- Orange cabling
- The industry-standard color code for high-voltage conductors. Treat any orange cable or connector as energized at dangerous levels (200-450 V) until verified at zero.
- High voltage (HV)
- On a light-duty hybrid/EV, the propulsion voltage — roughly 200-450 V DC — high enough to be lethal. Always treated as live until de-energized and verified.
- De-energize (de-power) sequence
- Power down and remove the key/fob, disconnect the 12-volt battery, remove the HV service disconnect, wait the OEM bleed-down time, then verify zero energy.
- HV service disconnect (service plug / MSD)
- A removable device on or near the battery pack that manually breaks the HV circuit, usually by splitting the pack into two non-lethal halves.
- Live-dead-live test
- Verifying zero energy with a CAT III meter: read a known live source, then the HV test points (~0 V), then the live source again to prove the meter still works.
- CAT III meter
- A multimeter and leads rated to withstand the transient overvoltage spikes of high-energy circuits. CAT III or higher is required for measuring HV on hybrids/EVs.
- Class 0 insulating gloves
- Rubber insulating gloves rated to 1,000 V AC (ASTM D120) — the minimum for light-duty HV service. Worn with leather protectors and inspected before each use.
- Class 1 / Class 2 gloves
- Higher-rated insulating gloves: Class 1 to 7,500 V AC, Class 2 to 17,000 V AC — for utility-grade work beyond light-duty automotive HV.
- Leather protectors
- Leather overgloves worn over rubber insulating gloves to protect them from cuts and abrasion that would destroy their insulating value.
- Arc flash
- A rapid release of electrical energy when current arcs across a gap or short, producing intense heat, light, and a pressure blast capable of severe burns.
- Arc-rated clothing / face shield
- PPE that protects against the heat and pressure of an arc flash; worn with insulating gloves per the OEM and job hazard assessment when contacting exposed HV.
- DC-link capacitor bleed-down
- After the HV circuit opens, the inverter's DC-link capacitors still hold a lethal charge; the OEM-specified wait (often 5-10 min) lets them discharge before verifying zero.
- Isolation (loss of isolation)
- The HV system is electrically isolated from the chassis. A loss of isolation — insulation resistance far below spec — is a shock hazard and sets an isolation fault.
- Megohmmeter (insulation tester)
- Measures isolation resistance between HV components and chassis ground. Healthy readings are high megohms; a low (kilohm) reading signals loss of isolation.
- Isolation fault indication
- A resistance reading much lower than the manufacturer's minimum (often low kilohms) between the HV pack and chassis indicates a loss of HV isolation.
- Why disconnect the 12-volt first
- Removing the low-voltage supply prevents the HV contactors from being commanded closed again while you work on the de-energized system.
- Verifying zero before touching HV
- Never rely on a dashboard warning light. Proof of a safe system is a live-dead-live measurement with a CAT III meter after de-power and bleed-down.
- HV battery chemistry (light-duty)
- Nickel-metal hydride (NiMH) or, increasingly, lithium-ion cells in series (and parallel) to reach pack voltage. Lithium-ion is denser but more heat/charge sensitive.
- NiMH battery
- Nickel-metal hydride — rugged, common in early hybrids, less energy-dense than lithium-ion, and not prone to a true memory effect.
- NiMH memory effect myth
- Unlike NiCad, NiMH is not prone to true memory effect. Reduced capacity is far more likely from cell aging, internal resistance, or imbalance.
- Lithium-ion battery
- Higher energy density than NiMH but sensitive to heat, over-charge, and over-discharge; relies on careful thermal management and BMS control.
- Battery management system (BMS)
- Electronics that monitor cell voltages and temperature, balance cells, manage state of charge, and limit charge/discharge to protect the HV battery.
- State of charge (SOC)
- Remaining HV battery energy as a percent of usable capacity, kept by the BMS within a protective window (not 0-100% of physical capacity).
- Cell balancing
- The BMS equalizes individual cell charge so no cell is over- or under-charged, protecting pack life and capacity.
- Reduced capacity / poor charge acceptance
- Usually from cell aging, increased internal resistance, or cell imbalance — not from NiMH memory effect.
