-voltage signal connector (usually 4-6 pin) for water ingress or oxidation. Measure the connector pin voltages (constant power, IG signal, PWM control, feedback signal).
  • 3
    Refrigeration system pressure check: Connect the manifold gauge set. Verify static pressure is 0.8-1.2 MPa (at 25°C ambient). Verify operating pressure is 1.3-1.8 MPa on the high-pressure side and 0.15-0.25 MPa on the low-pressure side. Confirm no ice blockage or severe refrigerant shortage.
  • 4
    Insulation and high-voltage check: Use a megohmmeter to measure the insulation resistance between the compressor high-voltage terminal and the housing (must be >500 MΩ). Use a multimeter to verify bus voltage stability (must be within ±10% of the rated value).
  • 5
    Signal waveform measurement: Use an oscilloscope to capture the PWM speed control signal received by the compressor controller (usually 100-200Hz duty cycle) and the speed feedback signal (Hall square wave or resolver sine wave). Confirm the waveforms show no distortion or noise.
  • 6
    Compressor replacement verification: If the above checks are normal but the fault recurs, disconnect the refrigerant lines (recover refrigerant), replace the electric compressor assembly, re-evacuate the system (hold vacuum level <-0.1 MPa for 15 minutes), and charge the standard amount of R134a or R1234yf refrigerant and PAG compressor oil.
Real-World Cases
BYD DTC AI Analysis

Qin EV compressor Hall sensor failure causing intermittent speed fluctuation

✓
A 2019 Qin EV with 80,000 km had intermittent AC cooling failure; the instrument cluster displayed a thermal management fault. DTC B2AB774 (abnormal speed) set. Freeze frame showed compressor commanded speed at 3000 rpm with feedback at 0 rpm. Disconnected the compressor low-voltage connector; the Hall sensor supply measured 5V (normal), but the signal line showed no pulse. Disassembled the compressor rear cover and found the Hall sensor corroded from refrigerant leakage. Replaced the compressor assembly; fault resolved.
BYD DTC AI Analysis

High-voltage distribution box contactor burnout causes sudden compressor speed drop

✓
AC stopped cooling after fast charging with DTC B2AB774 logged. Data showed compressor bus voltage dropped sharply from 650V to 180V during operation, causing speed to fall from 4000rpm to 800rpm and trigger the fault. Found burnt contacts on the compressor circuit contactor inside the high voltage distribution box; contact resistance measured 0.8Ω. Replaced the high voltage distribution box assembly. Fault resolved.
BYD DTC AI Analysis

Refrigerant overcharge caused compressor speed fluctuation

✓
The vehicle set DTC B2AB774 the day after air conditioning service at an independent workshop. The scan tool showed the compressor actual speed fluctuating severely between 2000-3500rpm with abnormally high bus current (>15A). Recovered the refrigerant and found the charge was 850g (standard 650g). Excess refrigerant overloaded the compressor and destabilized the speed closed loop. Recovered the excess refrigerant and recharged to specification. Fault cleared.
BYD DTC AI Analysis

Compressor control board IPM module overheated and damaged

✓
Extended air conditioning use in high summer temperatures triggered a fault that cleared after the vehicle was parked and cooled. Data showed the compressor IGBT temperature reached 110 °C (normal < 85 °C). Opening the compressor controller revealed dried thermal grease in the IPM module. The power transistors had overheated and failed, reducing drive capability so the compressor could not maintain speed. Replaced the compressor controller (or the assembly), reapplied thermal grease, and cleared the fault.
BYD DTC AI Analysis

Low voltage control harness shield damaged, causing interference

✓
When driving through a specific section (near high-voltage power lines), the AC suddenly stopped working and set DTC B2AB774. Found the compressor speed feedback signal wire shielding damaged; external electromagnetic interference caused the ECU to miss pulses in the speed signal. Re-wrapped the harness shielding and ensured single-point grounding. Fault resolved.
Other Seal U Fault Codes
B1108For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2.5 air quality detection module (dust sensor) or its wiring harness. The sensor uses laser scattering to detect in-cabin and outside PM2.5 concentrations and transmits data to the HVAC ECU via the LIN bus or an analog signal. Short circuit conditions include: 1) Sensor power supply wire (+5V/+12V) shorted to ground 2) Signal wire shorted to power or ground 3) Short circuit in the sensor's internal photoelectric detection circuit This fault prevents the automatic climate control from switching between fresh air and recirculation based on air quality. The system enters fail-safe mode (typically forced recirculation). Severe short-circuit current can destroy the HVAC ECU sampling circuit. The system classifies this as a Level 3 severe fault and restricts vehicle operation to prevent electrical fires or controller damage. For other BYD models (such as the Qin EV, E5, and Song DM), B1108 indicates a driver-side sunload sensor fault, reflecting differences in DTC definitions across models.B110811This DTC indicates a short circuit in the air conditioning system PM2.5 air quality sensor (rapid detector). Specifically, abnormal continuity exists between the sensor power supply circuit or signal output circuit and body ground or battery positive. This causes the control module (ACU/air conditioning controller) to detect abnormal current or a voltage signal outside the valid range (typically 0V or supply voltage). The PM2.5 sensor monitors internal and external particulate concentrations, providing the data the automatic air conditioning system uses to switch between fresh air and recirculation modes. A short circuit disables the air quality monitoring function. This fault may force the air conditioning system into recirculation protection mode and trigger related thermal management system faults, as the air conditioning system acts as a key actuator in vehicle thermal management. In severe cases, the short circuit current can damage the internal sampling circuit of the air conditioning controller or overheat the wiring harness.B1109DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2.5 air quality sensor (dust concentration sensor) and the air conditioning control unit (AC ECU), or an open circuit within the sensor internal circuitry. Typically located in the air conditioning intake duct, this sensor monitors real-time PM2.5 particle concentration inside and outside the vehicle. It provides data to the automatic air conditioning system for intelligent switching between fresh air and recirculation modes and for controlling the air purification device. During an open circuit fault, the AC ECU cannot receive the air quality signal, triggering the system fail-safe mode. Symptoms typically include failure of the automatic recirculation function, abnormal air purification indicator light operation, or abnormal air quality data displayed on the air conditioning panel. Severe cases may compromise the cabin environment control strategy, but do not directly affect vehicle driving safety.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.
Data confidence: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.