- Voltage (V)
- The electrical pressure (potential difference) that pushes current through a circuit, measured in volts. In Ohm's law, V = I × R.
- Current (I)
- The flow of electric charge through a circuit, measured in amperes (amps). In Ohm's law, I = V ÷ R.
- Resistance (R)
- Opposition to current flow, measured in ohms (Ω). Higher resistance lets less current flow at a given voltage. R = V ÷ I.
- Ohm's law
- The relationship between voltage, current, and resistance: V = I × R. Rearranged: I = V ÷ R and R = V ÷ I.
- Power formula (watts)
- Electrical power equals voltage times current: P = V × I, measured in watts. Also P = I² × R.
- Ampere (amp)
- The unit of electric current — the rate of charge flow. One amp equals one coulomb of charge per second.
- Volt
- The unit of electrical potential difference (pressure). It drives current through resistance.
- Ohm (Ω)
- The unit of electrical resistance. One ohm allows one amp to flow under one volt of pressure.
- Watt
- The unit of electrical power — the rate of doing electrical work. P = V × I.
- Conductor
- A material (like copper) that allows electrons to flow easily because of low resistance.
- Insulator
- A material (like rubber or plastic) that resists electron flow and is used to contain and direct current.
- Semiconductor
- A material (like silicon) that conducts under some conditions and not others — the basis of diodes, transistors, and modules.
- Series circuit
- A circuit with one path. Current is the same everywhere, voltage divides across loads, and resistances add (R = R₁ + R₂ + …).
- Parallel circuit
- A circuit with multiple branches. Voltage is the same across each branch, current divides, and total resistance is less than the smallest branch.
- Series-parallel circuit
- A circuit combining series and parallel sections — common in real vehicle wiring with shared and branched paths.
- Open circuit
- A break in the current path so no current flows. The device is dead; a voltmeter reads source voltage up to the break, 0 beyond it.
- Short circuit
- An unintended low-resistance path. A short to ground bypasses the load, spikes current, and blows the fuse.
- Short to ground
- An unintended path to ground before the load. Current spikes and the fuse blows (or keeps blowing).
- Short to power (B+)
- An unintended connection to battery voltage that can backfeed a circuit and operate a device when it should be off.
- High resistance
- Unwanted resistance from corrosion, a loose/dirty connection, or a damaged wire. The device runs weak; voltage-drop testing locates it.
- Voltage drop
- The voltage lost across a part while current flows. Measured under load, a high drop reveals unwanted resistance.
- Voltage drop test
- Measuring voltage across a wire/connection/ground with the circuit on and under load. Good connection drops near 0 V; under ~0.1 V is the goal.
- Digital multimeter (DMM)
- The primary electrical test tool — measures voltage, current, and resistance. Its high input impedance loads the circuit very little.
- Input impedance (DMM)
- A meter's internal resistance. High input impedance means the DMM draws almost no current, so it won't disturb sensitive circuits.
- Measuring resistance (ohms)
- Done with the circuit de-energized and the component ideally isolated. The meter supplies its own test current; a live circuit gives a false reading.
- Measuring current (amps)
- The meter is connected in series so all current flows through it, or read with an inductive (clamp) ammeter around the wire.
- Measuring voltage
- The meter is connected in parallel (across) the part. Reads potential difference; done on a live circuit.
- Inductive (clamp) ammeter
- A meter that reads current by sensing the magnetic field around a wire — no need to break the circuit.
- Continuity test
- Checks for an unbroken path. The meter beeps/reads near 0 Ω if continuous; an open reads infinite (OL).
- Fuse
- A sacrificial link that melts to open the circuit when current exceeds its rating, protecting the wiring from a short.
- Circuit breaker
- A resettable protective device that opens on overcurrent and either resets automatically or must be reset manually.
- Fusible link
- A short length of smaller-gauge wire that melts to protect a high-current circuit when a fuse would be impractical.
- Ground
- The common return path (usually the vehicle body/chassis) back to the battery negative. A bad ground causes high resistance and odd symptoms.
