Your FREE ASE T6 Flashcards 2026 – 200+ Cards

Voltage = current × resistance (V = I × R). Rearranged: I = V ÷ R and R = V ÷ I. Knowing any two values lets you solve for the third.

Electrical pressure (volts) that pushes current through a circuit. Cars and light/medium trucks use a nominal 12-volt system; some heavy trucks crank on 24 volts.

The rate of electron flow through a circuit, measured in amperes (amps). The source and the load determine how much current flows.

Opposition to current flow, measured in ohms (Ω). Unwanted resistance from corrosion or loose connections drops voltage and weakens the circuit.

P = V × I (watts = volts × amps). Power is the rate at which a circuit does work or dissipates heat.

The voltage lost across a wire, connection, or component while current flows. The best way to find high resistance.

Energize and load the circuit, set the DMM to DC volts, and connect it in parallel across the wire or connection. The reading is the voltage lost there.

Under load, a single connection should drop less than about 0.1 volt. More than that signals unwanted resistance to clean or repair.

Under load, a full cable or main ground should drop less than about 0.2 volt. More indicates a high-resistance connection.

Resistance only reveals itself as lost voltage under current flow. An ohmmeter (circuit off) can miss a fault that only appears under load.

The core T6 tool. Reads DC/AC volts, ohms, and amps. Measure volts in parallel, ohms with the circuit off, and amps in series.

Set the meter to DC volts, put the red lead on positive and black on negative, and read the display. A rested 12-volt battery reads about 12.6 volts.

Set the meter to amps and connect it in series so all the circuit current flows through it. Never connect an ammeter across (in parallel with) a load.

Disconnect the component from the circuit, set the meter to ohms, and place a lead on each terminal. Applied voltage gives false readings or damages the meter.

Set the meter to ohms/continuity with the circuit off. A good path reads near zero ohms or beeps; an open reads OL (infinite). Never check continuity on a live circuit.

A break in the path so no current flows and the load is dead. A voltmeter across the open in a live circuit reads near source voltage.

An unwanted low-resistance path to ground. Excessive current blows the fuse or fusible link, often repeatedly.

An unwanted connection to a power source that feeds a circuit it shouldn't, often blowing a fuse or back-feeding other circuits.

Corroded or loose connection. It does not blow a fuse but weakens the load (dim lights, slow motors) and shows up as voltage drop under load.

One current path; the same current flows through every component and resistances add. Source voltage divides across the components.

Two or more branches; each branch sees the same voltage and current divides. Total resistance is always less than the smallest branch.

Add the resistances: 2 Ω + 4 Ω + 6 Ω in series = 12 Ω.

One resistance divided by the number of branches: two 4-ohm lamps in parallel = 2 Ω.

The return path carrying current back to the battery negative, often through the chassis or engine block. A poor ground causes the same symptoms as a bad feed.

Dim or flickering lights, slow-operating motors, and erratic electronics on the affected circuits.

A protective device that opens when current exceeds its rating, protecting the wiring. Test it for voltage on both exposed tabs with the circuit live.

With the circuit powered, touch a test light or voltmeter to both exposed test points. Voltage on only one side means the fuse is open.

Usually a short to power or to ground drawing excessive current. Never install a larger fuse — find the short.

A length of smaller-gauge wire that burns open under heavy overload to protect a major circuit. Replace with the correct rated link, never a plain fuse or jumper.

Check for battery voltage on both ends with the circuit live, or remove it and check continuity. A blown link may look intact but feel bubbled or smell burnt.

An electromagnetic switch: a small control current energizes a coil that closes (or opens) heavy-current contacts.

Contacts are open at rest and close only when the coil is energized.

Contacts are closed at rest and open when the coil is energized.

85 and 86 are the coil; 30 and 87 are the load contacts (87a is the normally closed contact).

Apply battery voltage and ground to the coil (85/86); listen for a click and check continuity between the load terminals (30/87).

