H6 Transit Bus — Electrical / Electronic — practice test

High-yield H6 topics

The material that shows up year after year. If you're short on time, start here.

• Voltage drop, measured under load. Know the numbers cold: about 0.1V across a connection, 0.2V across a cable, roughly 0.5V total on the insulated side of a 12V starter circuit and about 0.2V on the ground side. Cranking floor is 9.6V at 70F on 12V, 19.2V on a 24V system. A corroded ground strap reads a perfect 12.6V key off and only shows up when current flows - that is exactly how they write the question.
• 24V systems, series/parallel, and battery equalizers. Coaches that crank on 24V but run 12V accessories pull the 12V leg through an equalizer (Vanner and similar). Classic symptom: one battery pair chronically undercharged while the other gasses or cooks. Also know AGM group 31 banks - charge-voltage limits, thermal runaway, and that you must remove surface charge before you judge state of charge. Load test at 1/2 CCA for 15 seconds, 9.6V minimum at 70F.
• High-output charging: pad-mount brushless alternators (Delco 50DN/40SI, Leece-Neville, Niehoff) at 200-300A with external regulators, remote voltage sense, and isolated-ground cases. Expect AC ripple questions - more than about 0.5V AC or a scope pattern with a missing hump means a bad diode or open phase, and it can look like a dead battery or a network that misbehaves at idle. Also know why a full battery makes a good alternator read low output.
• J1939 and J1708/J1587 datalinks. 250k baud (some 500k), twisted pair, 120 ohm terminating resistor at each end, so you measure about 60 ohms across CAN H and CAN L with the key off and batteries disconnected. 120 ohms = one terminator open or missing. Know that a single node with a shorted transceiver can drop the whole bus, and that J1708 is a different animal (two-wire, no terminators, 9600 baud).
• Multiplexing: I/O Controls Dinex, Weldon V-MUX, Vansco/Parker. On these buses a dash switch is a low-current network input, not a load-carrying switch, and the node drives the load. When an output won't come on, verify at the node whether the input is being seen before you touch the wiring. Nodes have status LEDs and a laptop tool for a reason. Know load shedding, node addressing, and what a lost node does to unrelated circuits.
• Interlocks and ADA accessory circuits: door-open interlock applying the brake and inhibiting throttle/fast idle, kneeling, wheelchair ramp/lift interlocks, stop request, destination signs (Luminator, Twin Vision), GFI fareboxes, APC, AVL, and cameras. These are transit-only and they show up. Ground loops and RFI from radio/DVR installs are fair game, as is parasitic draw - a coach with a DVR, farebox and telematics has a legitimately higher key-off draw than a pickup, and you clamp the ground cable rather than break it.

Where techs lose points on H6

Good technicians miss these — not from lack of skill, but because the question is built to catch them.

• "The meter says 12 volts, so the wire is good." A high-resistance connection will show near source voltage on a DVOM with no load on it, and it will phantom-read through corrosion. The answer they want is loaded testing - voltage drop or a test light - not an unloaded DVOM reading. Half the Tech A/Tech B items in general diagnosis are built on this.
• Bad ground vs bad bulb. When the turn signal makes markers flicker or interior lights dim in step with another circuit, that is current backfeeding through a shared ground, not a failed lamp or a failed switch. Techs replace components; the test wants the common ground point. Same trap with LED retrofits - low current draw can defeat a multiplex output's open-load detection or an old flasher, and the "obvious" answer of a bad module is wrong.
• 60 ohms vs 120 ohms on J1939. Plenty of techs read 60 and call the network shorted because 60 sounds low. Sixty is correct - two 120 ohm terminators in parallel. Also watch the setup: some questions describe measuring with the key on or the batteries connected, which invalidates the reading.
• Condemning the wrong half of a 24V/12V system. Reading only the 12V accessory pair and calling those batteries bad, when the equalizer or an unbalanced series/parallel bank is the actual fault. Same family of trap: a good alternator that reads low output because the batteries are already fully charged.

A study plan that works for H6

Roughly two weeks of real preparation, in the order that actually builds on itself.

1. Days 1-3: Fundamentals and diagnosis. Ohm's law, series vs parallel, and voltage drop until you can do it in your head. Read real transit wiring diagrams (Gillig, New Flyer, Nova) and trace circuits end to end. This is the largest task area and it feeds every other one.
2. Days 4-6: Battery, starting, charging. 12V and 24V, series/parallel, equalizers, AGM specifics, load and cranking specs, pad-mount alternators, AC ripple, isolated ground, remote sense. Drill the numbers - 9.6V, 1/2 CCA for 15 sec, 0.5V starter-circuit drop.
3. Days 7-9: Lighting, gauges/warning devices, and the accessory bucket. FMVSS 108 basics, LED behavior, door and lift interlocks, kneeling, destination signs, farebox, cameras, parasitic draw. Do not skim this - it is where the transit-specific points live.
4. Days 10-12: Datalinks and multiplexing, then two timed full-length practice tests. Log every miss by task area and go back into that section instead of retaking the whole test. If you miss anything in general diagnosis, that is a fundamentals gap, not bad luck.

Sample H6 questions

Straight from the bank — answers highlighted, with the explanation underneath.