EVT Diagnostics & Repair Procedures 2 — Questions and Answers
Question 1: When diagnosing an EV that shows a high-voltage interlock loop (HVIL) fault, what is the FIRST step a technician should take?
- Replace the HV battery pack immediately
- Verify all HV connectors are fully seated and service plugs are installed (Correct answer)
- Clear the DTC and perform a test drive
- Measure the 12V auxiliary battery voltage
Correct answer: Verify all HV connectors are fully seated and service plugs are installed
An HVIL fault typically indicates an open circuit in the interlock loop, often caused by an unseated HV connector or removed service plug.
Question 2: An EV's onboard charger (OBC) is not accepting AC power. After confirming the EVSE output is correct, what should the technician check next?
- Coolant level in the thermal management system
- AC inlet pilot signal and proximity detection circuits (Correct answer)
- DC-DC converter output voltage
- Regenerative braking calibration data
Correct answer: AC inlet pilot signal and proximity detection circuits
The pilot signal and proximity detection circuits between the EVSE and OBC must be within spec for the OBC to enable charging.
Question 3: A technician measures isolation resistance on an EV HV system and gets 150 kΩ. What does this indicate?
- Normal isolation; no action needed
- Slightly degraded isolation that may require monitoring
- A serious isolation fault requiring immediate HV system inspection (Correct answer)
- A failed isolation monitoring device (IMD)
Correct answer: A serious isolation fault requiring immediate HV system inspection
Acceptable HV isolation resistance is typically above 500 kΩ (often 1 MΩ+), so 150 kΩ indicates a serious isolation fault requiring inspection.
Question 4: Which tool is used to confirm HV bus capacitors are fully discharged before working on an EV drive system?
- Clamp-on ammeter
- Calibrated high-voltage rated digital multimeter (Correct answer)
- Standard 12V test light
- Oscilloscope with 10:1 probe
Correct answer: Calibrated high-voltage rated digital multimeter
A calibrated high-voltage rated DMM is required to verify that HV bus voltage has dropped to a safe level (typically below 60V DC).
Question 5: An EV shows reduced regenerative braking performance. Which component failure is MOST likely responsible?
- Worn friction brake pads
- Degraded HV battery state-of-charge acceptance (high SOC) (Correct answer)
- Faulty 12V battery
- Low power steering fluid
Correct answer: Degraded HV battery state-of-charge acceptance (high SOC)
Regen braking requires the HV battery to accept charge; if the battery is near full SOC or has degraded acceptance, regen performance is reduced.
Question 6: When performing a motor resolver calibration on a permanent magnet AC motor, what is the purpose of this procedure?
- To balance the motor windings for equal resistance
- To align the resolver's electrical zero position with the rotor's magnetic pole position (Correct answer)
- To measure peak torque output of the motor
- To reset the motor's thermal protection threshold
Correct answer: To align the resolver's electrical zero position with the rotor's magnetic pole position
Resolver calibration ensures the inverter knows the exact rotor position relative to the stator to achieve efficient torque control.
Question 7: A BMS fault code indicates 'cell voltage imbalance exceeds threshold.' What is the BEST corrective action?
- Immediately replace the entire battery pack
- Perform a cell balancing cycle via the BMS and retest (Correct answer)
- Top off the 12V auxiliary battery
- Reflash the inverter control module
Correct answer: Perform a cell balancing cycle via the BMS and retest
Many BMS systems support passive or active cell balancing routines that can reduce imbalance before determining if physical cell replacement is needed.
EVT Diagnostics & Repair Procedures 2
When diagnosing an EV that shows a high-voltage interlock loop (HVIL) fault, what is the FIRST step a technician should take?