An automotive wiring diagram draws each circuit as a path from power, through protection and controls, to a load and back to ground. The symbols below cover the parts you trace on a car schematic; the sections after them show how to read one with a meter, the way the A6 electrical test asks. In the 2026 study guide from ASE, 3 of the 9 sample questions are built on a circuit diagram, two of them with a meter on it.
3 of 9ASE samples built on a diagram
8circuit questions here
26%of A6 is general diagnosis
Wiring diagram symbols
Drawing styles differ between manufacturers, so check the legend in the service information. These are the shapes to know:
Battery. A stack of long and short plates; the long plate is positive.
Ground. A short line over shorter parallel lines. A ground drawn on a component means it grounds through its case or mounting.
Fuse. An S-curve or a small box with a line through it, labeled with its rating. Circuit breaker. A bowed contact between two terminals; it resets. Fusible link. A short length of wire thinner than the circuit it protects, labeled as a fusible link.
Resistor. A zigzag. With an arrow through it, a variable resistor: a potentiometer (three terminals) or a rheostat (two).
Switch. A hinged line drawn in its rest position, open (gap) or closed (touching). A dashed line between contacts means they move together.
Relay. A coil beside a switch, linked by a dashed line. On an ISO mini relay the coil is 85 and 86, the common contact 30, the normally open output 87 and the normally closed output 87a.
Diode. A triangle pointing at a bar, with current flowing the way the triangle points. An LED adds two outward arrows; a zener diode has a bent bar.
Motor. A circle with an M. Lamp. A circle with a loop or an X inside. Solenoid or coil. A looped line.
Splice and connector. A dot where wires join (wires that cross without a dot are not connected); connectors are arrowheads or boxes with a connector and pin number, such as C201 pin 4.
Twisted pair and shield. Two wires drawn twisted, as for the two lines of a CAN bus; a dashed outline around wires is a shield.
Wire labels. A color code such as BLK, or RED/WHT for red with a white stripe, plus a gauge or circuit number where the maker prints one.
Reading a circuit: power, control, load, ground
Trace one circuit at a time, in the direction current flows:
Power. The battery, or a fuse fed from it. The diagram says whether that fuse is hot at all times, in run, or in start.
Protection. The fuse, breaker or fusible link for this circuit.
Control. A switch, a relay or a module output. Controls can sit on the power side or the ground side of a load, and the readings differ for each.
Load. The lamp, motor or module that does the work. In a healthy series circuit it uses nearly all of the source voltage.
Ground. Back to battery negative through a ground point (G101 and so on). When several circuits share one ground, one bad ground makes unrelated items act up together.
Then ask what each reading would be if the circuit were good. A diagram question compares that with the reading you are shown.
What the meter readings mean
With the circuit powered and switched on:
Voltmeter from a point to ground shows the voltage available there: full source voltage before the load, close to 0 V after it.
Voltmeter across a closed switch, a connection or a wire should read close to 0 V. A reading above the maker's limit is a voltage drop: resistance that only shows under current.
Voltmeter across an open reads close to source voltage, because the meter bridges the break through the rest of the circuit. That jump is how you find the break. The brake-light sample question in the 2026 guide asks you to place an open from three meter readings.
Ohmmeter only on an unpowered, isolated part. It finds opens and shorts but can pass a connection that fails under load.
Ammeter in series: higher current than spec points to a short or a dragging motor, lower current to added resistance.
Opens, shorts and high resistance
Open. Nothing works past the break. Voltage is present up to it and missing after it, and the fuse is fine.
Short to ground on the power side. The fuse blows or the breaker cycles as soon as the circuit is switched on. Put a test lamp across the fuse terminals and unplug connectors down the circuit until the lamp goes out.
Short to ground on a ground-switched control wire. The load comes on by itself, such as horns that sound without the button pressed.
Short to power. One circuit feeds another, such as a lamp glowing with its switch off.
High resistance. The load works, but weakly: a dim lamp, a slow motor. A voltage drop test with the circuit on finds it.