- Weak or low module diagnosis
- Measure individual module voltages with a CAT III meter (system de-energized) and compare each to the rated value to find a weak or shorted module.
- Bus bar connection
- The conductive bars linking battery cells/modules. Loose or corroded bus bars raise resistance and hurt performance; torque to spec and keep clean.
- Liquid-cooled HV battery
- Many lithium-ion packs use a liquid cooling loop to control cell temperature; coolant level/condition and pump flow affect battery health and life.
- Lithium-ion pack storage
- Store a removed lithium-ion pack per the OEM: at a moderate state of charge, in a cool, dry, fire-safe location away from ignition sources.
- Reduced regen acceptance (battery)
- A full, cold, or near-SOC-limit battery limits how much regenerative energy it can accept — often normal, BMS-protective behavior.
- HV warning labels
- High-voltage components carry warning labels and orange marking; observe them and the OEM procedure before any contact.
- Why CAT III, not just voltage rating
- The category rating, not just max voltage, decides survival of transient spikes. A high-voltage but low-CAT meter can fail catastrophically on HV.
- Two main HV hazards
- Electric shock (current through the body) and arc flash (a rapid energy release that burns and blasts). Both demand correct PPE and de-energizing.
- Wheel chocks / parking brake
- Before HV service, set the parking brake and chock the wheels so the vehicle can't move while powered down for work.
- Pack splits into non-lethal halves
- Removing the service disconnect typically divides the pack so neither half is at the full lethal pack voltage during service.
- First-responder / service precautions
- Treat the HV system as live, use insulated tools and PPE, follow OEM cut/loop locations, and never bridge HV terminals or improvise grounding.
- Insulated tools
- Tools rated and labeled for high-voltage work used when there is any risk of contacting energized HV; part of layered HV protection.
- Measuring an individual module
- After verifying zero, probe across module terminals with a CAT III meter set to DC volts and compare to the rated nominal voltage.
- Healthy isolation reading
- A pack-to-chassis insulation reading well above the OEM minimum, in the high megohm range, indicates good isolation.
- Thermal management importance
- Heat is the enemy of lithium-ion cells; the cooling system and BMS thermal limits protect life, capacity, and safety.
- Inverter
- Power-electronics module that converts battery DC to three-phase AC to drive the traction motor, and AC back to DC during regen and charging.
- IGBT
- Insulated-gate bipolar transistor — the high-current switching device inside the inverter that synthesizes the AC waveform. Overheats if cooling fails.
- Three-phase AC traction motor drive
- The inverter feeds three AC phases to the motor; controlling their timing and amplitude sets motor torque and speed.
- DC-to-DC converter
- Steps the high-voltage battery (200-450 V) down to about 12-14 V to charge the 12-volt battery and run accessories — replaces the alternator.
- DC-DC converter failure
- If it fails, the 12-volt battery discharges and low-voltage systems (lights, modules, contactor coils) lose power.
- HV contactors
- Relays inside the battery pack that connect or isolate the HV battery from the rest of the vehicle.
- Pre-charge circuit
- Limits the inrush current as the HV contactors close, protecting the contacts and the DC-link capacitors from damage.
- HV interlock loop (HVIL)
- A low-voltage safety circuit run in series through HV connectors and covers. Opening any of them breaks the loop and de-energizes the HV system.
- HVIL fault
- A damaged or open interlock loop (e.g., a bent connector pin) can set a false interlock/isolation fault and keep the HV system from energizing.
- Power inverter module (PIM)
- The assembly housing the inverter (and often the DC-DC converter and controls) that manages high-current motor drive.
- Inverter cooling
- Often a dedicated liquid loop; poor heat-sink contact or failed thermal paste causes IGBT overheating — a common inverter failure.
- Capacitor discharge before service
- DC-link capacitors smooth the DC bus and hold a lethal charge; the bleed-down wait must elapse before verifying zero in the power electronics.
- Symptom: no electric drive, battery OK
- Often points to the inverter or its motor-control outputs rather than the pack; verify three-phase outputs and inverter cooling.
- Rectification (regen)
- During regen the inverter rectifies the motor-generator's AC output to DC to charge the HV battery.
- Inverter diagnosis essentials
- De-energize and verify zero, check cooling, inspect HV connectors/HVIL, then test the inverter's outputs and control signals per OEM.
