ASE A2 Electronic Systems Diagnosis — Questions and Answers
Question 1: A technician disconnects a shift solenoid connector and measures across the solenoid terminals with a DMM set to ohms. The meter displays OL. What does this reading indicate?
- The solenoid winding is shorted to ground
- The solenoid winding has an internal short between coils
- The solenoid winding is open (Correct answer)
- The solenoid is within specification
Correct answer: The solenoid winding is open
OL means infinite resistance, so there is no continuous path through the winding. The coil is open and the solenoid cannot be energized.
A solenoid is a coil of wire, so its resistance should fall within a specified range, often a few ohms to about 15 ohms depending on the design. Check it with the solenoid disconnected from the harness. OL means the DMM sees no path, so the winding is broken. A shorted coil would read well below specification, not OL. An open solenoid is replaced, and the harness and connector are still checked for the original cause.
Question 2: A technician tests a permanent-magnet (variable reluctance) output speed sensor with the transmission output shaft turning. Which DMM test result indicates a properly working sensor?
- AC voltage that increases as shaft speed increases (Correct answer)
- DC voltage that stays steady at about 5 volts
- A fixed 12 volts DC at all speeds
- Resistance that rises steadily as speed increases
Correct answer: AC voltage that increases as shaft speed increases
A magnetic-pickup sensor generates its own AC signal, and the amplitude and frequency rise with rotational speed.
A permanent-magnet speed sensor has a coil and magnet. Teeth on a tone ring or reluctor passing the sensor change the magnetic field and induce an AC voltage. The faster the teeth pass, the higher the voltage and frequency. With the DMM on AC volts, a good sensor shows a rising AC signal as speed rises. Static resistance of the coil can also be checked against specification. A weak or absent AC signal points to a bad sensor, an excessive air gap, or a damaged tone ring.
Question 3: Which statement correctly describes a Hall-effect input speed sensor used in an electronically controlled transmission?
- It generates its own AC voltage without any supply
- It is a simple coil whose resistance is checked with an ohmmeter to verify operation
- It is a thermistor whose resistance changes with fluid temperature
- It requires a supply voltage and produces a digital on/off signal (Correct answer)
Correct answer: It requires a supply voltage and produces a digital on/off signal
Hall-effect sensors contain electronics that need a reference or battery supply and output a square-wave digital signal.
A Hall-effect sensor has an integrated circuit that switches its output as a magnetic field or toothed target passes it. Because of that circuit, the sensor needs a supply voltage, a ground and a signal wire. The output is a square wave whose frequency rises with speed. Unlike a magnetic-pickup sensor, it still produces a signal at very low speed. Diagnosis includes checking supply and ground at the connector and viewing the signal with a scope or a DMM frequency function.
Question 4: With the ignition off, a technician measures resistance between CAN High and CAN Low at the data link connector and reads about 120 ohms. A normal two-terminator high-speed CAN network should read about 60 ohms. What is the most likely cause?
- The CAN High wire is shorted to battery voltage
- One of the network's terminating resistors or its circuit is open (Correct answer)
- The CAN Low wire is shorted to ground
- Both terminating resistors are working normally
Correct answer: One of the network's terminating resistors or its circuit is open
Two 120-ohm terminating resistors in parallel read about 60 ohms. A reading near 120 ohms means only one resistor is in the circuit.
A high-speed CAN network has a 120-ohm terminating resistor at each end of the bus. Measured at the DLC between CAN High and CAN Low with power off, they act in parallel and read about 60 ohms. A reading near 120 ohms means one resistor or its wiring is open or disconnected. A short between the two wires would read near 0 ohms. A short to power or ground would not show as 120 ohms across the pair. An open terminator can cause intermittent communication faults and U-codes.
Question 5: A technician measures the voltage drop across the ground circuit of a transmission control module with the circuit operating and reads 0.8 volt. What does this indicate?
- Normal ground circuit condition
- A shorted solenoid winding
- Excessive resistance in the ground circuit (Correct answer)
- A failed terminating resistor
Correct answer: Excessive resistance in the ground circuit
A good ground circuit should drop only a small fraction of a volt, commonly 0.1 to 0.2 volt or less. A drop of 0.8 volt means high resistance.
A voltage drop test measures the voltage lost across a portion of a circuit while current is flowing. Ground circuits should have very little resistance, so the drop should be minimal. A reading of 0.8 volt shows that a connection, terminal or wire is corroded, loose or damaged. Poor module grounds can cause erratic shifting, false sensor readings and communication faults. Repair the high-resistance point and retest rather than replacing the module.
