BYD Tang DM-i EHS Solenoid Valve Troubleshooting Guide

Fault Background

Inside the EHS (Electro-Hybrid System) assembly of the DM-i hybrid system, there are several key solenoid valves—the main pressure solenoid valve, clutch pressure solenoid valve, and sub-cool protection solenoid valve. They are directly driven by the VCU through the through-case wiring harness, forming the "oil pressure heart" of the entire hybrid drivetrain. Solenoid valve fault codes fall into three categories: open circuit, short to ground, and short to power. Each category has different judgment conditions and troubleshooting paths. This article compiles the diagnostic methods for these three fault types into a field-usable troubleshooting guide (applicable to Qin PLUS DM-i, Song PLUS DM-i, Tang DM-i, etc.).

Fault Symptoms

Cause Analysis

• Open circuit fault: The solenoid valve itself is open, the connecting wiring harness is open, or the VCU internal circuit is open—one of these three must be the cause.
• Short to ground: The solenoid valve winding is shorted to the housing, or the connecting wiring harness is chafed and shorted to the housing/ground.
• Short to power: The solenoid valve is burnt, or the solenoid valve control wire is shorted to the power wire.

Diagnostic Procedure

1. Read DTC and classify: Use VDS to read the fault code, first determine whether it is open circuit, short to ground, or short to power; clear the fault code and road test to confirm whether the fault reproduces. If it reproduces, continue troubleshooting.
2. Visual inspection: Check whether the VCU female connector and the through-case wiring harness connector are fully seated, whether pins are deformed or bent, and whether terminals have backed out or sockets are enlarged. If no visual abnormality, proceed to measurement.
3. Open circuit—VCU side measurement: Power down the vehicle, disconnect the VCU female connector, and measure the resistance of the main pressure solenoid valve, clutch pressure solenoid valve, and sub-cool protection solenoid valve from the VCU female side. During measurement, wiggle the wiring harness and observe whether the resistance changes—resistance fluctuation is a typical sign of an internal broken wire in the harness.
4. Open circuit—EHS side localization: If the resistance at the VCU side is abnormal, disconnect the EHS female connector and measure the three solenoid valve resistances from the EHS male side. If the resistance at the EHS male side is abnormal, the EHS assembly is faulty; if the EHS male side is normal, the fault is in the vehicle wiring harness.
5. Short to ground measurement: Disconnect the VCU female connector, and measure the resistance from the VCU female side of each solenoid valve to vehicle ground. Normal reading should be tens of kΩ or more up to infinity. If abnormal, repeat the measurement at the EHS male side to localize: abnormal at EHS male side indicates assembly issue; normal indicates vehicle wiring harness issue.
6. Short to power measurement: The judgment condition is based on ON-state current; the troubleshooting path is the same as for open circuit—first measure resistance at the VCU female side (with harness wiggling), then re-measure at the EHS male side, localizing step by step to assembly or harness.
7. Harness visual re-inspection: Regardless of fault type, after localizing to the vehicle wiring harness, inspect the harness along its route for wear, chafing, or crushing, especially near the through-case area and moving parts.
8. Intermittent fault focus: If the fault is intermittent or occasional, focus on the terminal fit clearance at the VCU female and EHS female connectors. Loose terminal fit can cause intermittent signal break.

Repair Plan

1. Classify the fault code first (open/short to ground/short to power) and select the corresponding measurement items; do not mix measurements across types.
2. Follow the two-level localization sequence: "VCU female side → EHS male side". Two measurements are enough to distinguish EHS assembly fault from vehicle wiring harness fault.
3. When measuring resistance, wiggle the wiring harness to expose intermittent broken wires.
4. After localizing to the harness, replace or repair the harness (repair must follow OEM standards for waterproofing and abrasion protection); if localized to the EHS assembly, replace the assembly and perform oil circuit bleeding and adaptive learning.
5. After repair, clear the DTC and road test to confirm the fault code does not reappear.

Key Parameter Quick Reference

Safety Warnings

• EHS involves high-voltage systems. Before any measurement, the vehicle must be powered down and voltage verified. Do not disconnect or connect VCU and EHS connectors while powered.
• Replacing the EHS assembly is high-voltage component repair. Disconnect the service disconnect switch and take safety precautions. After completion, restore and self-check per procedure.
• When measuring resistance to ground, use properly insulated test leads to prevent the human body from entering the measurement circuit.

Repair Insights

Solenoid valve fault codes may seem simple, but they actually represent three completely different troubleshooting paths. Three key points: First, classify before acting—open circuit requires resistance measurement, short to ground requires insulation check, short to power requires current observation; choosing the wrong method means wasted effort. Second, use the two-level localization method—measure once at the VCU female side and once at the EHS male side; "assembly faulty or harness broken" becomes immediately clear. Third, don't forget to wiggle the harness during measurement—intermittent broken wires only reveal themselves under stress. What determines fault diagnosis is never luck, but the choice of measurement points.

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【This article is for reference only】