- Wiring diagram (schematic)
- A drawing of how a circuit is connected, with components, colors, connectors, and grounds — essential for systematic diagnosis.
- Scan tool
- A device that communicates with vehicle modules to read DTCs, live data, freeze frame, and run bidirectional controls.
- Diagnostic trouble code (DTC)
- A standardized fault code a module stores when it detects a problem; read with a scan tool to point toward the affected circuit.
- Bidirectional control
- A scan-tool function that commands a module to operate an output (e.g., cycle a relay or motor) to confirm it works.
- Test light
- A simple lamp probe that lights when voltage is present. Useful for power/ground checks but cannot measure or load sensitive circuits.
- Logic probe
- A tool that indicates whether a point is high, low, or pulsing — useful on digital and module circuits.
- Oscilloscope (lab scope)
- Displays voltage over time as a waveform, revealing glitches, ripple, and signal patterns a DMM averages away.
- Diode
- A semiconductor that allows current in one direction only. Used in rectifiers and to block backfeed.
- Transistor
- A semiconductor that switches or amplifies current; modules use transistors to control outputs.
- Capacitor
- A device that stores electrical charge and can smooth voltage or suppress noise in a circuit.
- Resistor
- A component that adds a specific resistance to limit current or divide voltage in a circuit.
- Kirchhoff's voltage law (concept)
- The sum of voltage drops around a series loop equals the source voltage — why voltage divides across series loads.
- Polarity
- The positive/negative orientation of a circuit. Correct polarity matters for diodes, modules, and battery connections.
- Backprobe
- Probing a connector from the wire (back) side without disconnecting it, so the circuit can be tested live and under load.
- Available voltage
- The voltage actually reaching a load. Voltage drops in the wiring reduce available voltage and weaken the device.
- Load
- The device that does the work in a circuit (lamp, motor, solenoid) and provides the resistance that limits current.
- Electromagnetic induction
- Generating voltage by moving a conductor through a magnetic field — the principle behind alternators and many sensors.
- Lead-acid battery
- The common automotive battery using lead plates in sulfuric-acid electrolyte. Provides ~12.6 V fully charged.
- State of charge
- How full a battery is, judged by open-circuit voltage (~12.6 V = full) or specific gravity. Separate from capacity.
- Open-circuit voltage
- A rested battery's voltage with no load. ~12.6 V = full, ~12.4 V = ~75%, ~12.0 V = discharged.
- Specific gravity
- The density of battery electrolyte vs. water, read with a hydrometer. ~1.265–1.280 indicates a full cell.
- Hydrometer
- A tool that measures electrolyte specific gravity to judge state of charge per cell on non-sealed batteries.
- Cold cranking amps (CCA)
- Current a battery delivers at -18°C (0°F) for 30 seconds while holding ≥7.2 V. Rates cold-start ability.
- Reserve capacity
- Minutes a fully charged battery can supply ~25 amps at 27°C (80°F) before dropping to 10.5 V.
- Load (capacity) test
- Applies ~half the CCA for 15 seconds; voltage must stay above ~9.6 V at 21°C (70°F) to pass.
- Conductance tester
- An electronic tester that estimates battery capacity/CCA without applying a heavy load.
- Charge before load test
- A battery must be charged to at least 75% before load testing, or the result unfairly fails it.
- AGM battery
- An Absorbed Glass Mat battery — sealed, spill-proof, with electrolyte held in glass mats; handles deep cycling and many accessories.
- Parasitic draw
- Key-off current the vehicle draws to keep memories alive. Excessive draw drains the battery overnight.
- Parasitic draw test
- Ammeter in series with the battery (or low-current clamp) read after modules sleep; pull fuses to find the offending circuit.
- Module sleep time
- Modules may take 20–60 minutes to power down after key-off; wait before reading parasitic draw or the value is falsely high.
- Battery sulfation
- Lead-sulfate crystals building on plates when a battery sits discharged, reducing capacity and the ability to take a charge.
