ASE H6 Electrical/Electronic Systems (Transit Bus) cheat sheet

A 12 V lead-acid battery has six cells at about 2.1 V each; a rested, fully charged battery reads about 12.6 V at 77°F (25°C).

Open-circuit voltage approximates state of charge: about 12.4 V is roughly 75%, 12.2 V roughly 50%, and 12.0 V roughly 25%; temperature affects readings.

Batteries in series add voltage while amp-hour capacity stays the same; batteries in parallel add capacity and current capability while voltage stays the same.

A capacity test loads a 12 V battery to half its CCA rating for 15 seconds; at 70°F the voltage should remain above 9.6 V.

Excessive resistance in a cranking circuit is found by voltage-drop testing under load; more than about 0.5 V across a cable or connection is generally excessive.

The starter solenoid uses a low-current control circuit to close high-current contacts; a no-crank condition requires checking control power, ground, neutral/start interlock, and solenoid.

A slow crank with bright lights suggests high starter-circuit resistance or a failing starter; dim lights and slow crank suggest a discharged or failed battery.

When servicing batteries, disconnect the negative/ground first and reconnect it last to reduce short-circuit risk; preserve keep-alive memory if required.

A battery disconnect switch isolates the bank; verify open-circuit voltage and load capability before condemning a battery, because surface charge can mislead.

A charging system on a 12 V bus typically regulates about 13.5–14.5 V; a 24 V system about 27–29 V, with values temperature-dependent.

The alternator rectifier converts AC stator output to DC; a failed diode can cause AC ripple, dim or pulsing lights, and a discharged battery.

The voltage regulator controls field current; overcharging raises electrolyte gassing and water loss, while undercharging leaves batteries sulfated.

The charge indicator/excite circuit must supply initial field current; a burned-out bulb or open excite wire can prevent charging until high rpm.

Check alternator belt condition and tension; glazing or slipping causes low output, noise, and overheating.

A remote voltage-sense wire senses battery voltage; high resistance in that wire makes the regulator overcharge the system.

Measure voltage drop on the alternator B+ cable and ground return under load; excessive drop reduces charging voltage at the battery.

Full-load charging test: bring engine to a fast idle, turn on major electrical loads, and confirm output holds within specification without discharging.

A battery that drains overnight with a good alternator suggests parasitic load or a shorted diode; isolate circuits and perform a current-draw test after modules sleep.

Incandescent lamp brightness depends on current; excessive voltage drop in feed or ground makes lamps dim and increases heat at poor connections.

LED lamps are polarity-sensitive and draw much less current; a standard thermal flasher may not flash them, so an electronic/no-load flasher is required.

A turn-signal circuit has a flasher, switch, and lamps; one fast-flashing side usually indicates a failed lamp on that side.

Brake lamps are fed through the brake switch; turn and hazard lamps use a separate flasher and switch path, so both circuits must be tested separately.

A poor ground can cause backfeed through other lamps, producing dim or cross-illuminated bulbs; test ground voltage drop under load.

Headlamp switches and some accessory circuits use thermal circuit breakers; repeated tripping indicates overload or a short, not a weak breaker.

Relay coils are low-current control devices; relay contacts carry load current, so coil tests and contact voltage-drop tests are separate.

Multiplexed lighting may be controlled by a power distribution module; a switch input can be a data message rather than a direct lamp feed.

Accessory circuits often use a fuse or breaker sized to protect the wire; installing a larger fuse risks wire overheating and fire.

Most temperature and pressure senders are variable resistors; gauge reading depends on sender resistance plus feed and ground circuit resistance.

If a gauge reads full-scale with the sender disconnected, the gauge or wiring is likely shorted to ground; if it stays low, the sender or feed is open.

Warning lamps should prove out at key-on; a lamp that never illuminates indicates a bulb, LED, cluster, or power/ground fault.

A cluster that loses data-bus communication can show multiple inoperative gauges or warning lamps at once; check network power, ground, and terminating resistors.

Speedometer and odometer inputs may come from a vehicle speed sensor, ABS wheel-speed data, or ECM data; tire size and axle ratio affect calibration.

Low-air-pressure warning must operate before air pressure falls below the manufacturer's specified threshold; verify switch and warning circuit operation.

Coolant temperature senders commonly use negative temperature coefficient resistance; higher temperature lowers resistance and moves the gauge.

An instrument voltage limiter supplies reduced voltage to gauges; a failed limiter can make all gauges read high or low simultaneously.

Use a gauge tester or resistance substitution box to separate sender error from gauge error; do not bend the gauge needle to correct calibration.

J1939 CAN typically runs at 250 kbps or 500 kbps on a twisted pair; two 120-ohm terminating resistors in parallel measure about 60 ohms.

Measure CAN termination with the network unpowered; about 60 ohms is normal, about 120 ohms suggests one terminator, and open suggests a missing or broken terminator.

J1708/J1587 networks are slower serial links; J1708 defines the physical layer and J1587 defines message content.

A shorted CAN_H or CAN_L to power or ground can pull the whole network down and make many modules appear offline.

Multiplex control modules receive switch inputs and send messages; output modules or power distribution modules drive the actual loads.

A module that stays awake can drain batteries; check for network activity and sleep command after the ignition is off.

Replacing a multiplex module may require configuration or address learning; a blank module can power up but not control its loads.

Module power and ground voltage drops must be tested under load; a module can communicate but fail to drive outputs if its ground is poor.

Shielded or twisted-pair network wiring should be repaired to maintain twist and shielding; poor splices cause intermittent communication faults.

Ohm's law: V = I × R; power: P = V × I. In a series circuit current is common, while in a parallel circuit voltage is common.

Voltage-drop testing is performed with the circuit loaded and the meter in parallel across the connection; it measures resistance without disconnecting the circuit.

A fuse protects the wiring, not the load; replace a blown fuse with the same rating and find the cause of the overload.

Parasitic draw is measured in series with the battery after modules sleep; compare the reading with the manufacturer's maximum specification.

A short to ground blows a fuse or trips a breaker; a short to power can energize a circuit when it should be off.

Connector terminal tension is checked with a drag test or mating-terminal gauge; loose terminals cause intermittent voltage drops and heat.

Corrosion and fretting increase resistance; clean terminals, repair with proper crimp or solder splices, and seal with heat-shrink.

Relay testing includes coil resistance/current, contact voltage drop, and coil suppression; a relay can click and still have burned contacts.

Use the wiring diagram to identify wire numbers, colors, splices, and connector pins; isolate circuits by disconnecting loads and fuses one at a time.