Five free practice questions on general electrical diagnosis: content area A of the ASE T6 Electrical/Electronic Systems test, 14 of its 50 scored questions (28%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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Why is a high-impedance digital multimeter, rather than a test light, used to read the signal voltage of a 5-volt sensor at a truck's engine ECM?
- It feeds a small test voltage that confirms the circuit
- It loads the circuit enough to expose a weak connection
- Its fused input protects the ECM if a probe slips
- It draws too little current to pull the signal down
Show the answer
D. Correct. On DC volts a digital multimeter's input resistance is millions of ohms, so it takes almost no current from the sensor circuit and reads the signal the ECM sees. A test light draws enough current to pull a sensor signal down and can overload an ECM's low-current circuits.
Why not the others
A. A meter feeds its own test current only on its resistance and diode-test ranges, into an unpowered circuit. On DC volts it only measures, and its value here is that it takes almost no current from the signal.
B. Loading the circuit is what a test light does, and on a sensor signal it pulls the voltage down and gives a false reading. The high-impedance meter is chosen because it does not load the circuit.
C. A meter's fuses protect its current inputs, not its voltage input, and they do nothing for the ECM. The meter is chosen because its high input resistance draws almost no current from the signal.
ID T6-A-004 · Task T6-A-1 (ASE task list)
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A truck's windshield wiper motor draws more current than the manufacturer's specification while it runs. Any of these could cause the high current EXCEPT:
- a binding pivot in the wiper linkage.
- shorted turns in the armature windings.
- high resistance in its feed circuit.
- dry or worn armature bushings.
Show the answer
C. Correct. Extra resistance in the feed circuit is in series with the motor, so it lowers the voltage at the motor and reduces the current. It would make the motor run slow with lower current, not draw high current.
Why not the others
A. A binding pivot makes the motor work harder to move the arms, and a permanent-magnet motor draws more current as its load rises, so this could cause the high reading.
B. Shorted turns lower the armature's resistance and its counter-voltage, so the motor draws more current than specified; this could cause the high reading.
D. Dry or worn bushings add friction and can let the armature drag, which loads the motor and raises its current draw, so this could cause the high reading.
ID T6-A-009 · Task T6-A-2 (ASE task list)
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A tractor's telematics unit is suspected of waking now and then overnight and draining the batteries, but a key-off draw reading taken after the modules sleep looks normal. Which meter setup can catch the wake-ups?
- A clamp set to AC amps to show the wake-up pulses
- One reading taken right after the key is turned off
- A min/max recording clamp left on for several hours
- One reading with the telematics fuse removed
Show the answer
C. Correct. A unit that wakes for short periods between long sleeps is missed by spot readings; a clamp meter recording its maximum, or a logging meter, left on the negative cable while the truck sits catches each wake-up and its size. Comparing what is recorded with the unit's normal behavior shows whether it is waking when it should not.
Why not the others
A. Key-off draw is direct current, and a wake-up is a rise in that DC current, not an AC signal. A DC clamp recording its maximum over several hours catches it.
B. Right after key-off the modules are still awake, so the reading is high for a normal reason and says nothing about later wake-ups. The current has to be recorded over the hours the truck sits.
D. A single reading with the unit unpowered cannot show the unit waking up, and pulling its fuse may reset it. Recording the DC draw over several hours with everything connected is what catches the wake-ups.
ID T6-A-022 · Task T6-A-5 (ASE task list)
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A small capacitor is connected between the power feed terminal of a truck's electric fuel transfer pump motor and ground. What does the capacitor do?
- Passes high-frequency noise to ground but blocks steady DC
- Stores enough charge to run the motor briefly after shutoff
- Lets current flow to ground in one direction only
- Adds resistance in series to limit the starting current
Show the answer
A. Correct. A capacitor stops DC once it has charged but offers a low-impedance path to fast voltage changes, so the electrical noise the motor makes as it runs is shunted to ground instead of traveling along the feed wire into radios and modules. Because it blocks DC, it does not drain the battery or bypass the motor.
Why not the others
B. A small noise-suppression capacitor holds far too little energy to keep a motor turning; it discharges in a tiny fraction of a second. Its job is to shunt high-frequency noise to ground while blocking DC.
C. Conducting in one direction only is what a diode does. A capacitor has no forward and reverse direction for steady current: it blocks DC in both directions and passes rapidly changing (noise) voltages to ground.
D. The capacitor is connected from the feed to ground, in parallel with the motor, not in series with it, so it cannot limit the motor's current. It shunts the motor's noise to ground and blocks steady DC.
ID T6-A-028 · Task T6-A-7 (ASE task list)
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On a truck's wiring schematic, a dashed line is drawn from the moving blade of one switch to the moving blade of another switch. What does the dashed line mean?
- One switch is built into a control module
- Both switches share one ground splice
- The wire between them is shielded cable
- Both switches are moved by one control
Show the answer
D. Correct. A dashed line joining switch blades shows a mechanical link: the two switch sections are operated together by one knob, lever or plunger, as in a multi-section headlamp or ignition switch. Each section can still switch a different circuit.
Why not the others
A. A component that is part of a larger assembly is shown inside a dashed outline box, not by a line joining two switch blades. The line between blades means the two switches are mechanically linked.
B. Electrical connections, including shared grounds, are drawn as solid lines meeting at a splice dot. A dashed line between switch blades shows that the switches move together mechanically.
C. Shielding is drawn around the conductors it covers, not as a line between two switch blades. A dashed line linking the blades shows that one control moves both switches.
ID T6-A-038 · Task T6-A-9 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | D |
| Q2 | C |
| Q3 | C |
| Q4 | A |
| Q5 | D |
03What area A covers
This content area takes in the meter work and circuit logic behind every truck and tractor electrical repair: voltage drop, current and resistance readings, tracing shorts, grounds and opens, key-off drain on multi-battery banks, fuses, breakers, PTC devices and fusible links, suppression diodes and capacitors, relays and solenoids, schematics, J1939 data link faults, service software and calibrations, and lab scope waveforms.
The ASE test specification calls this content area “General Electrical/Electronic System Diagnosis”. It carries 14 of the 50 scored questions, 28% of the test.
- Take loaded voltage drop and clamp-on current readings
- Check resistance and continuity with the circuit unpowered
- Trace shorts to ground, opens and high resistance
- Track down key-off drain on multi-battery trucks
- Test fuses, breakers, PTC devices and fusible links
- Check suppression diodes, resistors, capacitors, relays and solenoids
- Read schematics and diagnose J1939 data link faults
- Use service software, check calibrations and read waveforms
04What the questions turn on
Where the voltage is lost
Readings taken to battery negative with the load running show where voltage is lost. A drop split evenly along a feed wire means the wire is too small for its current, while a drop across one connector's mated halves points to its terminals.
Counting terminators on the backbone
With the batteries disconnected, the two 120-ohm terminators at the ends of a J1939 backbone read 60 ohms from CAN high to CAN low. A 40-ohm reading means a third terminator is on the bus, such as one brought in by an added telematics unit.
Clamp every return path
Key-off current returns through every negative cable, so on a bank with one cable to the starter and one to the frame, both go inside the clamp jaws together. A unit that wakes now and then overnight needs a recording clamp left on for hours.
Suppression protects the driver
A coil switched off sends a voltage spike up its control wire. A diode across the coil clamps that spike lower than a resistor does, so an ECM that calls for a diode-suppressed relay can lose its driver to a relay without one.
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.
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