- Surface charge
- A temporary higher voltage on a recently charged battery. Remove it (apply a brief load) before an accurate voltage reading.
- Battery terminal corrosion
- White/blue buildup that adds resistance, causing voltage drop, hard starting, and charging complaints.
- Starter motor
- A high-current DC motor that cranks the engine for starting, drawing heavy amperage from the battery.
- Starter solenoid
- An electromagnet that engages the starter drive into the flywheel and switches the heavy starter current.
- Starter current draw test
- Measures cranking amps. High draw + slow crank = worn starter/dragging engine; low draw + slow crank = high cable resistance.
- Cranking voltage drop test
- Measures drop on the positive cable, ground, and solenoid while cranking to find resistance in the starting circuit.
- Neutral safety / range switch
- A switch that allows cranking only in Park or Neutral; a fault can cause a no-crank condition.
- Starter relay
- A relay that lets the ignition switch's small current control the heavy starter/solenoid current.
- Clicking, no-crank
- Often a low battery or high-resistance connection — the solenoid pulls in but can't sustain the heavy starter current.
- Slow cranking causes
- Discharged/weak battery, high-resistance cables or grounds, a dragging starter, or a mechanically tight engine.
- Overrunning clutch (starter drive)
- A one-way clutch in the starter drive that lets the engine spin faster than the starter without driving it back.
- Battery cable sizing
- Heavy-gauge cables carry hundreds of cranking amps; undersized or corroded cables add resistance and cause slow cranking.
- Memory saver
- A device that maintains module/clock/radio memory while the battery is disconnected for service.
- Battery disconnect precautions
- Remove the negative (ground) terminal first and reconnect it last to avoid sparks and shorts.
- Maintenance-free battery
- A sealed lead-acid battery you cannot add water to; state of charge is checked by voltage or a built-in eye, not a hydrometer.
- Charging system
- Keeps the battery topped up and powers loads while the engine runs, using the alternator and voltage regulator.
- Alternator (AC generator)
- A belt-driven generator that spins a rotor in a stator to make AC, then rectifies it to DC for the vehicle.
- Rotor (field)
- The spinning electromagnet inside the alternator; field current through it controls how much voltage the alternator makes.
- Stator
- The stationary windings in the alternator where AC voltage is induced as the rotor's magnetic field sweeps past.
- Rectifier (diodes)
- The diodes that convert the alternator's AC into DC. A failed diode causes high AC ripple and low output.
- Voltage regulator
- Controls the rotor field current to hold charging voltage in range (~13.5–14.5 V). Often built into the alternator or PCM.
- Normal charging voltage
- Roughly 13.5–14.5 V at the battery with the engine running and loads on.
- AC ripple
- Leftover AC in the alternator's DC output. Excessive ripple (DMM AC scale, often >0.5 V) = a failed rectifier diode.
- Charging output test
- Measure voltage and amperage at the battery with loads on. Too low = undercharge; too high = overcharge.
- Undercharging
- Charging voltage too low, leaving a discharged battery. Causes: bad regulator, worn brushes, slipping belt, high circuit resistance.
- Overcharging
- Charging voltage too high, boiling/gassing the battery and shortening its life. Usually a faulty voltage regulator.
- Charging-circuit voltage drop
- Drop on the alternator's output (B+) wire and ground that reduces voltage reaching the battery; tested under charging load.
- Drive belt (serpentine)
- Drives the alternator. A loose, glazed, or worn belt slips and causes low or erratic charging.
- Belt tensioner
- Keeps the serpentine belt at correct tension automatically; a weak tensioner causes belt slip and charging problems.
- Alternator 'S' (sense) terminal
- Lets the regulator sense system/battery voltage so it can adjust charging output accurately.
- Charge indicator lamp
- The dash light that warns of a charging fault; it should go out once the engine runs and the alternator charges.
- Diode (alternator) failure
- An open diode lowers output and raises AC ripple; a shorted diode can drain the battery (a key-off current path).