- Contactor function
- Open contactors isolate the battery for safety; closed contactors connect it to the inverter and the rest of the HV system.
- Why HV connectors de-energize the system
- They sit in the HVIL; opening one breaks the interlock loop, which commands the contactors open and drops the high voltage.
- Onboard charger
- Converts AC from a charging station to DC to charge the HV battery; in some EVs it is bidirectional for vehicle-to-load/grid.
- DC fast charging
- External DC charging feeds the pack directly, bypassing the onboard AC charger, for much faster charging on capable vehicles.
- Thermal paste / heat sink (power module)
- Transfers heat from the IGBTs/power module to the cooling system; degraded paste or poor contact causes overheating and failure.
- Motor-generator (MG)
- An electric machine that acts as a motor (adding drive torque) and as a generator (producing electricity during regen or engine-driven charging).
- Two motor-generators (typical hybrid)
- Hybrids often use two: one as a starter/generator (engine start, charging) and one as the main traction motor.
- Three-phase AC machine
- Most traction motors are three-phase AC (permanent-magnet or induction) fed by the inverter; the phase timing sets torque and speed.
- Power-split transaxle (eCVT)
- A planetary-gear transaxle that blends engine and electric power continuously, behaving like a CVT so the engine runs in its efficient range.
- Planetary gearset (power split)
- The sun, planet carrier, and ring gears combine engine and two motor-generators to split power between driving the wheels and generating.
- Single-speed reduction gear (BEV)
- Many battery EVs use a single fixed-ratio reduction gear from the motor to the wheels — no multi-speed transmission needed.
- Electric creep in Drive
- A power-split hybrid can creep forward in Drive using electric motor torque alone, with the engine off.
- Winding-to-ground insulation test
- A megohmmeter test between each motor winding and the case checks for loss of isolation; a low reading means insulation breakdown.
- Loss of winding isolation
- Near-zero resistance between a winding and the motor case means the insulation has broken down — a shock hazard and motor failure.
- Rotor / resolver position sensor
- Reports the motor rotor's position so the inverter can time the AC phases; a fault causes rough running or no drive.
- Transaxle whine rising with speed
- A steady whine that rises with road speed usually points to gear or bearing wear inside the hybrid transaxle.
- Transaxle / drive-unit fluid service
- Use the OEM-specified fluid and procedure; the electric drive unit and gears rely on correct lubrication and, often, shared cooling.
- De-energize before opening a motor
- Always de-power and verify zero before removing or opening an HV motor or its inverter connections.
- Shared liquid cooling (drive unit)
- The electric drive unit may share a cooling circuit with the power electronics; a coolant problem can affect both.
- FWD hybrid clicking on turns
- A clicking noise from the front on turns points to a worn outer CV joint, just as on a conventional FWD vehicle.
- No-move complaint (BEV)
- Diagnose systematically: confirm HV ready, check for faults, verify inverter output and the reduction-gear/drive-unit mechanically.
- Permanent-magnet motor
- Uses magnets on the rotor; efficient and compact, common in EV traction motors. Back-EMF from spinning magnets is present even unpowered.
- AC induction motor
- Uses induced rotor currents instead of magnets; rugged and used by some EVs for traction.
- Bearing / mechanical noise vs electrical fault
- Mechanical noises (whine, growl, click) point to gears, bearings, or CV joints; electrical faults show as no/rough drive or isolation codes.
- Motor as generator (engine charging)
- The engine can spin a motor-generator to recharge the HV pack, separate from regen — a hybrid maintains its own state of charge.
- Auto start-stop
- Shuts the internal-combustion engine off when it isn't needed (idle, low load) to save fuel; restarted quickly by a motor-generator.
- Integrated starter-generator (ISG)
- A motor-generator that starts the engine quickly and smoothly and can also generate, replacing a conventional starter and alternator.
- Electric coolant pump
- Keeps engine coolant circulating while the engine is auto-stopped (a belt-driven pump would stop with the engine).
- Engine cycling under computer control
- The hybrid engine starts and stops automatically; interpret performance data knowing the engine may not be running continuously.
