BYD Tang EV Rear Drive Electric Assembly Disassembly, Air Tightness, and Fault Diagnosis Guide

Fault Background

The four-wheel-drive Tang EV is equipped with an independent rear drive electric assembly on the rear axle: the drive motor controller, drive motor, and single-speed reducer are integrated into one unit, converting the DC power from the traction battery into rear wheel power. The maintenance rules for this assembly are much stricter than those for a traditional rear axle—before disassembly, drain the gear oil and residual coolant; after replacing the motor or controller, perform three-level air tightness tests on the assembly, coolant passage, and transmission; and after replacing the controller alone, also perform a VDS zero-position calibration. This article compiles disassembly requirements and torque specs, three-level air tightness standards, disassembly sequence, and common fault handling into a field guide.

Disassembly Requirements and Torque Specs

Disassembly requirements:

• Drain gear oil first: Before removing the electric assembly from the vehicle, use a torque wrench (H10) to open the drain plug assembly, drain the lubricating oil from the transmission housing completely, then reinstall the drain plug to prevent foreign objects from entering the transmission cavity.
• Disconnect high voltage first: Before disassembling components, disconnect the high-voltage wiring harness and wait 5 minutes before proceeding with assembly disassembly.
• Busbar connector: Remove the 2 mounting bolts (5±0.5N·m) of the controller busbar connector on the battery pack, pull the connector apart (do not pull the cable); when assembling, insert until the mating surfaces contact, then tighten the 2 bolts.
• Coolant hoses: Due to vehicle space constraints, when removing the electric assembly, you may keep a section of the coolant hose on the assembly, only removing the clamps and clips of the coolant hoses (flexible hoses) to disconnect the assembly from the cooling system.
• Drain residual coolant: Before disassembling the assembly body, use an air gun at the water inlet to blow into the coolant passage, forcing the water out through the outlet.

Three-Level Air Tightness Testing (After Replacing Motor or Controller)

Before testing, perform full inspection: drain bolt (set to 50N·m), controller bolts (set to 35N·m), torque wrench with red ink, mark all inspected bolts with red marks.

Operation points: For assembly air tightness—connect the air tightness adapter to the controller vent plug hole and the transmission vent valve hole, use harness sealing tools to seal the controller high-voltage busbar harness port and the motor adapter harness port, first remove the differential oil seal dust plug and replace with the differential oil seal plug, then use the housing sealing clamp tool to clamp both plugs; for coolant passage air tightness—block the lower end of the water pipe interface, connect the upper end to the air tightness tester; for transmission air tightness—connect the air tightness adapter to the rear housing air tightness component mounting hole. All plugs must be clamped tightly without leakage; the differential oil seal plug must be installed slowly with rotation, do not force it; the high-voltage positive/negative harness sealing tools must not be installed incorrectly; if the water seal ring comes out during removal, it must be properly seated. If leakage exceeds the limit, use soapy water to check for leaks at the fill/drain bolts, oil passage bolts, and quick connectors.

Disassembly Sequence

Drive motor controller (also remove the controller first when the transmission or motor has issues requiring factory return):

1. Remove the 12 M5 bolts and 1 stud bolt securing the motor end cover, then remove the drive motor end cover;
2. Loosen the mounting bolts for the shield and three-phase terminals;
3. Remove the 5 M10 bolts securing the drive motor controller, separating the controller from the motor and transmission;
4. Protect the water inlet from contacting the drive motor, and use an air gun to dry any residual water at the inlet and outlet.

Drive motor: Before disassembly, drain the oil and residual coolant from the water passage; press the clip to disconnect the resolver and temperature sensor connector; remove the 2 M5 hexalobular socket head bolts securing the shield; remove the 3 M6 bolts securing the stator lead wires—at this point, you can perform the motor insulation and withstand voltage test: clamp one end to one of the three-phase terminals and the other end to the motor housing. The stator lead wire copper terminals are in U, V, W phase sequence from left to right. When reinstalling the end cover, apply sealant: scrape off old sealant from the mating surfaces, apply sealant along the inside of the bolt holes, align the bolt holes, install bolts in sequence first, then tighten in a crisscross pattern, and apply paint marks.

Transmission assembly: Disassembly and repair must be returned to the factory; the front housing is integral with the drive motor, so only the rear housing can be removed, then disassemble the corresponding components per repair requirements.

