Electrical and Electronic Systems, Batteries, and Lighting
Voltage, ground, load
Electrical questions on transit buses usually test fundamentals rather than exotic electronics. Know the battery bank: 12-volt batteries in series make 24 volts, while parallel connections add capacity. A fully charged battery rests near 12.6 volts, about 25.2 volts for a 24-volt bank; 12.4 volts is roughly 75 percent state of charge, and below 12.2 volts is effectively discharged. Test with a conductance or load tester, not just a voltmeter, and replace batteries in matched sets, because mixing a new battery with old ones in a bank kills the new one.
Charging checks are done under load: start the bus, turn on headlights and HVAC blowers, and read charging voltage at the battery terminals; compare with the OEM specification and check for AC ripple. A low reading at the battery with normal output at the alternator means voltage drop, so perform a drop test on the positive and ground paths. A useful rule is that total cranking voltage drop should stay under about 1 volt.
Lighting and accessories are a ground-hunting exercise. A lamp that is dim, or that lights when another circuit is switched on, usually has a corroded ground rather than a failed bulb. LED conversions draw far less current, so a flasher designed for incandescent lamps may hyperflash.
Worked rule of thumb: test the circuit, not the part. Verify power and ground at the connector before replacing a component.
Trap: never assume a multiplexed bus has a simple switch-to-lamp circuit; the body controller may be the real fault.