Electronic Systems Diagnosis Flashcards

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?

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 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?

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.

Which statement correctly describes a Hall-effect input speed sensor used in an electronically controlled transmission?

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.

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?

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 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?

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.

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?

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.

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?

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.

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?

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.

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?

Answer: Resistance that decreases

An NTC thermistor's resistance falls as temperature rises.

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?

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.