- PCM-controlled charging
- On many vehicles the PCM commands the alternator's field for smart charging based on load, temperature, and battery state.
- Full-fielding (test)
- Bypassing the regulator to force maximum alternator output, used to isolate a regulator fault from an alternator fault.
- Charging system load test
- Turn on accessories and verify the alternator maintains voltage and supplies rated current under load.
- Halogen bulb
- A headlamp that heats a filament inside a halogen-gas capsule — a simple resistive load.
- HID (xenon) lamp
- A high-intensity-discharge headlamp that strikes an arc using a high-voltage ballast/igniter. Don't touch the capsule; respect the high voltage.
- LED lamp
- A light-emitting-diode lamp using semiconductors and a driver; draws little current. One failed LED/driver can dim or kill it.
- Ballast (HID)
- The unit that supplies the high-voltage startup pulse and regulated current to ignite and run an HID lamp.
- Headlamp aiming
- Adjusting headlight beam aim to spec so the road is lit without blinding oncoming drivers.
- Hyperflash
- A rapid turn-signal flash signaling a burned-out (or low-current) bulb on that side. LED conversions can trigger it.
- Turn-signal flasher
- The device that cycles the turn signals on and off. Many modern systems flash via the BCM rather than a thermal flasher.
- Load resistor (LED)
- A resistor added to an LED turn signal so the system sees normal current and doesn't read 'bulb out' (hyperflash).
- Brake light switch
- A switch at the brake pedal that turns on the brake lights (and signals other modules) when the pedal is pressed.
- Daytime running lights (DRL)
- Lights that run automatically when the vehicle is on, for visibility; often dimmed headlights or dedicated lamps.
- Multifunction (combination) switch
- The steering-column switch that operates turn signals, high/low beam, headlights, and often wipers.
- Dimmer (high/low beam) switch
- Selects between low and high headlight beams; built into the multifunction switch on most vehicles.
- Courtesy/interior lights
- Dome and door lights, often controlled by door switches and the BCM with a timed delay.
- Bulb out, fuse good
- An open in that branch — a burned bulb, bad socket, corroded connector, or broken wire.
- Dim single headlight
- Usually a high-resistance connection or ground; confirm with a voltage-drop test rather than replacing the bulb.
- All lights affected
- Points to a shared cause — the BCM, a common ground, a master switch, or a network fault — not one bulb.
- Headlight relay
- A relay that switches the high headlight current so the switch only carries the small control current.
- Composite headlamp
- A headlamp assembly with a replaceable bulb in a fixed housing/lens, rather than a one-piece sealed beam.
- Sealed beam
- An older one-piece headlamp where the filament, reflector, and lens are sealed together and replaced as a unit.
- Auto headlamp (photocell)
- A light sensor that switches headlights on automatically in darkness via the BCM.
- Instrument cluster
- The networked module that displays speed, RPM, temperature, fuel level, and warnings, reading most data over a data bus.
- Sending unit
- A sensor (e.g., a fuel-tank float-and-rheostat) whose changing resistance reports a value to a gauge or module.
- Fuel level sensor
- A float and variable resistor in the tank; its resistance changes with level to drive the fuel gauge.
- Coolant temperature gauge
- Driven by a temperature sender (thermistor) whose resistance changes with engine coolant temperature.
- Thermistor
- A temperature-sensitive resistor used in temperature senders; resistance changes predictably with temperature.
- Speedometer / VSS
- Vehicle speed comes from a vehicle speed sensor or wheel-speed data sent over the bus to the cluster.
- Tachometer
- Displays engine RPM, taken from the engine/ignition signal and sent to the cluster.
- Warning lamp (telltale)
- An indicator light (oil, charge, ABS, airbag) that alerts the driver to a system condition or fault.
- Bulb check (prove-out)
- The brief lighting of warning lamps at key-on to prove the bulbs/LEDs work; a missing telltale may be a burned indicator.
- Whole cluster dark
- Usually a power, ground, or data-bus fault — not a single gauge.
- Single bad gauge
- Usually the gauge's sender or wiring, not the cluster — confirm the sender's resistance against spec.
