Geely Emgrand EV300 Unable to Power On: VCU High-Voltage Interlock Circuit Inspection

Fault Background

Before the Emgrand EV300 powers on, the vehicle control unit (VCU) must complete a high-voltage system self-check, among which the high-voltage interlock detection is a safety red line: the VCU sends a 5V signal from CA55/73, which flows sequentially through the motor controller (PEU), air conditioning compressor, PTC heater, and the high-voltage wiring harness connectors of the VCU and the PEU cover, forming a series loop before returning to CA55/51. The VCU determines whether the interlock loop is complete based on whether the feedback voltage is 5V—an open loop indicates a risk of high-voltage electric shock, and the VCU will refuse to close the traction battery relay, preventing the vehicle from powering on. This article dissects a case where the interlock broke inside the "motor controller."

Fault Symptoms

Cause Analysis

1. High-voltage interlock circuit or component damaged or loose—any open point in the interlock loop will trigger the fault.
2. High-voltage wiring harness connector fault—loose connector or backed-out terminal pin.
3. PEU internal interlock circuit fault—the interlock signal passes through the PEU cover interlock socket; an internal open circuit will also break the loop.
4. VCU itself faulty—abnormality in the signal sending/receiving end.

Diagnostic Procedure

1. Read codes to confirm: The diagnostic scan tool reads current DTCs P0A0A11 "High-voltage interlock open" and P105763 "High-voltage interlock detection timeout during charging preparation" from the VCU, and combined with the symptoms, the interlock loop is identified as the focus.
2. First segment measurement: Referring to the wiring diagram, disconnect connector EP01 and measure the resistance between EP01/1 and EP01/14; it reads infinite, while the standard value should be less than 2 Ω—this segment is abnormal.
3. Second segment measurement: Disconnect the PEU module and measure the resistance between EP11/4 and EP01/14; it reads 0.3 Ω (standard less than 2 Ω)—this segment is normal.
4. Third segment measurement: Measure the resistance between EP11/1 and EP01/1; it reads 1.8 Ω (standard less than 2 Ω)—this segment is also normal. At this point, all external circuits are ruled out; the only unmeasured part is the PEU internal: the interlock signal enters the PEU from EP11/1, passes through the cover high-voltage interlock socket, and exits from EP11/4.
5. Internal localization: Remove the PEU cover and measure the resistance between pin 1 of the internal module and the cover interlock socket; it reads infinite, with a standard of less than 1 Ω—it is determined that the motor controller module internal circuit is damaged.
6. Repair verification: After replacing the motor controller (PEU) with a new one, depress the brake pedal and press the ignition switch; the instrument cluster shows READY, the vehicle powers on normally, and the fault is resolved.

Repair Plan

1. After reading codes to confirm an interlock-type fault, first consult the wiring diagram to understand every module and connector in the interlock loop.
2. Use the segmented measurement method: Starting from the connectors at both ends, measure the interlock wire resistance segment by segment; the standard value is less than 2 Ω (less than 1 Ω for the PEU internal segment), and use "which segment is infinite" to directly pinpoint the faulty segment.
3. When all external circuits are normal, decisively disassemble and inspect the PEU cover, measuring both ends of the internal interlock socket.
4. After confirming internal PEU damage, replace the entire motor controller; do not perform temporary internal wiring repairs.

Key Parameter Quick Reference

Safety Warnings

• A high-voltage interlock fault means the system has a high-voltage safety risk; perform all work according to high-voltage repair specifications: complete vehicle power loss, disconnect the service disconnect switch, and perform voltage verification.
• Before disassembling the PEU, confirm there is no residual voltage on the high-voltage bus, and use insulated test leads with proper insulation rating when measuring.
• After replacing the PEU, re-run the power-on self-check to confirm the interlock status is normal and no DTCs are present before returning the vehicle.

Repair Insights

The interlock loop is a series "single wire," which makes troubleshooting divisible: EP01/1-EP01/14 infinite → indicates an open point in the loop; EP11/4-EP01/14 at 0.3 Ω and EP11/1-EP01/1 at 1.8 Ω → proves the external circuits are intact; only the PEU internal remains. Three segment measurements, narrowing down layer by layer, and finally lifting the cover hits the mark. The secret to troubleshooting a series loop is: don't replace parts wholesale; break the "single wire" into "several segments" to measure.

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【This article is for reference only】