Five free practice questions on truck and trailer lighting: content area D of the ASE T6 Electrical/Electronic Systems test, 6 of its 50 scored questions (12%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
-
A truck has auxiliary fog lamps and auxiliary driving lamps mounted in its bumper. How does the fog lamps' beam differ from the driving lamps' beam?
- It is wide and flat with a sharp top cutoff
- It is narrow and reaches far down the road
- It is aimed higher to light signs and overpasses
- It has the same pattern behind a yellow lens
Show the answer
A. Correct. A fog lamp is mounted low and throws a wide, flat beam with a sharp top cutoff, so it lights the road surface close ahead without lighting up the fog in front of the driver's eyes. A driving lamp does the opposite job: a concentrated beam that reaches far ahead to add to the high beams.
Why not the others
B. A narrow, long-reaching beam describes the driving lamp, which adds distance to the high beams. The fog lamp's beam is wide and flat with a sharp top cutoff, aimed low under the fog.
C. Light aimed upward would reflect off the fog and glare back at the driver, the opposite of what a fog lamp is for. Its beam is aimed low, wide and flat, with a sharp top cutoff.
D. Lens color does not make a lamp a fog lamp; its optics shape a wide, flat beam with a sharp top cutoff. Fog lamps are made with clear or selective-yellow lenses.
ID T6-D-005 · Task T6-D-2 (ASE task list)
-
Many trucks feed the left and right headlamps through a separate fuse for each side instead of one fuse for both. What is the main reason?
- Each fuse can be smaller, so the bulbs last longer
- It lets the dimmer switch carry less current
- A short on one side leaves the other lit
- It keeps both headlamps at the same brightness
Show the answer
C. Correct. With one fuse for both headlamps, a short to ground on either side would open it and leave the driver with no headlamps at all; with a fuse for each side, only the faulted side goes dark. The headlamp that still works also tells the technician which side to search for the short.
Why not the others
A. A fuse protects the wiring and passes whatever current the bulb draws until it opens; its rating has no effect on bulb current or bulb life. The reason for a fuse per side is that a short opens only that side's fuse, so the other headlamp stays lit.
B. The dimmer switch, or the relay it controls, still feeds both headlamp circuits, so splitting the fuses downstream does not change the current it handles. The split exists so a short on one side cannot put out both headlamps.
D. Brightness is set by the voltage each lamp receives and by the bulb itself; a good fuse drops almost no voltage and does nothing to balance the two sides. Separate fuses are there so a fault on one side leaves the other headlamp working.
ID T6-D-008 · Task T6-D-3 (ASE task list)
-
On a trailer, the right-side marker feed wire runs back along the side rail from the nose box, with a short branch from it to each of eight LED marker lamps, and each lamp has its own ground lead. With the markers on, the first three lamps from the front light and the five behind them do not. Where is the fault?
- In the feed between the third and fourth lamps
- In the trailer cord's marker conductor
- In the fourth lamp, whose LEDs have failed open
- In a short to ground at the eighth lamp's branch
Show the answer
A. Correct. Each lamp hangs on its own branch from the feed wire, so lamps ahead of a break stay lit and every lamp behind it goes dark. The open is in the feed after the third lamp's branch and before the fourth's, splices included; voltage checks at the third and fourth lamps' branches close in on it.
Why not the others
B. A break in the cord's marker conductor would cut the feed to every marker lamp on the trailer, including the first three, which still light. The fault lies on the side rail, past the third lamp.
C. Each lamp is on its own branch, so a lamp that fails open goes dark by itself; the lamps behind it are fed by the feed wire, not through that lamp, and would still light. Five dark lamps in a row point to the feed wire.
D. A short to ground would draw heavy current and open the circuit's fuse or protection, putting out every marker lamp on the circuit; the first three still light normally. A run of dark lamps behind lit ones points to an open in the feed.
ID T6-D-011 · Task T6-D-4 (ASE task list)
-
On an older truck, the instrument panel bulbs are fed through a wire-wound rheostat in the headlamp switch, which sets their brightness. Why can panel bulbs of a higher wattage than specified burn out the rheostat?
- Their higher resistance forces more voltage across it
- They draw more current through its resistance wire
- They add the parking lamp current to the rheostat
- Their extra heat in the cluster overheats the rheostat
Show the answer
B. Correct. Higher-wattage bulbs have lower resistance and draw more current, and the heat made in the rheostat's resistance wire rises with the square of that current. The rheostat was sized for the current of the specified bulbs, so the extra current overheats its wire until it burns open.