Trap: the jumper across 30 and 87
If a load works with a jumper between relay terminals 30 and 87, the feed, the output wire, the load and its ground are all good. The fault is in the relay or its control side: the 85/86 feed, the switch or the module. The horn sample question in the guide tests exactly this split.
8 circuit questions
0 of 8 answered · 0 correct
Question 1 of 8 · General Electrical/Electronic System Diagnosis · 26%Not answered
Bulb Location
Quantity
Wattage Spec (per bulb)
Headlights (Low Beam)
2
55W
Marker Lights
4
5W
Tail Lights
2
10W
Given a 12.0V system and the installed bulbs listed in the table, what is the total expected continuous amperage draw on this lighting circuit?
Explanation
Incorrect: 14.45 A corresponds to about 173 W, more than the 150 W the bulbs draw.
Correct: The bulbs total 110 + 20 + 20 = 150 W, and 150 W ÷ 12 V = 12.50 A.
Incorrect: 10.42 A matches only 125 W, so part of the bulb load was left out.
Incorrect: 18.25 A would need about 219 W, well above the 150 W total.
Incorrect: 15.00 A would need 180 W; it overstates the table's total wattage.
Question 2 of 8 · General Electrical/Electronic System Diagnosis · 26%Not answered
A technician measures resistance between CAN_H and CAN_L at the diagnostic connector with the battery disconnected and reads 60 ohms. After unplugging the anti-lock brake module, the resistance reading changes to 120 ohms. What does this change in resistance indicate?
Explanation
Incorrect: An open backbone would read about 120 Ω with everything connected, but the bus started at a normal 60 Ω.
Correct: Two 120 Ω terminators in parallel read 60 Ω; unplugging the ABS module removed one and left the other at 120 Ω.
Incorrect: A short to ground is checked from each line to chassis and does not produce a clean step from 60 to 120 Ω.
Incorrect: Unplugging a non-terminating node leaves the reading at 60 Ω, so the jump to 120 Ω rules it out.
Incorrect: Removing a shorted module raises a reading that was below 60 Ω; this bus started at a normal 60 Ω.
Question 3 of 8 · General Electrical/Electronic System Diagnosis · 26%Not answered
A technician is diagnosing a vehicle whose wiper motor runs slowly whenever the driver turns on the headlights. Schematic review shows that both circuits share a common ground splice at the chassis. Which of the following conditions is the most likely cause of this symptom?
Explanation
Incorrect: With the headlight feed open, no headlight current flows to load the shared ground.
Incorrect: A socket short to power affects the headlight circuit and does not load the shared ground.
Correct: Headlight current through a resistive shared ground raises the splice voltage, leaving less across the wiper motor, so it slows.
Incorrect: A short to ground would blow the wiper fuse or run the wiper constantly, with no link to the headlights.
Incorrect: An internally shorted wiper motor would act up whether or not the headlights are on.
Question 4 of 8 · Battery and Starting System Diagnosis and Repair · 18%Not answered
A vehicle will not crank. Battery voltage is 12.4 V at rest and 10.7 V during the crank attempt. During cranking, a voltmeter reads 10.5 V at the starter B+ terminal and 3.2 V on the starter motor case, both measured relative to the negative battery post. What is the most likely cause?
Explanation
Incorrect: The positive side drops only 0.2 V (10.7 V at the battery, 10.5 V at B+), which is acceptable.
Correct: The starter case should sit within a few tenths of a volt of battery negative while cranking; 3.2 V means the ground return is resistive.
Incorrect: A failed armature does not lift the case voltage; the 3.2 V reading is in the ground return outside the motor.
Incorrect: The control side works, because 10.5 V reaches the starter B+ terminal during the attempt.
Incorrect: The battery holds 10.7 V while cranking, so it is supplying voltage; the loss is on the ground side.
Question 5 of 8 · Lighting Systems Diagnosis and Repair · 12%Not answered
A vehicle's headlight circuit has a 15 A fuse F1 feeding the headlight switch. The headlight switch has separate output terminals for low beam and high beam. Fuse F1 blows immediately when the headlight switch is turned to low beam, but does not blow when switched to high beam. Which wire segment is most likely shorted to ground?
