EVT Diagnostics & Repair Procedures 3 — Questions and Answers
Question 1: During an EV high-voltage system repair, a technician notices an orange cable running along the vehicle frame. What does this cable indicate?
- 12V accessory power circuit
- High-voltage traction system wiring (Correct answer)
- CAN bus communication wiring
- Chassis ground return
Correct answer: High-voltage traction system wiring
Orange cables are the industry-standard color coding for high-voltage traction system wiring in electric and hybrid vehicles.
Question 2: What does a 'negative temperature coefficient' (NTC) thermistor reading that is significantly lower than ambient temperature indicate in a battery thermal sensor circuit?
- The battery pack is functioning normally
- An open circuit or disconnected sensor (Correct answer)
- The battery cells are overheating
- Low refrigerant charge in the thermal system
Correct answer: An open circuit or disconnected sensor
An NTC thermistor resistance rises as temperature drops; a reading far below ambient typically indicates an open circuit rather than extreme cooling.
Question 3: When replacing an EV traction motor, which additional component should always be inspected or replaced due to shared operational stress?
- The 12V auxiliary battery
- The inverter/motor controller and its gate drive circuitry (Correct answer)
- The cabin air filter
- The DC-DC converter output capacitors only
Correct answer: The inverter/motor controller and its gate drive circuitry
The inverter drives the motor directly; a motor failure caused by an electrical fault can damage the inverter's IGBTs or gate drivers, and vice versa.
Question 4: An EV fails to power on and shows no 12V supply to the main control modules. The HV battery SOC is adequate. What is the MOST likely cause?
- Failed HV contactor coil
- Failed DC-DC converter (Correct answer)
- Open HV service disconnect
- Faulty motor resolver
Correct answer: Failed DC-DC converter
The DC-DC converter steps down HV to 12V to power vehicle control modules; its failure would cut 12V supply and prevent the vehicle from powering on.
Question 5: A scan tool shows the main positive contactor closes but the precharge contactor code remains active. What is the MOST likely fault?
- Shorted HV battery cell
- Failed precharge resistor or precharge contactor (Correct answer)
- Open circuit in the HVIL loop
- Faulty coolant pump
Correct answer: Failed precharge resistor or precharge contactor
The precharge circuit uses a resistor and contactor to slowly charge the inverter bus capacitors; a failure in either component will set a precharge fault.
Question 6: When using a bi-directional scan tool to actuate HV contactors for testing, what safety precaution is CRITICAL?
- Ensure the vehicle is in Drive mode first
- Confirm no HV cables are disconnected and all safety shields are installed (Correct answer)
- Discharge the 12V battery to prevent contactor cycling
- Remove the coolant reservoir cap to reduce system pressure
Correct answer: Confirm no HV cables are disconnected and all safety shields are installed
Actuating contactors with HV cables disconnected or shields removed exposes personnel to lethal voltage arcing when the contactors close.
Question 7: An EV produces a grinding noise during regenerative braking at low speeds only. What is the MOST likely cause?
- Motor bearing wear caused by high regenerative torque
- Brake caliper dragging due to incomplete pad retraction after blended braking (Correct answer)
- HV bus overvoltage during regen
- Motor resolver calibration error
Correct answer: Brake caliper dragging due to incomplete pad retraction after blended braking
Blended braking systems combine regen with friction brakes; if calipers don't fully retract after regen phases, they can drag and grind at low speeds.
EVT Diagnostics & Repair Procedures 3
During an EV high-voltage system repair, a technician notices an orange cable running along the vehicle frame.
What does this cable indicate?