An electrical roadmap showing how components connect — wire colors, connector and pin numbers, splices, and the path from power to ground.

A circle with a wavy sine-wave line inside represents an AC voltage source or signal.

Shown as a series of shrinking horizontal lines or a downward triangle, marking the circuit's return path.

Replace the fuse with a circuit breaker or self-resetting load so current keeps flowing, then trace the harness with a meter or gauss probe while wiggling it.

A meter that graphs voltage over time. Best tool for catching intermittent or fast electrical events (and bus signals) a DMM averages out.

A simple bulb-and-probe tool that shows whether voltage is present. Useful for quick power/ground checks but not for precise voltage-drop measurement.

A tool that indicates high, low, or pulsing logic levels in a digital circuit, useful on electronic and data circuits.

Sharing data and control over a single data bus so several modules communicate without a separate wire per signal — cutting harness size and connections.

The standard high-speed CAN data network on modern medium/heavy trucks — 250 kbps over a twisted two-wire pair (CAN High and CAN Low).

The protocol J1939 is built on, using differential signaling over a twisted pair to reject electrical noise.

Nominal 250 kbps over a twisted two-wire pair (CAN High and CAN Low).

J1939 is CAN-based at 250 kbps on a twisted pair; J1708/J1587 is the older RS-485 link at 9.6 kbps. J1939 replaced it in the mid-2000s.

A 120-ohm resistor at each end of the J1939 backbone that absorbs signal reflections; the two in parallel measure about 60 ohms with power off.

About 60 ohms between CAN High and CAN Low with power off (two 120-ohm terminators in parallel).

One terminating resistor is missing/disconnected or there is an open in the backbone.

CAN High and CAN Low are shorted together.

An extra (third) terminating resistor is on the bus, in parallel with the normal two.

Disconnect battery power first, then read resistance between CAN High and CAN Low at the diagnostic connector.

Twisting and differential signaling reject the electrical noise common around a truck's high-current circuits.

An oscilloscope (or a min/max recording meter) captures transient events a test light or basic DMM misses. Wiggle-test the harness to provoke it.

Poor ground connections, which cause voltage fluctuations and intermittent behavior across modules.

Often indicates an overloaded circuit drawing higher-than-normal current.

Inserting the meter lead at the back of a connector to read voltage with the circuit live and connected, without piercing the wire.

Records the highest and lowest readings over time, useful for catching a momentary drop or spike on an intermittent circuit.

A larger fuse removes the wiring's overcurrent protection and can cause overheating or a fire.

The percentage of on-time in a repeating signal, used to read pulse-width-controlled outputs like some solenoids and motors.

Switching a load on and off rapidly and varying the on-time to control its average power (e.g., motor speed, lamp dimming).

A one-way electrical check valve that allows current in only one direction; the basis of the alternator rectifier.

Stores an electrical charge and can smooth voltage or suppress spikes in a circuit.

An electromagnet with a movable core that converts current into linear motion to move a valve, plunger, or relay contact.

A device that passes a signal between circuits using light, electrically isolating one side from the other to block noise and voltage differences.

A resistor whose value can be changed, used for things like dash dimming and as a sensor element.

A three-terminal variable resistor that outputs a voltage proportional to position — used in throttle and level sensors.

Conventional flow assumes current goes from positive to negative; electron flow is negative to positive. Most diagrams use conventional flow.

Because the same current flows through feed and ground, a bad ground causes the same voltage loss and symptoms as a bad power feed.

Stores electrical energy and supplies cranking power, stabilizing system voltage. A fully charged 12-volt battery reads about 12.6 volts at rest.

A sulfuric-acid-and-water solution that conducts current between the plates and stores charge chemically.

How fully a battery is charged, judged by open-circuit voltage (~12.6 V = full) or electrolyte specific gravity.

About 12.6 volts at rest for a healthy 12-volt battery; ~12.4 V ≈ 75%, ~12.2 V ≈ 50%.