Why not the others
A. A higher-wattage bulb has lower resistance, not higher; that is why it draws more current. The extra current through the rheostat's resistance wire is what overheats it.
C. Changing panel bulbs does not reroute any other circuit through the rheostat; it carries only the panel lamps' current. Bigger bulbs raise that current, and the extra current heats its resistance wire.
D. The rheostat sits in the headlamp switch, away from the bulbs' heat. It is the larger current flowing through its own resistance wire that makes it run hot and burn out.
ID T6-D-012 · Task T6-D-5 (ASE task list)
-
A truck's cab sits on rubber-isolated mounts, and ground straps or cables connect the cab to the frame or engine. What is the main purpose of these ground connections?
- To give the batteries a backup ground if the main cable fails
- To carry cranking current from the batteries to the starter
- To carry cab circuits' ground current past the mounts
- To stop galvanic corrosion between the cab and the frame
Show the answer
C. Correct. Rubber mounts insulate the cab from the frame, so the cab's lamps, gauges and switches need straps or cables to return their current to battery negative. A loose or corroded strap shows up as dim or flickering cab lamps, erratic gauges, or current forced through cables and hoses never meant to carry it.
Why not the others
A. The main battery ground cable is the batteries' intended ground path, and a failed cable is repaired, not backed up by a cab strap. Cab straps exist because the rubber mounts insulate the cab, leaving its circuits without a ground return.
B. Cranking current flows through the main battery cables to the starter and the engine block; cab ground straps are not sized or routed for it. Their job is to return the cab circuits' current past the insulating mounts.
D. Bonding two different metals with a strap does not stop galvanic corrosion; keeping them apart and sealed does. The straps are there to return cab circuit current past the insulating mounts.
ID T6-D-013 · Task T6-D-6 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | A |
| Q2 | C |
| Q3 | A |
| Q4 | B |
| Q5 | C |
03What area D covers
This content area covers lighting on the tractor, the truck body and the trailer: headlamps and daytime running lamps, auxiliary fog and driving lamps and their aim, LED and incandescent marker, tail and stop lamps, air-pressure stop lamp switches, turn signal and trailer turn relays, panel and cab lighting, backup lamps and alarms, and the trailer cord, plug and receptacle.
The ASE test specification calls this content area “Lighting Systems Diagnosis and Repair”. It carries 6 of the 50 scored questions, 12% of the test.
- Find why lamps burn out, flicker or stay dark
- Aim headlamps and auxiliary fog and driving lamps
- Test headlamp switches, dimmers, relays and fuses
- Repair truck and trailer marker, tail and stop circuits
- Check air-operated stop lamp switches and LED lamps
- Diagnose turn signals, hazards and trailer turn relays
- Test backup lamps and alarms, panel and cab lighting
- Inspect the trailer cord, plug pins and receptacle
04What the questions turn on
LEDs can hide a voltage drop
Regulated LED marker lamps stay fully bright over a wide voltage range, so a trailer whose lamps all look normal can still be under the fleet's minimum at the last lamp. Voltage drop tests on the feed and ground, section by section, find the loss.
The branch layout locates the open
On a trailer side rail, each marker lamp hangs on its own branch from one feed wire. Lamps ahead of a break stay lit and every lamp behind it goes dark, so the open sits between the last working branch and the first dead one.
Trailer relays share a feed
Trailer turn relays carry the trailer lamp current on their own fused feed, so the turn switch and flasher handle only the tractor lamps and relay coils. When both trailer sides stay dark while the tractor lamps flash, look at that shared contact feed.
Faults that follow one trailer
A tractor's solid-state lamp output shuts off and logs overcurrent when a short lets too much current flow, so a fault seen with only one trailer coupled is in that trailer. Taillamps that blink only as the plug rocks mean a loose pin fit.
05The other content areas
The T6 test has five content areas. Each has its own page of free questions:
The ASE T6 Electrical/Electronic Systems practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.
ToolboxPrep is an independent study site, not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original, written from ASE’s published test specifications and task lists and other public sources. They are not actual ASE test questions and not recalled or “brain dump” questions. ASE is a trademark of its owner, and we never use the ASE logo. Standards and service information are cited, not reproduced.