- Lean fuel-trim code
- A long-term fuel trim well into positive (lean) territory points to a vacuum/air leak or low fuel delivery — same diagnosis as a conventional engine.
- Relearn / verification after service
- After engine or hybrid service, a relearn or verification of auto start-stop and emissions monitors may be required for correct restarts.
- Atkinson-cycle engine
- Many hybrids use an Atkinson-cycle engine — more efficient but lower power density — because the electric motor fills in torque.
- Engine-off electric driving
- Under light load or low speed a hybrid can drive on the motor alone with the engine off, then restart the engine when more power is needed.
- Why the engine still matters on L3
- Most hybrids still carry a gasoline engine; conventional engine-performance diagnosis (misfires, trims, sensors) applies within the hybrid context.
- Engine restart smoothness
- A motor-generator spins the engine to speed before fueling for a smooth restart; a rough restart can signal an MG, sensor, or control fault.
- Emissions monitors and auto-stop
- Frequent engine stops can affect when OBD-II monitors run; a relearn or drive cycle may be needed to set readiness after service.
- Engine vs electric heat source
- On a hybrid the engine can be a heat source for the cabin, but with frequent engine-off operation, electric heating is often needed.
- Vacuum source with engine off
- Brakes and other systems that relied on engine vacuum may use an electric vacuum pump or brake-by-wire since the engine isn't always running.
- Battery state-of-charge maintenance
- A hybrid runs the engine as needed to keep the HV battery within its target SOC window, independent of plugging in.
- Engine cooling integration
- Hybrid engines may share or coordinate cooling with the power electronics and drive unit; check the correct loop for a given complaint.
- Regenerative braking
- Recovers kinetic energy during deceleration by back-driving the motor-generator as a generator and charging the HV battery instead of wasting heat.
- Brake-by-wire / brake blending
- Electronically blends regenerative and hydraulic friction braking to deliver exactly the deceleration the driver requests.
- Regen blend-out at low speed
- As the vehicle nears a stop, regen torque fades out and friction braking takes over; poor blending feels inconsistent to the driver.
- Regen limited by battery
- A full, cold, or near-SOC-limit battery reduces how much regen energy it can accept, so the friction brakes do more of the work.
- Diagnosing weak regen
- Use a scan tool to compare commanded versus actual regenerative torque to find why regen is weaker than normal.
- Electric A/C compressor
- Driven by its own HV motor (not a belt), so it can cool the cabin while the engine is off; speed is controlled electronically.
- Non-conductive refrigerant oil
- Electric A/C compressors require special electrically non-conductive (POE-type) oil; the wrong oil can cause a loss of HV isolation.
- PTC (resistance) cabin heater
- Passes high-voltage current through a ceramic element to make heat for the cabin; effective but a heavy drain on range in cold weather.
- Heat-pump HVAC
- Moves heat rather than generating it, so it heats the cabin far more efficiently than a PTC resistance heater, reducing cold-weather range loss.
- Electric power steering (EPS)
- Provides steering assist electrically; it depends on a healthy 12-volt supply, so a weak 12-volt system can cause heavy or lost assist.
- Onboard charger (supporting view)
- Converts station AC to DC to charge the HV battery; Level 1/2 AC charging uses it, while DC fast charging bypasses it.
- Vehicle-to-grid / vehicle-to-load (V2G/V2L)
- A bidirectional onboard charger lets the vehicle discharge HV battery energy back to the grid or to power external loads.
- Cabin heating range cost
- Resistance (PTC) heating draws a lot of HV power; weak heat plus high cold-weather range loss often points to PTC heater operation or a fault.
- Level 2 charging fault example
- A vehicle that charges on Level 2 AC but reports a charging fault may have an onboard-charger, communication, or connector problem.
- Regen primary advantage
- Compared with friction braking, regen recovers kinetic energy and returns it to the HV battery, improving efficiency and reducing brake wear.
- Friction brakes still required
- Regen can't supply all braking (low speed, full/cold battery, hard stops), so hydraulic friction brakes blend in to complete the stop safely.
- Brake-by-wire service caution
- Servicing a brake-by-wire system requires the correct OEM procedure and bleed/initialization steps so blending works correctly afterward.
- Customer 'regen stopped working'
- Check for a full/cold battery or a BMS limiting regen acceptance, and compare commanded versus actual regen torque with a scan tool.