A measure of electrolyte density (with a hydrometer) that indicates state of charge; about 1.265 fully charged.

Marked with a plus (+) symbol; connects toward the starter and the charging/distribution system.

Marked with a minus (−) symbol; the ground/return connection for the vehicle's electrical system.

The current a battery can deliver at 0°F for 30 seconds while staying above a minimum voltage — a cranking-capacity rating.

How long a fully charged battery can run a defined load without the engine, in minutes — a backup-power rating.

Applies a heavy load (about half the CCA) and checks that voltage stays above ~9.6 V at room temperature, proving cranking capacity.

Measures the battery's internal conductance to estimate its health and CCA without a heavy discharge load.

Tests the high-current cables and connections under cranking load by reading drop across each; excess drop means high resistance.

Current the truck draws with the key off. A small draw is normal for memory; an excessive draw drains the battery overnight.

With everything off and modules asleep, connect a meter in series in the battery circuit (or use an amp clamp), then pull fuses to isolate the draw.

Heavy trucks wire multiple batteries in parallel to multiply cranking amperage at the same system voltage; one bad battery or cable can drag the bank down.

A weak or discharged battery, or high resistance in the cables, connections, or grounds — confirmed with a voltage-drop test.

Usually a weak/discharged battery or high-resistance cables; the solenoid pulls in but can't supply enough current to spin the starter.

Verify battery voltage and state of charge first — a weak battery is the most common cause — before condemning the starter.

The high-current DC motor that cranks the engine, drawing hundreds of amps. It depends on a good battery and low-resistance cables.

A heavy relay that pushes the drive pinion into the flywheel and closes the high-current contacts to the starter motor when the key is turned to start.

Engages the starter pinion with the flywheel/flexplate ring gear to crank, then overruns so the running engine doesn't drive the starter.

Part of the starter drive that lets the pinion spin freely once the engine starts, protecting the starter.

Measures cranking amperage; a higher-than-spec draw points to a dragging/failing starter or a mechanical engine problem.

Voltage-drop from the battery negative to the starter case while cranking; over about 0.2 V means a bad ground path.

Adds resistance that causes hard or slow cranking, dim lights, and excessive voltage drop under load.

Sealed lead-acid battery with calcium alloy plates that minimize water loss; you can't add water.

Absorbed Glass Mat battery that holds electrolyte in glass mats — spill-resistant, vibration-resistant, and tolerant of deep cycling.

Batteries vent explosive hydrogen gas and contain acid — wear eye protection, avoid sparks, and disconnect the negative cable first.

Disconnect the negative (ground) cable first and reconnect it last to avoid shorting a wrench to ground.

Use a charger matched to the battery type; slow charging is gentler on the plates than fast charging.

Hard lead-sulfate crystals that build on the plates of a chronically undercharged battery, reducing capacity.

A battery that goes dead overnight points to a parasitic draw; one that won't hold a charge points to the battery itself — load/conductance test it.

A switch that allows cranking only in Park or Neutral (or with the clutch pressed); a fault here can cause a no-crank.

Worn start contacts add resistance or open the starter control circuit, causing intermittent or no cranking.

A relay that carries the starter solenoid's control current, sparing the ignition switch from heavy current.

Should stay high (near battery voltage minus a small drop); a large drop indicates resistance in the feed or ground.

System voltage shouldn't sag below roughly 9.6 volts while cranking a 12-volt system at normal temperature.

A tool that reads electrolyte specific gravity to judge each cell's state of charge in a serviceable battery.

A single weak cell (much lower than the others) indicates an internal fault — the battery should be replaced.

Connect positive to positive, then negative to a good engine ground on the dead vehicle (not the battery) to keep sparks away from battery gases.

Converts engine mechanical power into electrical power to recharge the battery and run the truck's electrical loads while the engine runs.

The belt-driven generator that produces three-phase AC, rectifies it to DC, and recharges the battery while powering the loads.

The alternator GENERATES electrical power while running; the battery STORES it and supplies cranking and stabilizing power.

