EVT Diagnostics & Repair Procedures 5 — Questions and Answers
Question 1: Which CAN bus identifier range is typically reserved for HV powertrain communication in an EV architecture?
- The same bus as body control modules to reduce wiring
- A dedicated powertrain or HV CAN bus isolated from lower-priority body networks (Correct answer)
- The LIN bus shared with seat memory modules
- A Bluetooth link between BMS and inverter
Correct answer: A dedicated powertrain or HV CAN bus isolated from lower-priority body networks
HV powertrain components communicate on a dedicated high-speed CAN bus, separated from body and convenience networks to ensure message priority and integrity.
Question 2: A technician is performing a high-voltage system leak test after replacing a coolant fitting on the battery thermal circuit. What fluid property makes a HV coolant leak especially hazardous?
- High viscosity that clogs coolant passages
- Electrical conductivity if contaminated — the coolant used must be non-conductive (dielectric) (Correct answer)
- High flammability compared to water
- Reaction with aluminum fittings causing expansion
Correct answer: Electrical conductivity if contaminated — the coolant used must be non-conductive (dielectric)
EV battery coolant must be non-conductive (dielectric); a contaminated or wrong coolant can become conductive and create a shock hazard or short circuit within the battery pack.
Question 3: During EV motor stator winding resistance testing, a technician finds one phase reading 0.0 Ω while the others read 0.3 Ω each. What does this indicate?
- Normal variation due to measurement lead resistance
- A shorted winding on the phase reading 0.0 Ω (Correct answer)
- An open circuit fault in the winding
- Correct motor balance; all phases should read equal
Correct answer: A shorted winding on the phase reading 0.0 Ω
A reading of 0.0 Ω on one phase while others show normal resistance indicates a shorted winding, which would cause excessive current draw and motor damage.
Question 4: When an EV is towed after a collision, what towing method should be used if the drive wheels cannot be lifted?
- Flat-bed towing only, or use a dolly under the drive wheels (Correct answer)
- Tow normally with drive wheels rolling on the ground
- Lock the drive wheels with chocks and drag the vehicle
- Flat tow using a tow bar connected to the frame
Correct answer: Flat-bed towing only, or use a dolly under the drive wheels
Rolling drive wheels spin the motor and can generate back-EMF into the inverter or charge the HV battery unexpectedly; a flatbed or dolly eliminates this risk.
Question 5: A scan tool live data PID shows motor inverter temperature at 105°C during a performance test. The vehicle enters 'limp mode.' What is the CORRECT diagnosis?
- The temperature sensor is faulty and should be replaced
- Inverter overtemperature protection has activated — the thermal management system or cooling circuit should be inspected (Correct answer)
- The HV battery needs immediate replacement
- The motor resolver has failed
Correct answer: Inverter overtemperature protection has activated — the thermal management system or cooling circuit should be inspected
Most inverters derate or enter limp mode at high temperatures to prevent IGBT damage; the cooling circuit (pump, coolant level, heat exchanger) must be inspected.
Question 6: What is the purpose of the 'service disconnect' or 'manual service disconnect (MSD)' in an EV?
- To disconnect the 12V auxiliary battery from the chassis
- To break the HV battery circuit mid-pack, creating a safe working voltage on each side (Correct answer)
- To isolate the onboard charger from AC mains
- To disable the vehicle's CAN network during servicing
Correct answer: To break the HV battery circuit mid-pack, creating a safe working voltage on each side
The MSD divides the HV battery pack into two halves, each below lethal voltage thresholds, allowing safer handling of the pack during service procedures.
Question 7: A technician is verifying an EV's regenerative braking system after a brake modulator replacement. Which functional test BEST confirms correct blending operation?
- Measure brake pedal travel with a ruler at rest
- Perform a controlled stop from 30 mph while monitoring regen current and wheel speed data simultaneously (Correct answer)
- Check the parking brake cable tension
- Verify the ABS modulator solenoid resistance
Correct answer: Perform a controlled stop from 30 mph while monitoring regen current and wheel speed data simultaneously
Monitoring regen current and wheel speed together during a deceleration event confirms the system is correctly blending regenerative and friction braking without wheel slip.
EVT Diagnostics & Repair Procedures 5
Which CAN bus identifier range is typically reserved for HV powertrain communication in an EV architecture?