- 12-volt system importance
- Many supporting systems (EPS, contactor coils, modules) need a healthy 12-volt supply kept charged by the DC-DC converter.
- Electric compressor speed control
- Because it is electronically driven, the electric A/C compressor's speed (and cooling capacity) is controlled by the HVAC system, not engine RPM.
- Heat pump vs PTC efficiency
- A heat pump uses much less energy than a PTC heater for the same cabin heat, preserving more driving range in cold weather.
- Supporting-system isolation risk
- HV-powered accessories (compressor, heater) are on orange cabling; de-energize and verify zero before servicing them.
- Power assist loss diagnosis (EPS)
- Heavy or absent steering assist often traces to a low/weak 12-volt system causing the EPS module to reduce or drop assist.
- Bidirectional charging requirement
- V2G/V2L needs a bidirectional onboard charger and compatible equipment to safely export HV battery energy.
- Supporting systems overview
- The HV-powered or HV-aware accessories: regenerative braking, electric A/C and heating, electric power steering, and the charging system.
- Top dead center (engine context)
- On the hybrid's engine, conventional timing references like TDC still apply for valve timing and engine-performance diagnosis.
- Hybrid powertrain control module
- Coordinates engine, motor-generators, battery, and braking so power flows seamlessly; many faults are network/communication-related.
- Contactor welding
- A stuck-closed (welded) contactor keeps the HV connected when it should isolate — a serious safety fault detected by the BMS.
- Pre-charge resistor fault
- If pre-charge fails, contactors see high inrush; the system may refuse to close the main contactors to prevent damage.
- Inverter no DC-to-AC conversion
- Loss of DC-to-AC conversion (a failed main inverter module) most directly stops the traction motor from receiving drive power.
- IGBT overheating cause
- Insufficient cooling — failed coolant flow, poor heat-sink contact, or degraded thermal paste — leads to IGBT overheating and failure.
- DC-DC output check
- Verify the DC-DC converter output (about 12-14 V) when the 12-volt battery won't stay charged on a hybrid/EV with no alternator.
- HV connector and dash warning
- Disconnecting an HV connector can trigger the HVIL, set a fault, and display an HV warning — by design, to protect the technician.
- Power module reinstallation
- When reinstalling a power module after replacing thermal interface material, follow torque and seating specs so cooling contact is correct.
- Inverter mismanagement of power
- A power inverter module may mismanage motor power from a control/sensor fault, overheating, or a failed switching device (IGBT).
- Why orange marks supporting HV parts too
- Any HV-powered accessory (A/C compressor, heater, cables) is marked orange and must be treated as live until verified at zero.
- Isolation test before HV work
- Confirm HV isolation to chassis is healthy (high megohms) as part of diagnosing shock-hazard or isolation-fault complaints.
- Smart key out of range
- Moving the key/fob out of range during de-power ensures the vehicle can't wake and close the HV contactors.
- Why not a 12-volt test light on HV
- A 12-volt test light can't confirm HV module voltage or zero energy; use a CAT III meter set to DC volts for HV measurements.
- Capacitor charge after circuit break
- Opening the HV circuit does not instantly make it safe — DC-link capacitors hold a lethal charge until the bleed-down time passes.
- PPE inspection
- Inspect insulating gloves for damage before each use; a pinhole or cut destroys their protection. Periodic dielectric testing is also required.
- Battery thermal runaway
- An uncontrolled exothermic reaction in a lithium-ion cell; managed by thermal control and BMS limits, and a reason for careful storage/handling.
- Module voltage comparison
- Comparing each module's voltage to its rated value (after verifying zero pack hazard at the test point) finds a weak or shorted module.
- Coolant in HV battery loop
- A liquid-cooled pack relies on correct coolant and flow; low or degraded coolant can let cells overheat and lose performance.
- HV system isolation by design
- The HV system is intentionally isolated (ungrounded) from the chassis so a single fault doesn't energize the body; isolation monitoring guards this.
- Traction motor back-EMF
- A spinning permanent-magnet motor generates voltage (back-EMF) even when unpowered, so its terminals can be live while the wheels turn.