Explanation
Incorrect: A short on the switch feed would blow F1 in every position, high beam included.
Incorrect: F1 survives on high beam, which clears the high-beam output wire.
Correct: The fuse blows only when the low-beam output is energized, so the short to ground is downstream of that output.
Incorrect: The ground return is already at ground potential, so contact with ground there cannot blow the fuse.
Incorrect: A short ahead of F1 bypasses the fuse, so F1 would not blow.
Question 6 of 8 · Battery and Starting System Diagnosis and Repair · 18%Not answered
In a series circuit: battery positive to closed ignition switch, then to relay coil terminal 85, coil terminal 86 to neutral safety switch, and from there to ground. The relay coil resistance is 80 ohms. The ignition switch is closed; the neutral safety switch is open. What is the voltage at relay coil terminal 86?
Explanation
Incorrect: 1 V would mean the coil drops nearly all the voltage, which takes current that an open circuit does not allow.
Correct: With the neutral safety switch open no current flows, the coil drops nothing, and terminal 86 sits at full battery voltage.
Incorrect: 6 V assumes the voltage splits between two loads, but with no current the coil drops nothing.
Incorrect: 0 V at 86 needs a path to ground, and the open neutral safety switch removes it.
Incorrect: Current in an open series circuit is zero, so the voltage at 86 can be worked out without an ammeter.
Question 7 of 8 · Body Electrical Systems Diagnosis and Repair · 34%Not answered
A rear window defroster has 10 parallel grid lines, each rated at 10.0 ohms. A technician measures the total resistance of the entire grid at the main connector without disconnecting any lines. The multimeter displays a reading of 1.11 ohms. What does this measurement indicate?
Explanation
Incorrect: A parallel path drags a reading down, and this one is higher than the 1.0 Ω expected.
Correct: Ten 10 Ω lines in parallel read 1.0 Ω; 1.11 Ω is the nine-line value (10 ÷ 9), so one line is open.
Incorrect: A feed short to ground would blow the fuse or read near 0 Ω, not slightly above normal.
Incorrect: A healthy grid reads 1.0 Ω; 1.11 Ω means the grid is not normal.
Incorrect: Shorted grid lines would pull total resistance below 1.0 Ω, not raise it.
Question 8 of 8 · Body Electrical Systems Diagnosis and Repair · 34%Not answered
A power sunroof fails to open, and the technician verifies the motor lacks power. Before replacing the motor, the technician notices the interior map lights and illuminated vanity mirrors are also entirely inoperative. What is the most likely cause of this combined system failure?
Explanation
Incorrect: A shorted winding acts on the sunroof alone unless it opens the shared fuse, which is the part to check first.
Incorrect: An open map light switch kills only the map light, not the sunroof motor.
Incorrect: A sunroof-only relay cannot explain dead map lights and vanity mirrors.
Incorrect: A BCM ground fault would hit many BCM functions, not one overhead circuit.
Correct: The sunroof, map lights and vanity mirrors all quit together, which points to the one feed they share.
Answer every question to see your score
More on this test
ASE A6 →Format, the five content areas and what makes the electrical test different
ASE A6 exam facts →Format, passing score, results, retakes and recertification, sourced
ASE A6 cost →A6 alone, a retake, A6 with A8 in one order, and keeping A6 current
ASE A6 study guide →What to study for A6, in order of weight, and the meter skills behind every area
Wiring diagrams, quick answers
How do you read an automotive wiring diagram?
Pick one circuit and follow it from its fuse through the switch or relay to the load, then to its ground point. Note where each connector and splice sits, then compare the voltage you expect at each point with what you measure.
What do the numbers on a relay mean?
On an ISO mini relay, 85 and 86 are the coil, 30 is the common contact, 87 the normally open output and 87a the normally closed output. Check the diagram printed on the relay, since some differ.
What does a dot mean on a wiring diagram?
A splice or connection. Wires drawn crossing without a dot are not connected.
Are wiring diagrams on the ASE A6 test?
Yes. Reading schematics and symbols is a task in the general diagnosis area, and 3 of 9 sample questions in the 2026 study guide are built on a diagram.