Question 6: While a technician slowly moves the throttle from closed to wide open, the scan tool throttle position value drops to zero for an instant at one point in the sweep and then recovers. The transmission shows harsh, erratic shifts. What is the most likely cause?
- A worn spot in the throttle position sensor resistance track (Correct answer)
- A restricted transmission cooler line
- A low reading from the fluid temperature sensor
- A missing terminating resistor on the CAN bus
Correct answer: A worn spot in the throttle position sensor resistance track
A smooth sensor should give a continuous, gradual signal. A momentary dropout points to a worn spot or open in the sensor's resistive track.
The throttle position sensor is a potentiometer that gives the PCM and TCM a variable signal from low to high voltage as the throttle opens. Shift scheduling and line pressure depend on this input. A worn track causes signal dropouts at commonly used throttle positions. The control module then briefly sees a false throttle value and changes line pressure and shift points, producing harsh or erratic shifts. A sweep test with a scan tool or a DMM or scope on the signal wire reveals the dropout.
Question 7: A technician replaces a valve body and clutch pack during an overhaul and the new components shift harshly even though no DTCs are stored and the pressure control solenoid tests good. What should the technician do next?
- Replace the TCM immediately
- Install a larger-value terminating resistor
- Replace the fluid temperature sensor
- Clear the old adaptive values and perform the manufacturer's adaptive learn procedure (Correct answer)
Correct answer: Clear the old adaptive values and perform the manufacturer's adaptive learn procedure
Adaptive shift strategies store correction values for the old clutch wear. After mechanical repairs these must be reset and relearned.
Many electronic transmissions adjust pressure and fill timing based on learned clutch application times. After an overhaul or valve body replacement the stored values no longer match the transmission and can cause harsh, flared or delayed shifts. The correct step is to clear or reset the adaptives with the scan tool and run the specified drive or learn procedure. Replacing the TCM or other parts without that step wastes time and does not address the cause.
Question 8: A vehicle sets several unrelated DTCs at once for the throttle position sensor, a pressure sensor and another 5-volt sensor, all with low-voltage signals. Which fault is most likely?
- A failed transmission fluid pump
- A shorted 5-volt reference circuit shared by those sensors (Correct answer)
- An open shift solenoid winding
- A failed output shaft speed sensor
Correct answer: A shorted 5-volt reference circuit shared by those sensors
Sensors that share a 5-volt reference all lose their supply if that circuit is shorted to ground, so several sensors fail together.
Control modules often supply one 5-volt reference line to several sensors. If that line is shorted to ground, or the module's regulator fails, every sensor on it reads low and sets codes at the same time. Diagnose by checking the reference voltage at the sensors and disconnecting them one at a time to find a shorted sensor or harness. Multiple unrelated sensor codes should lead you to look for a shared circuit before condemning individual sensors.
Question 9: A technician tests a transmission fluid temperature sensor that is a negative temperature coefficient (NTC) thermistor. As the sensor is heated, what should the DMM show?
- Resistance that stays constant
- Resistance that increases
- Resistance that decreases (Correct answer)
- Resistance that changes to OL
Correct answer: Resistance that decreases
An NTC thermistor's resistance falls as temperature rises.
Fluid temperature sensors are commonly NTC thermistors. Resistance is high when the fluid is cold and drops as it warms, which the TCM reads as a changing voltage signal. A technician can check the sensor by measuring resistance at known temperatures and comparing to the specification. A sensor that does not change, or that reads OL or zero, is faulty. The TCM uses the temperature signal for shift timing, torque converter clutch operation and fluid warm-up strategies.
Question 10: A scan tool cannot communicate with the transmission control module, although it communicates with other modules on the vehicle. What should the technician check first?
- Replace the TCM and reprogram it
- Replace the valve body solenoid pack
- Flush and refill the transmission fluid
- Power, ground and fuse supply circuits to the TCM (Correct answer)
Correct answer: Power, ground and fuse supply circuits to the TCM
A module that does not respond often has no power or ground. These simple circuits should be verified before condemning the module.
A control module needs battery voltage, ignition feeds where applicable, and good grounds to function and communicate. If other modules talk normally, the network is probably intact and the problem is local to the TCM or its feeds. Check fuses, test for power and ground with a test light or DMM, and perform voltage drop tests. Check the data line connection as well. Replace or reprogram the module only after the supply circuits, connector and wiring are proven good.
ASE A2 Electronic Systems Diagnosis
A technician disconnects a shift solenoid connector and measures across the solenoid terminals with a DMM set to ohms.
The meter displays OL.
What does this reading indicate?
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