- Resolver
- A rotary position sensor on the traction motor that tells the inverter the rotor angle for precise AC phase control.
- Drive-unit gear reduction
- Reduction gearing matches the high-speed electric motor to wheel speed and multiplies torque; wear shows as whine or growl.
- CV joint vs gear noise
- Clicking on turns = CV joint; a steady speed-related whine = gear/bearing wear — distinguish before disassembly.
- Power-split creep behavior
- A power-split hybrid can creep on electric torque alone in Drive; the engine starts only when more power or charging is needed.
- Motor insulation health
- High winding-to-ground resistance (megohms) = healthy; a low reading means insulation breakdown to the case and a failed or unsafe motor.
- Three-phase balance
- The motor's three phases should be balanced; large resistance or insulation differences between phases indicate a winding fault.
- Electric drive efficiency
- Electric drive is efficient and provides instant torque; regen recovers energy on deceleration, improving overall vehicle efficiency.
- Single reduction-gear BEV no-move
- On a BEV with one reduction gear, a no-move fault means checking HV readiness, inverter output, and the drive unit mechanically.
- Atkinson vs Otto cycle
- Atkinson trades peak power for efficiency by effectively shortening the compression stroke; hybrids use it because the motor adds torque.
- Engine auto-stop coolant flow
- An electric coolant pump maintains flow during auto-stop so the engine and heater core stay at temperature for the next restart.
- Verifying a clean restart
- After an auto-stop, confirm the engine restarts smoothly and on time; a delayed/rough restart can mean an MG, sensor, or control issue.
- Engine-off accessory power
- With the engine off, the HV battery and DC-DC converter power accessories that an engine-driven belt would normally run.
- Misfire diagnosis on a hybrid
- Conventional misfire diagnosis applies, but interpret freeze-frame data knowing the engine may stop/start under hybrid control.
- Fuel system on a hybrid
- Hybrids still have a conventional fuel system; sealed/evaporative systems may differ slightly to handle long engine-off periods.
- Engine as range extender
- In some plug-in hybrids the engine mainly drives a generator to recharge the pack (series operation) rather than the wheels directly.
- Series vs parallel vs series-parallel
- Series: engine drives a generator only. Parallel: engine and motor drive the wheels. Series-parallel (power-split): both, blended by a planetary set.
- Mild hybrid
- Uses a small motor-generator (often 48-volt) to assist and enable start-stop, but can't drive on electric power alone.
- Full (strong) hybrid
- Can drive on electric power alone for short distances, blend power, and recharge via regen and the engine — e.g., a power-split hybrid.
- Plug-in hybrid (PHEV)
- A hybrid with a larger battery that plugs in for meaningful electric-only range, then operates as a hybrid once the charge is used.
- Battery electric vehicle (BEV)
- An all-electric vehicle with no internal-combustion engine; propulsion is entirely from the HV battery, inverter, and traction motor.
- Regen one-pedal driving
- Strong regen on lift-off lets the driver slow the car with the accelerator alone; friction brakes still finish hard or low-speed stops.
- Brake fluid in brake-by-wire
- Brake-by-wire still uses hydraulic friction brakes; correct fluid, bleeding, and initialization per OEM are required after service.
- Cold-weather range loss
- Cabin heating (especially PTC), reduced battery performance, and higher rolling losses cut range in cold weather.
- Heat pump operation
- Uses a refrigerant cycle to move ambient or waste heat into the cabin, delivering more heat per unit of energy than resistance heating.
- 12-volt battery role on EV/hybrid
- Powers low-voltage controls and the contactor coils; kept charged by the DC-DC converter. A dead 12-volt can prevent HV start-up.
- Charging connector standards
- AC (Level 1/2) and DC fast-charging connectors differ; the onboard charger handles AC, while DC charging feeds the pack directly.
- Regen and brake light behavior
- Strong regenerative deceleration may activate the brake lights even without the friction brakes, for following-driver safety.
- Accessory belt (or lack of)
- Many hybrids/EVs eliminate engine-driven accessory belts, using electric pumps and compressors so accessories run with the engine off.
- Scan-tool HV data
- A scan tool reads pack voltage, SOC, cell/module data, temperatures, and commanded vs actual values to diagnose HV systems without exposure.