The spinning electromagnet inside the alternator; field current through it creates the magnetic field that induces voltage in the stator.

The stationary windings in which the spinning rotor's magnetic field induces three-phase AC.

The diode bank that converts the alternator's three-phase AC into the DC the truck uses.

Controls field (rotor) current to hold charging voltage in a safe band (~13.8–14.4 V) regardless of speed or load.

Roughly 13.8 to 14.4 volts at the battery with the engine running and the system warm.

Read battery voltage with the engine running; it should rise above the rested 12.6 V to about 13.8–14.4 V if charging.

Leftover alternating current on the DC output, caused by failed rectifier diodes. Excess ripple flickers lights and confuses modules.

Set the meter to AC volts at the battery with the engine running; more than a few tenths of a volt indicates bad diodes.

Charging voltage too high (boiling electrolyte, short battery life), usually a voltage regulator stuck high.

Charging voltage too low (dead battery), from a slipping belt, bad regulator, open field, or failed diode(s).

Inspect the drive belt: a loose, glazed, or worn belt slips and cuts output (and often squeals).

Often a loose or glazed serpentine belt slipping on the alternator pulley.

Can indicate failing alternator diodes producing AC ripple, or low alternator output at idle.

Insufficient alternator output at low engine speed when extra loads (HVAC, lights) are added.

High resistance in the alternator output cable or ground shows as voltage drop and reduced charging at the battery.

The circuit that supplies current to the rotor; an open field means no magnetic field and no output.

Converts AC to DC; a single failed diode lowers output and raises AC ripple.

A capacitor at the alternator suppresses radio/electronic noise from the charging circuit.

Correct serpentine or V-belt tension is required for full output; too loose slips, too tight wears bearings.

Look for glazing, cracks, missing ribs, and proper tensioner operation; a worn belt reduces alternator output.

Worn bearings cause a growl/whine and can let the rotor rub; they reduce output and eventually fail the alternator.

Carry field current to the spinning rotor; worn brushes or dirty slip rings cause intermittent or low output.

Bypassing the regulator to force maximum field current, used to confirm whether the alternator or the regulator is at fault.

Modern trucks let an engine/body module command the regulator/alternator output via the data bus for load and fuel management.

Connecting the battery backward can instantly destroy alternator diodes — always observe correct polarity.

If output can't keep up with total electrical demand, battery voltage falls and lights dim — check belt, output cables, and alternator capacity.

Indicates the charging system isn't maintaining proper voltage; verify with a voltmeter before condemning parts.

A diode that leaks in reverse drains the battery overnight (a charging-system parasitic draw) and shows on a diode test.

Loads the charging system and measures amps; output well below the alternator's rating points to diodes, stator, or belt slip.

Runs from a fuse, through a switch (often a relay or flasher), to the bulbs, and back to ground.

Emits light by passing current through a heated filament until it glows; shortest typical lifespan.

An incandescent bulb with halogen gas that runs hotter and brighter and lasts longer than a plain incandescent.

A bulb that creates an arc in gas; needs a ballast to supply the high voltage to start and run it.

A semiconductor light source — longest lifespan and lowest current draw of the common bulb types.

Provides the high starting voltage and regulates current for an HID bulb.

Lets a low-current switch control a high-current lighting load by switching battery power to the bulbs.

Controls the timing and on/off flashing of the turn signals.

Often a burned-out bulb or a bad ground in that turn-signal circuit, which changes the flasher's load.

Check the shared part of the circuit: the fuse, then the common power feed and ground.

That bulb, its socket, or its local wiring/ground — the rest of the circuit is fine.

A poor connection at that bulb's socket or in its wiring (corrosion, loose terminal, chafed wire).

Lamps glow dim or flicker, or current back-feeds through another lamp, rather than going fully dark.

Check the wiring harness between the switch and the lamps for an open, chafed wire, or corroded connector.

Adjusts the beam's left-to-right angle for proper alignment.

