Five free practice questions on motor/generators and drive units: content area C of the ASE L3 Light Duty Hybrid/Electric Vehicle Specialist test, 9 of its 45 scored questions (20%). 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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On an electrified vehicle, a resolver is mounted on each drive motor/generator (MG). What information does it give the module that controls the MG's inverter?
- The rotor's angle, speed and direction
- The current in each of the phase cables
- The temperature of the stator windings
- The MG's phase voltage, current and frequency
Show the answer
A. Correct. The resolver tells the inverter's control where the rotor's magnetic poles are, how fast the rotor turns and which way. The inverter needs that angle to switch current into the right stator windings at the right moment; without a valid signal it cannot produce controlled torque.
Why not the others
B. Phase current is measured by current sensors on the phase conductors at the inverter, not by the resolver. The resolver reports rotor position, speed and direction, which the inverter uses to time its switching.
C. Stator temperature comes from a temperature sensor (thermistor) built into the windings. The resolver is a rotor position sensor: it reports angle, speed and direction.
D. The inverter itself produces and measures the phase voltage, current and frequency it sends to the MG. The resolver is a small rotary transformer whose output tells the control the rotor's angle, speed and direction.
ID L3-C-001 · Task L3-C-7 (ASE task list)
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An insulation tester is being used to check a drive MG's stator windings for leakage, with the system de-energized and the phase cables removed from the MG's terminals. With the first lead on a phase terminal, where is the second lead connected?
- To clean bare metal of the MG's housing
- To the HV battery pack's negative terminal
- To the resolver connector's ground pin
- To a painted bolt head on the MG housing
Show the answer
A. Correct. The test measures leakage from the windings to the grounded case they sit in, so the second lead goes on the MG housing, on bare metal for a solid connection. A leak to the case then shows as a low reading.
Why not the others
B. The HV system floats: its negative side is isolated from the chassis, not tied to the case. Leakage is measured to the MG housing, where a failed winding would actually leak.
C. The resolver is a separate low-voltage circuit; testing to its pin checks winding-to-resolver insulation and puts the tester's voltage on sensor wiring. The second lead goes on the MG housing.
D. The housing is the right reference, but paint insulates, so the reading could look good even with a leak. The lead goes on clean, bare metal of the MG housing.
ID L3-C-017 · Task L3-C-6 (ASE task list)
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A drive motor's resolver has one excitation winding and two output windings, sine and cosine, set 90 degrees apart. Why are two output windings used?
- Together they fix rotor angle and direction
- One is a spare used if the other winding fails
- One reports speed and the other reports temperature
- Each one feeds a separate inverter for redundancy
Show the answer
A. Correct. As the rotor turns, the two output amplitudes vary as the sine and cosine of the rotor angle; read together, with their signs, they fix the angle uniquely over a full turn, and which one leads tells the direction. One output alone repeats its values and cannot tell 30 degrees from 150 degrees.
Why not the others
B. Both windings are used all the time; losing either one sets a resolver DTC because the angle can no longer be worked out. They are not a primary and a spare.
C. Speed is calculated from how fast the angle changes, and temperature comes from a separate sensor in the stator. The two outputs together give angle and direction.
D. Both outputs go to the same motor control circuit, which needs both to compute one angle. A resolver does not split its outputs between inverters.
ID L3-C-021 · Task L3-C-7 (ASE task list)
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On a battery electric vehicle, the MG is cooled by drive-unit fluid that an electric oil pump circulates through a heat exchanger cooled by the drive unit's coolant loop. After a long climb towing a trailer, it sets a 'Drive Motor Over Temperature' DTC and limits power. Any of these could cause it EXCEPT:
- a heat exchanger with blocked coolant passages.
- an electric oil pump that has stopped running.
- drive-unit fluid well below the specified level.
- an open in the resolver excitation circuit.
