Hybrid and Electric Vehicles Flashcards

Before working on the high-voltage system of a hybrid truck, a technician must de-energize the system. What is the FIRST step after donning appropriate PPE?

Answer: Remove the high-voltage service disconnect (manual service disconnect) plug

The correct sequence requires removing the high-voltage manual service disconnect (MSD) plug first — this physically breaks the HV circuit and isolates the battery pack from the rest of the system. After removing the MSD, the technician waits the manufacturer-specified time for capacitor discharge before measuring residual voltage. Disconnecting the 12 V battery first is done in some procedures but does not isolate the HV bus. Cutting orange cables is extremely dangerous and never a service procedure. Capacitor discharge via external load is performed only after the MSD is removed, not before.

Which color is universally used on hybrid and electric trucks to identify high-voltage cables and components?

Answer: Orange

SAE J1673 and vehicle OEM standards specify orange as the identifier for high-voltage (60–1000 V DC or AC) wiring in hybrid and electric vehicles, including trucks and coaches. Red is used for positive low-voltage wiring. Yellow is used in some safety contexts (e.g., airbag wiring) but not HV. Blue is sometimes used for 12 V auxiliary systems. Recognizing orange cables is a basic EV safety requirement for any technician working on these vehicles.

What is regenerative braking in a hybrid truck, and what is its primary benefit?

Answer: A system where the electric motor acts as a generator during deceleration, converting kinetic energy back into electrical energy stored in the battery

Regenerative braking uses the electric drive motor in generator mode during deceleration: the wheels drive the motor, which produces electricity that is fed back to the high-voltage battery (or supercapacitor). This recovers energy that would otherwise be lost as heat in friction brakes, improving overall efficiency. Air brakes store no energy; exhaust brakes use engine back-pressure, not energy storage; hydraulic accumulators exist (hydraulic hybrids) but are a separate technology from the electric regenerative braking being described.

A hybrid truck's high-voltage battery management system (BMS) triggers a fault and limits power output. Which parameter does the BMS monitor to protect the battery pack?

Answer: Cell voltage, state of charge (SOC), and cell temperature

The Battery Management System (BMS) continuously monitors individual cell voltage (to detect over- or under-voltage), state of charge (SOC) to prevent over-discharge or overcharge, and cell temperature (to prevent thermal runaway). When any parameter exceeds safe limits, the BMS reduces or cuts power to protect the pack. Engine oil pressure, transmission temperature, and 12 V charging are managed by separate control modules and are not primary BMS inputs.

A technician measures 450 V DC between the positive and negative HV bus terminals after removing the manual service disconnect. What should the technician do?

Answer: Immediately reinstall the MSD and consult the OEM service manual before continuing

Residual voltage of 450 V after MSD removal indicates that the capacitors have not yet discharged or there is an isolation fault — either way, the system is still lethally dangerous. The correct action is to reinstall the MSD (restore safety isolation), wait the full OEM-specified discharge time, and consult the service manual before re-approaching. Proceeding with repairs at 450 V risks electrocution. Shorting with a resistor without OEM authorization risks arc flash and component damage. Taping terminals and working quickly is entirely unsafe.

When performing routine maintenance on an electric transit coach, a technician notices the high-voltage interlock loop (HVIL) circuit has opened. What does this indicate?

Answer: A high-voltage connector or cover is not fully seated, triggering a safety shutdown

The High-Voltage Interlock Loop (HVIL) is a low-voltage safety circuit that runs through all HV connectors and access covers. If any connector is not fully mated or any cover is open, the HVIL circuit opens, signaling the vehicle's control system to disable HV power — preventing accidental exposure to live conductors. It is not a charge-level indicator, a thermal protection response, or related to the 12 V system voltage.