Auto Electricals cheat sheet

Ohm's law: V = I × R; power: P = V × I = I²R = V²/R.

Series circuit: R_total = R1 + R2 + …; current is the same through all components; voltage divides proportionally to resistance.

Parallel circuit: 1/R_total = 1/R1 + 1/R2 + …; voltage is the same across all branches; current divides inversely with resistance.

Kirchhoff's voltage law: the algebraic sum of voltages around any closed loop is zero.

Kirchhoff's current law: the sum of currents entering a node equals the sum leaving it.

A 12 V system at 14.4 V charging: a 0.1 Ω resistance in a high-current circuit drops 1.44 V per 10 A, causing excessive voltage drop and heat.

Conventional current flows from positive to negative; electron flow is from negative to positive.

Inductance (henries) opposes changes in current; capacitance (farads) opposes changes in voltage.

Voltage drop across a conductor: V_drop = I × R_conductor; target less than 0.5 V total for most 12 V accessory circuits.

Lead-acid battery nominal cell voltage is 2.1 V; a 12 V battery has six cells in series (12.6 V at rest, full charge).

State of charge (approximate, at 25°C): 12.6 V = 100%, 12.4 V = 75%, 12.2 V = 50%, 12.0 V = 25%, 11.8 V = 0%.

Cold cranking amps (CCA) is the current a battery can deliver for 30 s at −18°C while maintaining at least 7.2 V.

Charging system target: 13.5–14.5 V at the battery with engine running and accessories off; below 13.0 V or above 15.0 V indicates a fault.

Starter circuit voltage drop: maximum 0.5 V on the insulated (positive) side and 0.3 V on the ground side under cranking load.

Alternator output test: measure AC ripple voltage; more than 0.5 V AC suggests failed rectifier diodes.

Parasitic draw: after all modules sleep (typically 20–40 minutes), normal draw is usually under 50 mA; 50–85 mA may be acceptable on some vehicles.

Battery capacity: reserve capacity (RC) is minutes a battery can supply 25 A at 27°C before falling below 10.5 V.

Headlamp aim: SAE/E-code low beam cutoff should be adjusted per vehicle spec; a common baseline is 2 inches down at 25 feet for low beam.

Halogen bulb: do not touch the glass envelope with bare fingers; oils cause hot spots and premature failure.

LED retrofit lamps may cause hyper-flash or bulb-out warnings because current draw is lower than the vehicle expects.

Turn signal flasher: thermal flashers flash faster when a bulb is out; electronic flashers maintain rate regardless of load.

Brake light switch: normally open, closes when pedal is pressed; a faulty switch can prevent shift interlock release or cause cruise control dropout.

Horn circuit: typically 10–15 A fuse, relay-controlled; measure voltage at horn connector while pressing horn button.

Trailer lighting: 4-pin flat: white = ground, brown = tail/running, yellow = left turn/brake, green = right turn/brake.

Daytime running lights (DRL): often reduced-intensity high beam or separate lamp; may be disabled when park brake is applied.

Wire gauge: American Wire Gauge (AWG) - smaller number = larger conductor; 18 AWG is common for low-current signal, 10–12 AWG for high-current accessories.

Fuse rating: continuous load should not exceed 80% of fuse rating; select fuse to protect the smallest wire in the circuit.

Fuse types: blade (ATO/ATC), mini, maxi, cartridge, and glass; always replace with same rating and type.

Relay pin numbering (ISO): 30 = battery positive, 87 = load output (normally open), 86 = coil positive, 85 = coil ground, 87a = load output (normally closed).

Voltage drop testing is preferred over resistance testing for high-current circuits because it tests under load.

Connector terminal tension: a loose female terminal causes intermittent faults; drag test and visual inspection are required.

Harness routing: maintain at least 50 mm clearance from exhaust components and secure with OEM-style clips to prevent chafing.

Circuit protection devices: fuse (one-time), circuit breaker (resettable), fusible link (high-current, slow-blow).

CAN bus: differential pair CAN High and CAN Low; nominal 2.5 V each at idle, CAN High rises to ~3.5 V and CAN Low falls to ~1.5 V during dominant bit.

CAN termination: two 120 Ω resistors at each end of the bus; measured resistance across CAN H and CAN L with power off should be ~60 Ω.

LIN bus: single wire, 12 V dominant, 0 V recessive; typically 9.6–19.2 kbps, master-slave architecture.

Sensor types: potentiometer (throttle position), Hall effect (crank/cam), inductive (ABS wheel speed), thermistor (coolant temperature, NTC).

5 V reference circuit: most ECM sensors use a regulated 5 V supply; a short to ground or to voltage affects all sensors on that reference.

ECU power-up: battery positive (B+), ignition switched (IGN), and ground must all be present before communication is possible.

Network fault symptoms: multiple unrelated warning lamps, no-start with good battery, and scan tool unable to communicate with multiple modules.

OBD-II CAN pins: 6 = CAN High, 14 = CAN Low (ISO 15765-4).

Digital multimeter (DMM) input impedance is typically 10 MΩ; use auto-ranging or correct range to avoid damage.

Current measurement requires placing the meter in series or using a clamp; never measure current in parallel with a load.

Oscilloscope: set voltage scale and time base to display at least two complete cycles; use a 10:1 probe for high-voltage signals.

Fuel injector waveform: inductive kick of 60–100 V at turn-off indicates good coil; missing kick suggests open winding.

Scan tool data: compare live sensor values to expected ranges; freeze-frame data captures conditions at fault detection.

Bidirectional scan tool controls allow actuating relays, injectors, and motors to test circuits without disassembly.

Voltage drop test: measure across the connection while the circuit is loaded; more than 0.1 V per connection is excessive.

Insulation resistance test: use a megohmmeter; minimum acceptable is typically 10 MΩ for 12 V circuits.

Six-step diagnostic process: verify complaint, gather information, inspect, test, repair, verify repair.

Open circuit: no current flow; voltage present at source but not at load; resistance infinite.

Short to ground: fuse blows; measure resistance from load side to ground; should be infinite with load disconnected.

Short to voltage: circuit powered when it should be off; back-probe and disconnect components to isolate.

High resistance: voltage drop across connection; causes dim lights, slow motors, and heat.

Intermittent faults: use wiggle test, freeze-frame data, and event recording on scan tool; check for chafed harnesses and loose grounds.

Ground circuit testing: measure voltage from component ground to battery negative while loaded; more than 0.1 V indicates bad ground.

After repair: clear codes, perform road test, and verify all systems operate correctly; recheck for new codes.

Battery safety: wear eye protection and acid-resistant gloves; neutralize acid spills with baking soda and water.

High-voltage systems (EV/hybrid): orange cables indicate high voltage; qualified personnel only; use insulated tools rated 1000 V.

Lockout/tagout: disconnect 12 V negative and high-voltage service disconnect before working on airbags or high-voltage components.

Refrigerant and waste: recover refrigerant with certified equipment; do not vent to atmosphere; follow local regulations.

Personal protective equipment (PPE): safety glasses, insulated gloves, and arc-rated clothing for high-current work.

Fire safety: Class C extinguisher for electrical fires; Class D for lithium battery fires; never use water on live electrical fires.

Industry standards: SAE J1930 (terminology), J1962 (OBD-II connector), J2012 (DTC definitions), and ISO 11898 (CAN).

Electrical work area: keep area dry, use rubber mats, and verify meter leads are CAT III rated for automotive use.