- Fuel gauge stuck full
- Often an open or out-of-range sending-unit circuit or a bad sender ground; measure sender resistance at empty and full.
- Fuel gauge stuck empty
- Often a shorted sending-unit circuit or a sender grounded internally; check resistance and wiring.
- Odometer (electronic)
- Mileage stored electronically in the cluster (or another module) rather than mechanical gears.
- Driver information center (DIC)
- A display that shows trip data, warnings, and messages, fed by the cluster/BCM over the network.
- Cluster self-test
- A built-in diagnostic mode that sweeps gauges and lights telltales to confirm the cluster's operation.
- Data bus to cluster
- Most gauges get values as digital messages over a bus (e.g., CAN); a bus fault can blank or freeze the cluster.
- Oil pressure gauge/lamp
- Driven by an oil-pressure sender or switch; a lamp warns of low pressure, a gauge shows the value.
- Voltmeter (dash)
- A dash gauge showing system/charging voltage so the driver can spot a charging problem.
- Buzzer / chime module
- Provides audible warnings (key-in-ignition, seatbelt, door ajar), often driven by the BCM.
- Sender ground
- A poor sender ground raises resistance and skews the gauge reading — a common cause of inaccurate gauges.
- Stepper-motor gauge
- Modern analog gauges use a small stepper motor moved by the cluster; the needle is positioned electronically.
- Air-core gauge
- An older electromagnetic gauge whose needle position depends on current through its coils from the sender.
- Cluster power and ground
- Always verify the cluster's switched power, constant power, and ground before condemning it.
- Maintenance/oil-life reminder
- An algorithm in the cluster/PCM that estimates service intervals and displays reminders to the driver.
- Relay
- An electrically operated switch: a small coil current closes contacts to carry a much larger load current.
- Relay coil vs. contacts
- The coil (control) side is low current; the contact (load) side carries the high current to the device.
- Relay testing
- Swap an identical relay, check coil resistance, confirm the coil energizes, and verify switched voltage at the output.
- Solenoid
- An electromagnet that moves a plunger to do mechanical work, such as door locks or the starter drive.
- Body control module (BCM)
- The module managing many body accessories — lighting, locks, windows, wipers, chimes — and communicating over the network.
- Multiplexing
- Letting modules share data over a small two-wire network (a bus) instead of a separate wire per signal, cutting wiring weight.
- Controller Area Network (CAN)
- A common two-wire vehicle data bus for module communication. A bus fault can disable several systems at once.
- Network communication code (U-code)
- A DTC indicating a module has lost communication on the bus; read with a scan tool to find the dropped module.
- Pulse-width modulation (PWM)
- Switching a load on/off rapidly and varying the on-time (duty cycle) to control average power — used for motors and dimming.
- Duty cycle
- The percentage of time a PWM signal is on within each cycle. A higher duty cycle delivers more average power.
- Power window motor
- A reversible DC motor that raises/lowers a window, often PWM-controlled with anti-pinch protection.
- Anti-pinch (window)
- A feature that reverses a power window if it senses an obstruction while closing, for safety.
- Power door lock actuator
- A solenoid or small motor that locks/unlocks a door on command from the switch or BCM/remote.
- Blower motor
- The HVAC fan motor, controlled by a resistor pack (stepped speeds) or a PWM module (variable speed).
- Blower resistor pack
- Adds resistance to drop voltage for lower blower speeds. A burned resistor often leaves only high speed working.
- Wiper motor / park switch
- Drives the wipers; the park switch returns the blades to the rest position when switched off.
- Horn circuit
- Usually a relay-controlled circuit triggered by the horn switch; a dead horn is often a relay or its control.
- Remote keyless entry (RKE)
- A radio-frequency remote that locks/unlocks and arms the vehicle via a receiver and the BCM.
- Immobilizer / theft system
- A security system that prevents starting unless a recognized key/transponder is present.
- Transponder key
- A key with an embedded chip the immobilizer reads to authorize starting.