Show the answer
D. Correct. Without excitation the resolver gives no signal, so the inverter cannot time the MG's current: a resolver DTC sets and the MG cannot be driven to make torque, so it makes no extra heat. Over-temperature needs a cooling or load cause like the other three.
Why not the others
A. If coolant cannot flow through the heat exchanger, the fluid cannot shed the stator's heat and the motor runs hot. It could cause the DTC.
B. With the pump stopped, fluid no longer carries heat from the stator to the heat exchanger, so the motor overheats under load. It could cause the DTC.
C. Too little fluid means the pump cannot keep the stator supplied and cooled, so temperature climbs on a long, hard climb. It could cause the DTC.
ID L3-C-026 · Task L3-C-8 (ASE task list)
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A battery electric vehicle will not come out of P: the P indicator flashes and a 'Park Lock Actuator Supply Voltage Low' DTC is stored. At POWER ON the low-voltage battery reads 10.9 volts, and the manufacturer says the actuator needs at least 11.5 volts to operate. What should the technician do next?
- Replace the park lock actuator and run its learn
- Test and charge the battery and retest P
- Inspect the parking pawl and gear for damage
- Check the actuator motor windings for an open
Show the answer
B. Correct. The actuator runs on the low-voltage system, and 10.9 volts is below the 11.5 volts it needs, which matches the DTC. The low-voltage battery is tested and charged (or replaced) by its own type's procedure, the DTC cleared, and P release retested before any part is condemned.
Why not the others
A. The DTC and the 10.9-volt reading point to supply voltage, not the actuator. A new actuator would fail the same way on a weak battery.
C. A damaged pawl or gear is a mechanical problem and would not set a supply-voltage DTC. The battery below the actuator's 11.5-volt need is corrected first.
D. An open winding sets an actuator circuit DTC, not a supply-voltage one, and the measured 10.9 volts already explains the fault. The low-voltage battery comes first.
ID L3-C-032 · Task L3-C-9 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | A |
| Q2 | A |
| Q3 | A |
| Q4 | D |
| Q5 | B |
03What area C covers
This content area covers the motor/generators (MGs) that drive the wheels and the drive units that house them: the safety steps specific to MG work, insulation tests of windings and phase cables, drive motor codes, phase cable and resolver wiring faults, resolver and encoder position sensors, noise, shudder, overheating and no-drive complaints, the electric parking pawl, and drive-unit fluid and coolant requirements.
The ASE test specification calls this content area “Drive Systems”. It carries 9 of the 45 scored questions, 20% of the test.
- Verify zero volts at the MG phase terminals
- Insulation-test MG windings and phase cables
- Diagnose drive motor, resolver and park lock DTCs
- Find faults in phase cables and resolver wiring
- Test resolver windings and their signals
- Diagnose MG noise, shudder, overheating and weak output
- Diagnose electric parking pawl faults and learns
- Check drive-unit fluid type and level
04What the questions turn on
Test what the hands will touch
A zero at the inverter input does not clear the MG, since rotor movement can induce voltage in its windings. With the cover off, zero is confirmed phase to phase and phase to case at the terminals, with the meter proven before and after.
Ohms are not insulation tests
A DMM's ohms range works with only a few volts, so damaged insulation can still read OL. An insulation tester applies the manufacturer's test voltage to a de-energized circuit, and its leads stay on until the charge it built up has bled off.
Each phase reading holds two paths
A phase-to-phase resistance reading includes two phase paths, so extra resistance in one phase raises both readings that contain it, and the pair still in specification excludes the faulty phase. The windings are joined, so a leak to the case reads on every terminal.
The sensor reports the actuator
A park actuator's position sensor shows where the actuator went, not whether the pawl is holding the parking gear. A vehicle that rolls in P with the actuator reading engaged and no DTC points to a broken pawl in the drive unit.
05The other content areas
The L3 test has five content areas. Each has its own page of free questions:
The ASE L3 Light Duty Hybrid/Electric Vehicle Specialist 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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