Adjusts the beam's up-and-down angle so the pattern lights the road without blinding oncoming drivers.

Points to a weak battery or low charging output at idle (or high resistance in the feed).

A body controller reads switch inputs and commands lamps over the data bus, so a dead lamp can be a network/module fault.

Required truck/trailer lights that mark vehicle width and length; common corrosion-fault points from weather and vibration.

The 7-way (or similar) connector that carries power and grounds for trailer lights; a bad pin or ground causes multiple trailer-lamp faults.

Lights that run at reduced output whenever the engine is on, for visibility; often controlled by a module.

Composite headlamps use a replaceable bulb in a housing; sealed-beam units replace the whole lamp.

Total bulb wattage sets the current; adding lamps or higher-wattage bulbs can overload the circuit and blow the fuse.

Clean the contacts or replace the socket/pigtail; corrosion adds resistance that dims a lamp or causes flicker.

A rheostat or PWM control that varies instrument and panel light brightness.

A switch (often on the transmission) that powers the reverse lamps when the truck is shifted into reverse.

The body, cab, and chassis electrical systems plus the sensors and modules that report and act over the data network.

Displays speed, rpm, fuel, temperature, pressures, and warning lamps; on modern trucks it reads most data off the J1939 bus.

A dash indicator that alerts the driver to a system fault or status; some are driven directly, others by a module over the bus.

A variable-resistance sensor (fuel level, oil pressure, temperature) whose changing resistance moves a gauge or feeds a module.

A computer that reads sensors and commands outputs; the engine ECM and body/chassis modules share data over J1939.

System that monitors electronic components, stores trouble codes, and lights a warning lamp when a fault is detected.

A stored code naming the faulty circuit or system; retrieve it with a scan tool, but confirm the exact part with meter tests.

The J1939 code that identifies WHICH parameter or component has the fault.

The J1939 code that identifies HOW the parameter failed (short high, short low, open, out of range, etc.).

A diagnostic tool that reads codes, live data, and freeze-frame information from the truck's modules over the data connector.

A potentiometer that reports throttle/pedal position to the engine controller for fueling and idle control.

Reports engine position/speed for timing and fueling; a fault can cause a no-start or intermittent stall.

A thermistor whose resistance changes with temperature; tested by reading ohms while disconnected.

Measures intake air so the controller can set the correct fuel-to-air mixture.

A temperature-sensitive resistor whose resistance changes predictably with temperature; common for coolant and air-temp sensors.

A solid-state sensor that produces a digital signal from a passing magnetic field — used for speed and position.

Generates an AC signal as a toothed wheel passes — used for wheel speed and engine position.

Reports road speed to the speedometer and modules; a fault affects speed display, cruise, and ABS.

Prevents wheel lock-up during braking to keep steering control; monitors wheel-speed sensors and modulates brake pressure.

Assists vehicle stability and helps prevent skidding by selectively braking wheels and managing power.

Manages airbag deployment in a collision; service requires care with stored energy and proper disabling.

Monitors and warns of low tire pressure to maintain safe operation.

Manages engine and transmission operation together for optimal performance and emissions.

Powers the heater/AC blower motor, often through a resistor pack or module for speed control; a common high-current fault point.

Motor, switch/park circuit, and (on multiplexed trucks) a module that controls speed and intermittent timing.

A relay-controlled high-current circuit; a dead horn can be the relay, the horn, or its ground.

Power windows, mirrors, locks, and seats — motors and switches often run through a body controller on modern trucks.

A module that reads switch inputs and commands many body loads (lights, wipers, accessories) over the network.

May be a network or module fault, not a broken wire — diagnose with the wiring diagram and a scan tool, then confirm with the meter.

A DTC points to the affected circuit or system, not always the exact part — verify with meter tests before replacing components.

The diagnostic port where a scan tool connects to the J1939/J1708 network to read codes and data.

A snapshot of operating conditions captured when a fault set, used to recreate and diagnose the fault.