Installation Precautions

• Before inserting the drive shaft into the transmission differential port, use a differential oil seal protector to protect the oil seal, insert until the spline snap ring locks, ensuring the oil seal is not damaged;
• Fill with 1.0±0.1L of lubricating oil, confirm the fill plug (37±2N·m) and drain plug (50±3N·m) threads are normal, and apply paint marks;
• After installation, perform a 10-minute AC (or DC) charge, observe the coolant reservoir level change, ensure the level is above (or parallel to) the MAX mark, and top up if necessary.

Common Faults and Handling

Motor side:

• Resolver fault: Use an adapter harness to test the resolver sine, cosine, and excitation impedance, with test frequency set to 10kHz—sine 433Ω±87Ω (multimeter DC resistance 36±4Ω), cosine 433Ω±87Ω (42±4Ω), excitation 120Ω±24Ω (14.5±2Ω); if resistance is normal, check the controller; if abnormal, replace the resolver.
• Leakage: After removing the motor end cover and the stator lead wire mounting bolts, test insulation and withstand voltage—insulation resistance requirement DC 1000V, 10s, >50MΩ; withstand voltage requirement AC 2500V, 60s, <20mA; if not good, remove the motor and return to factory for inspection and repair.
• Abnormal noise: Confirm the operating conditions when the noise occurs and record audio, send to quality after-sales and simultaneously notify the technical department to confirm the handling method.

Controller side (diagnostic flow: battery 9~16V → read codes and record → clear codes and turn OFF → wait 3 minutes, turn ON again and read codes → check DC bus—bus positive to bus negative normal 420V~705.6V (rated 513.2V), if abnormal, check the charging/distribution three-in-one or traction battery → repair or replace the motor controller → test to confirm):

VDS Zero-Position Calibration (After Replacing Drive Motor Controller Only)

Procedure is the same as Yuan PLUS: Record the motor zero-position barcode (the last four digits of the barcode are the zero-position value) → Vehicle ON, P gear, stationary, connect VDS → Vehicle Diagnostic System → Passenger Car → EV Series → Select STE model → ECU Module Scan → Select "Rear Drive Motor Controller" to enter diagnostic page → Data Stream page 3 to view original zero-position value → Stop data stream → Click "Motor Zero-Position Calibration" → Enter the installed motor zero-position barcode (case-sensitive): if barcode correct, prompt "Operation Successful"; if incorrect, prompt "Negative Response" and revert to default value 07BC → After vehicle power-down and restart, read back data stream to confirm zero-position value matches barcode. If multiple "Negative Responses", record motor, controller barcodes and vehicle VIN and feedback to manufacturer.

Safety Warnings

• This article is for technical reference; high-voltage repairs must be performed by certified personnel per OEM specifications. Readers unfamiliar with these procedures are strictly prohibited from performing operations themselves.
• After disconnecting the high-voltage harness before disassembly, wait 5 minutes; before disassembly, drain gear oil and residual coolant from the water passage.
• All three air tightness test certificates are mandatory: assembly <50Pa, coolant passage <150Pa, transmission <50Pa; plugs must be clamped tightly without leakage; the differential oil seal plug must be installed slowly with rotation to prevent oil seal damage.
• Transmission disassembly and repair must be returned to the factory; the front housing is integral with the motor, so only the rear housing can be removed on site.
• After zero-position calibration, power down and restart to read back and verify; if default value 07BC appears, calibration was not successful.

Maintenance Insights

The Tang EV rear drive assembly service manual draws three "non-negotiable" lines for field technicians: First, "oil and water first"—gear oil and coolant must be drained before disassembly; the cost of foreign objects entering the transmission cavity is a full factory return; Second, "three-level air tightness as certificate"—assembly, coolant passage, and transmission each have their own standards (50Pa/150Pa/50Pa); details like "install differential oil seal plug slowly with rotation" are written into the manual because forcing it will damage the oil seal; Third, "match fault codes to components"—resolver resistance three sets of values (433Ω AC impedance corresponding to 36Ω DC resistance), winding temperature sensor resistance, CAN high/low voltage 2.5~3.5V/1.5~2.5V, are all benchmarks that turn the multiple-choice question of "controller or motor failure" into a fill-in-the-blank. Remembering these numbers is more valuable than remembering the process itself.

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【This article is for reference only】