- OBD-II on hybrids
- Hybrids use OBD-II for emissions monitoring and store DTCs; HV and hybrid-specific data may need an enhanced/OEM scan tool.
- Freeze-frame data
- A snapshot of sensor readings captured when a code set, used to recreate the conditions of an engine or hybrid fault.
- DTC retrieval on a hybrid
- Retrieve codes with a scan tool through the data link; do not improvise HV probing to chase a code — use the tool and OEM data.
- Service plug location
- The HV service disconnect is commonly under a rear seat, trunk/cargo floor, or an access panel near the battery pack.
- Personnel barrier / signage
- Cordon off an HV work area and post signage so others don't approach an energized or exposed HV system during service.
- Two-person rule (HV)
- Some OEMs require a trained second person nearby during exposed HV work to respond in an emergency.
- Dielectric glove testing interval
- Insulating gloves must be re-tested periodically (commonly every 6 months) and inspected before each use; failed gloves are removed from service.
- Metal jewelry removal
- Remove rings, watches, and metal jewelry before HV work; conductive items create a shock and arc-flash risk.
- HV cable routing/damage
- Inspect orange HV cables for chafing or damage; a compromised cable can cause loss of isolation or a shock hazard.
- Battery pack venting
- Lithium-ion packs have venting and thermal safeguards; never block vents, and handle damaged packs per OEM fire-safety procedures.
- Inverter three-phase output test
- With OEM procedure, verify balanced three-phase outputs from the inverter to confirm it can drive the motor.
- Communication (CAN) faults
- Many hybrid faults are network/communication issues between modules; check bus integrity before condemning HV hardware.
- Contactor coil power
- The 12-volt system energizes the contactor coils; a weak 12-volt supply can prevent the contactors from closing to enable HV.
- Power electronics liquid cooling
- Inverter and converter often share or have dedicated liquid cooling; check coolant level, flow, and the pump for overheating faults.
- Capacitor in DC bus
- Stabilizes the DC voltage feeding the inverter; its stored charge is the reason for the mandatory bleed-down wait.
- Generator mode (MG1)
- In many power-split hybrids one motor-generator (MG1) primarily generates and starts the engine, while MG2 mainly drives.
- Traction motor mode (MG2)
- The larger motor-generator (MG2) provides the main drive torque and the regenerative braking generation.
- Drive-unit insulation diagnosis
- Use a megohmmeter on de-energized windings; compare to spec to confirm isolation between windings and the case before reuse.
- Engine-off vacuum brakes
- Because the engine isn't always running, hybrids supply braking assist via an electric vacuum pump or brake-by-wire system.
- Refrigerant service (electric compressor)
- Use HV/EV-rated recovery equipment and the correct non-conductive oil; cross-contaminating oils can damage the compressor and isolation.
- HVAC and range tradeoff
- Heating and cooling draw from the HV battery, so HVAC use directly affects driving range — most in extreme temperatures.
- Onboard charger communication
- The charger negotiates with the station (e.g., for AC charging); a communication or connector fault can stop charging even if the charger is good.
- Regen torque blending diagnosis
- Compare commanded and actual regen torque and confirm brake-by-wire blending to diagnose inconsistent deceleration complaints.
- 12-volt vs HV systems boundary
- Low-voltage (12 V) runs controls/accessories; high-voltage (200-450 V) drives propulsion. The DC-DC converter bridges them.
- Why no alternator
- A hybrid/EV has no belt-driven alternator; the DC-DC converter charges the 12-volt battery from the HV pack instead.
- Pre-charge sequence on power-up
- On power-up the BMS typically closes a pre-charge path first to charge the DC-link capacitors, then closes the main contactors.
- Loss of isolation while driving
- An isolation fault can occur in service; the vehicle warns the driver and may reduce power to limit the shock hazard.
- Battery cooling fan/intake (air-cooled)
- Some packs are air-cooled; a blocked intake or failed fan lets the pack overheat and lose performance.
- Technician A / Technician B format
- The ASE question style: judge each technician's statement separately as true or false, then choose A only, B only, both, or neither.
- L3 study priority
- Master high-voltage safety first (tested everywhere), then weight study toward Power Electronics and the HV Battery System, the two largest scored areas.