- Heated seats/mirrors
- Resistive heating elements controlled by switches/relays or the BCM; diagnosed like any heated-load circuit.
- Rear window defogger
- A grid of resistive lines on the glass that heat to clear fog/frost; a break opens that line.
- Cruise control (electronic)
- A system that maintains set speed using throttle control and inputs over the network.
- Audio/infotainment system
- Networked modules and amplifiers; diagnosed for power, ground, and bus communication like other body electronics.
- Backup camera/sensors
- Driver-assist accessories tied into the BCM/network and a display, common on modern vehicles.
- Sunroof motor
- A reversible motor with a control module that slides/tilts the sunroof, often with pinch protection.
- Module reflash / programming
- Updating or configuring a module's software with a scan tool, sometimes required after replacement.
- Ground distribution
- Shared ground points serving multiple accessories; a bad shared ground causes odd, multi-system symptoms.
- Backfeed
- Voltage feeding a circuit through an unintended path (e.g., a shared ground), causing a device to operate oddly.
- Normally open (N.O.) relay contacts
- Contacts that are open until the coil energizes, then close to power the load.
- Normally closed (N.C.) relay contacts
- Contacts that are closed until the coil energizes, then open.
- Connector/terminal repair
- Repairing corroded or spread terminals and bad connections — a frequent root cause of body-electrical faults.
- Component locator
- Service-information data showing where modules, grounds, splices, and connectors are physically located.
- Splice pack / junction
- A point where several wires join; a failed splice can affect multiple circuits at once.
- Smart junction box (SJB)
- A fuse/relay box with built-in control electronics, often performing BCM-style functions.
- Open vs. short (summary)
- Open = no current (device dead, fuse OK). Short to ground = too much current (fuse blows).
- Why most vehicle circuits are parallel
- So each device gets full battery voltage and one failure doesn't kill the others.
- Total series resistance
- Add the resistances: R = R₁ + R₂ + R₃ + … More loads in series means more total resistance and less current.
- Total parallel resistance
- Less than the smallest branch resistance, because added paths give current more ways to flow.
- Ground-side voltage drop
- Voltage lost on the return (ground) path. Should be near 0 V; high ground drop dims or weakens a device.
- Battery negative ground
- Standard modern vehicles connect the battery negative to the chassis as the common return path.
- Jump-start polarity
- Connect positive to positive, then negative to a good engine ground (not the dead battery) to reduce spark risk.
- Charging belt slip symptom
- Low/erratic charging voltage and possible squeal — check belt tension and condition before condemning the alternator.
- Diode pattern on a scope
- Good rectification shows even humps; a missing/uneven hump indicates an open or shorted diode.
- Lamp-out detection
- Some systems sense bulb current; a low-current LED or an open bulb triggers a 'lamp out' or hyperflash warning.
- High-beam indicator
- A blue dash telltale confirming the high beams are on, driven by the headlight/multifunction circuit.
- Gauge sweep at key-on
- Stepper-motor clusters sweep the needles to confirm gauge operation during the prove-out.
- Intermittent cluster fault
- Often a loose connector, flexing wire, or marginal ground; wiggle-test and check connections under load.
- BCM as accessory hub
- The BCM commands many accessories and shares status over the bus, so its faults can affect several systems.
- Diagnosing a dead accessory
- Confirm power in, ground, the control signal (switch/module), and the device itself — in that order.
- Scan-tool network topology
- A scan tool can show which modules respond on the bus, helping pinpoint a lost module or wiring fault.
- Corrosion as resistance
- Corroded terminals and grounds add resistance, the most common cause of dim lights, slow motors, and odd faults.
- Voltage drop vs. ohmmeter
- Voltage drop tests under load and catches high resistance an ohmmeter on a dead circuit may read as fine.
- Fuse keeps blowing
- Indicates a persistent short to ground; trace the circuit with the load disconnected to isolate the shorted section.
- Service information first
- Always pull the wiring diagram, specs, and component locations before probing — systematic beats random.
