Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisEasy
A technician is diagnosing an intermittent issue where the vehicle's power windows occasionally stop working. The technician consults the wiring diagram and notes that the power window control module communicates with other modules via a Controller Area Network (CAN) bus. Which of the following multimeter readings, taken across the CAN-High and CAN-Low wires, would indicate a healthy CAN bus communication at rest (recessive state)?
- ACAN-High: 2.5V, CAN-Low: 2.5V
- BCAN-High: 0V, CAN-Low: 0V
- CCAN-High: 5V, CAN-Low: 0V
- DCAN-High: 3.5V, CAN-Low: 1.5V
Show answer & explanationAnswer & explanation
Correct answer: A. CAN-High: 2.5V, CAN-Low: 2.5V
In a healthy CAN bus system, when no data is being actively transmitted (recessive state), both the CAN-High and CAN-Low lines are typically biased to approximately 2.5 volts. This common voltage level ensures a robust signal for when data transmission begins.
Why the other options are wrong
- B. 0V on both lines would indicate a complete loss of power to the bus or a short to ground.
- C. These voltages are not characteristic of a standard CAN bus system in either recessive or dominant states.
- D. These voltages (3.5V and 1.5V) represent the dominant state, where data is actively being transmitted.
CAN Bus Recessive State
The idle or 'at rest' state of a Controller Area Network (CAN) bus, where no data is actively being transmitted, and both CAN-High and CAN-Low lines are at a common, biased voltage.
- Both CAN-High and CAN-Low are at approximately 2.5V.
- Represents a logic '1' or idle condition.
- Crucial for verifying bus integrity when not actively transmitting data.
Memory trick: Recessive is RESTING in the middle.