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Auto Electricals: Electronic Control Units, Sensors, and Multiplexed Networks

Auto Electricals · Work through your exam, one topic at a time.

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Electronic Control Units, Sensors, and Multiplexed Networks

This domain tests how electronic control units read sensors, command actuators, and communicate over multiplexed networks. Expect questions on 5 V references, sensor signal types, CAN and LIN operation, terminating resistors, gateways, and network diagnosis.

Core rules

  • An ECU provides a regulated 5 V reference and a sensor ground. Potentiometers divide the reference; thermistors change resistance with temperature; Hall-effect and inductive sensors produce pulses; pressure and position sensors may output analog voltage, frequency, PWM, or a digital message.
  • Sensor signal types include analog voltage, duty cycle, frequency, and serial data. A shorted 5 V reference can pull down every sensor sharing it.
  • CAN bus is a differential twisted pair. CAN High and CAN Low sit near 2.5 V in the recessive state; a dominant bit drives about 2 V differential. Two 120 Ω terminators in parallel give about 60 Ω at the DLC.
  • LIN is a single-wire, 12 V, master-slave bus used for low-speed accessories. Gateways translate between networks, and CAN FD carries more data per frame.
  • Network diagnosis requires battery stability, correct termination, and testing with the network asleep and awake as specified.

Worked example

A scan tool cannot communicate with several modules. With the battery disconnected, resistance across DLC pins 6 and 14 reads 120 Ω instead of about 60 Ω. That indicates one terminator or one side of the bus is open. With the battery connected, a scope shows CAN High stuck near 0 V and CAN Low near 5 V, suggesting a short. Isolate modules one at a time and inspect the harness. Do not add a second terminator to mask the fault; repair the open circuit.

Traps

Measuring network resistance with the battery connected gives false readings. Backprobing a sealed connector can spread terminals and create new faults. Assuming a sensor is bad because its signal is high ignores a missing sensor ground. A meter reading on a CAN line shows an average, not the differential waveform. Adding termination or swapping ECUs without proving the wiring wastes time and can introduce new faults.