ASE H6 Electrical/Electronic Systems (Transit Bus) key terms

The number of amps a 12-volt battery can deliver for 30 seconds at 0°F while keeping at least 7.2 volts. It shows how well a battery can crank a cold engine.

The amount of usable energy left in a battery, often shown as a percentage of full capacity. It is estimated from open-circuit voltage or measured with a conductance tester.

The ratio of battery electrolyte density to the density of water. A fully charged lead-acid cell reads near 1.265; lower readings mean discharge or a weak cell.

A test that applies a heavy current load to a battery while measuring terminal voltage, showing whether the battery can deliver cranking power under real demand.

Current that flows from the battery when the ignition and accessories are off, caused by module memory, stuck relays, or a fault. It is measured in milliamps.

An electromagnetic relay on or near the starter that switches high current to the motor and pushes the drive gear into mesh with the flywheel.

A switch that permits starter operation only when the transmission selector is in neutral or park, preventing the bus from cranking while in gear.

A manual or remote switch that opens the battery ground circuit to isolate the electrical system for service or emergency shutdown. Transit buses often use a rotary or solenoid type.

A belt-driven generator that converts engine rotation into DC charging current. It uses a rotor field, stator windings, and a rectifier to keep the battery charged.

A device that controls alternator field current to hold charging voltage within a set range, protecting the battery and electronic modules from overcharge or undercharge.

The small control current supplied to an alternator's rotor winding. Changing it alters magnetic strength and alternator output; the regulator adjusts it automatically.

A set of six diodes in an alternator that converts three-phase AC from the stator into DC for the battery and bus loads.

Alternating voltage superimposed on the DC charging output, caused by a failed diode or stator winding. Excessive ripple can disturb electronics and indicates alternator repair is needed.

A wire that feeds actual battery voltage back to the regulator so it can compensate for voltage drop in charging cables and hold the battery at the correct level.

A circuit that automatically turns on low-intensity forward lighting whenever the engine runs, improving bus visibility. Daytime running lamps usually bypass the normal headlamp switch.

A driver-operated switch that selects high or low beam by changing which headlamp filament receives power. It may be a floor button, column stalk, or multiplexed input.

A switch on the brake pedal linkage that closes when the brake is applied, feeding power to the brake lamps and often to the ABS or multiplex module.

An electronic or thermal unit that interrupts current to the turn lamps so they blink. It may also drive the dash indicator and provide the audible click.

An electronic module that regulates current to a light-emitting-diode lamp assembly. It keeps brightness stable as voltage varies and protects the LEDs from overcurrent.

An electrically operated switch that lets a small control current turn a larger load circuit on or off. Coil terminals are usually marked 85 and 86; contacts are 30, 87, and 87a.

A sensor that converts a physical value such as fuel level, oil pressure, or coolant temperature into an electrical signal for a gauge or control module.

The driver-facing assembly containing speedometer, tachometer, temperature and pressure gauges, and warning lamps. It may receive data over a network instead of direct wiring.

A symbol-illuminated warning lamp on the dash, such as brake, ABS, or engine warning, that alerts the driver to a system condition or fault.

A buzzer and lamp that activate when brake system air pressure drops below a safe threshold, warning the driver before braking performance is lost.

A sensor that reports road speed to the speedometer, engine, transmission, and ABS modules. It may be a magnetic pickup, hall-effect unit, or derived from wheel speed.

A wiring approach in which several devices share a data bus instead of each having dedicated wires. It reduces harness size and lets modules exchange status and commands.

A two-wire differential network used for high-speed control data between modules. It uses twisted-pair wiring and terminating resistors to reduce reflection and noise.

A high-speed CAN protocol used on heavy trucks and transit buses for powertrain and chassis data. It defines message identifiers, baud rate, and diagnostic services.

An older heavy-vehicle data link using two wires at 9600 baud. J1587 defines the message format, while J1708 defines the physical layer; many buses still carry it alongside J1939.

A resistor at each end of a CAN network that matches impedance and prevents signal reflections. It is typically 120 ohms, giving about 60 ohms across the pair.

A module that translates and routes messages between different networks, such as J1939 and J1708, so modules on separate buses can share information.

A stored code that identifies a fault detected by a module. Technicians read DTCs with a scan tool and use them to guide circuit and component testing.

A measurement of voltage lost across a cable, connector, or switch while current flows. More than a few tenths of a volt indicates excessive resistance that must be repaired.

A check using a meter or tone generator to confirm that a circuit path is complete and has low resistance, usually with power removed from the circuit.

A break in a circuit that stops current flow, such as a cut wire, unplugged connector, or failed switch. It reads infinite resistance and zero voltage across the load.

An unintended low-resistance path from a circuit to chassis ground, usually causing a blown fuse or breaker and preventing the load from operating.

Corrosion, loose terminals, or undersized wiring that limits current and causes voltage drop, often producing dim lights, slow motors, or intermittent operation.

The common return path through the bus frame and body. Poor ground connections cause strange symptoms because many circuits share this path.

A one-time protective device with a metal element that melts when current exceeds its rating, opening the circuit to protect wiring and components.

A standard sizing system for wire; lower numbers mean larger conductors. Correct gauge is chosen for current capacity and allowable voltage drop over the run length.