ASE T6 Electrical/Electronic Systems (Medium/Heavy Truck) cheat sheet

Ohm's law: E = I × R; power: P = I × E = I²R = E²/R.

Voltage is measured in parallel with the load; current is measured in series with the circuit; resistance is measured with the component isolated and power removed.

A voltage drop test measures the difference between two points while current flows; more than 0.5 V across a connection or cable on a 12 V circuit indicates excessive resistance.

Key-off battery drain is measured with an ammeter or clamp in series with the negative cable; typical acceptable parasitic draw on a heavy truck is roughly 50 mA or less (verify OEM spec).

Series circuit: total resistance is the sum of resistances; current is the same everywhere; voltage divides across loads.

Parallel circuit: voltage is the same across branches; total resistance is less than the smallest branch resistance; current divides.

High resistance in a circuit causes low current, dim or slow operation, and heat at the fault; an open circuit causes zero current and full source voltage at the open point.

A short to ground bypasses the load and typically blows a fuse or fusible link; a short to voltage feeds power where it does not belong.

Control modules communicate on data buses (J1939 CAN, J1587/J1708); a bus fault can set codes in multiple modules at once.

Battery state of charge approximates 12.6 V at 100%, 12.4 V at 75%, 12.2 V at 50%, and 12.0 V at 25% after rest (verify temperature correction).

A load test applies roughly half the CCA rating for 15 seconds; a good battery holds at least 9.6 V at 70°F (21°C).

Batteries in parallel keep voltage the same and add capacity; batteries in series add voltage and keep capacity the same.

Cranking current is highest at the first instant of cranking and drops as the engine spins; a clamp ammeter and voltage reading during crank diagnose high-resistance cables.

Excessive voltage drop on the cranking circuit (more than about 0.5 V per cable or connection) causes slow cranking and heat.

A starter solenoid that clicks but does not crank suggests low battery voltage, a bad ground, or an open hold-in/pull-in winding.

Starter draws no current and does not turn: check control circuit, neutral start switch, and solenoid; starter turns but engine does not: check drive, flywheel ring gear, and one-way clutch.

Battery testing order: visual inspection, state of charge, load or conductance test, then charging system check.

A battery that will not hold a charge after a full charge and load test should be replaced; always match CCA, group size, and chemistry.

Alternator output is typically 13.8 to 14.8 V at operating temperature with the engine running; readings outside OEM spec indicate a fault.

The charging warning lamp circuit often provides initial field excitation; a burned-out bulb can prevent charging on some designs.

Full-fielding the alternator bypasses the regulator to test maximum output; do not exceed OEM voltage limits or damage may result.

A diode trio or rectifier bridge failure causes low output, AC ripple, or a battery that discharges when the engine is off.

AC ripple voltage measured at the battery with the engine running should be low (typically under about 0.5 V AC); excessive ripple indicates failed diodes.

Belt condition and tension affect charging; a slipping serpentine belt causes low output, squeal, and glazing.

The voltage regulator controls field current; a failed regulator can cause overcharging (boiling electrolyte) or undercharging.

A charged battery that goes dead overnight with a good alternator points to parasitic draw, not the charging system.

Remote sense alternators use a sense wire at the battery; an open sense wire causes overcharging or low output.

Sealed beam and halogen headlamps are aimed per OEM or DOT procedure; improper aim causes glare and failed inspection.

A lamp that is dim on one side often has a poor ground at that lamp housing; check voltage drop on the ground circuit.

Turn signal and hazard circuits share the flasher; a fast flash usually indicates a burned-out bulb or wrong bulb wattage on that side.

Stop lamp circuits commonly use a brake pedal switch; a misadjusted switch causes no brake lights or a battery drain.

LED lamps draw less current; some flashers and lamp-out monitors require load resistors or a compatible flasher to function.

Marker and clearance lamp circuits are often tied to the headlamp switch; check the switch, relay, and fuse before condemning lamps.

Headlamp relays and daytime running lamp modules control high-current lamp circuits; a failed relay causes one or both lamps to be inoperative.

Corrosion at lamp sockets increases resistance, causing dim lamps and heat; clean or replace sockets as needed.

Check lamp circuit fuses first, then voltage at the socket, then ground; a lamp that receives voltage and ground but does not light is defective.

Most coolant temperature, oil pressure, and fuel gauges use a variable-resistance sending unit; high resistance usually reads low, low resistance reads high (verify OEM).

A gauge that pegs high or low with the sender wire grounded or open helps isolate sender versus gauge versus wiring.

Warning lamps are typically controlled by a module grounding the lamp circuit; a lamp that never lights may have a burned bulb or open ground.

The instrument cluster communicates on the data bus; loss of bus communication can blank multiple gauges at once.

Speedometer and odometer inputs come from a vehicle speed sensor or the ECM over the data bus; check sensor and wiring before the cluster.

Tachometer signals may come from the ECM or an alternator stator terminal; an erratic tach can indicate a charging system fault.

Driver information displays may show fault codes and menus; use the OEM diagnostic software to retrieve cluster and body controller codes.

A fuel gauge reading incorrectly may be a sender, float, or tank ground issue; verify sender resistance at the tank before replacing the gauge.

Audible and visual warnings (low air, low oil, coolant) are often tied to senders and the body controller; test the sender circuit before the module.

Horns are typically powered through a relay controlled by the horn button grounding the relay coil; check relay, fuse, and ground.

A horn that sounds weak or not at all may have a poor ground, corroded connector, or failed horn; test with direct battery voltage.

Wiper motors have high and low speed windings plus a park circuit; a motor that will not park has a faulty park switch or wiring.

Wiper linkage binding or seized pivots cause slow or stalled wipers and can blow the fuse; check linkage freedom before replacing the motor.

Intermittent wiper function is controlled by a module or the wiper switch; a failed module causes loss of delay and mist functions.

Washer pumps are usually small electric pumps; check fuse, switch, and pump connector for voltage before replacing the pump.

A wiper motor that runs on high but not low (or vice versa) indicates an open winding or switch contact for that speed.

Check wiper fuse and relay first; then verify voltage at the motor connector in each switch position.

Washer nozzle and hose blockages cause no spray with a good pump; verify pump operation and clear the lines.