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A 2015 Mercedes-Benz E260L is experiencing an issue where the engine won't start. Communication error codes are stored in multiple control modules. How can the cause of the CAN bus fault be accurately pinpointed?
A 2015 Mercedes-Benz E260L won’t start. Communication fault codes are stored in multiple control modules. How can we pinpoint the cause of the bus fault?
After picking up the vehicle, a test drive was conducted. When the ignition switch was turned on, several warning lights on the instrument cluster remained illuminated; upon attempting to start the engine, the starter motor did not engage. Using an automotive diagnostic scanner, it was discovered that multiple control modules contained fault codes indicating communication failures (missing data) with the engine control module, transmission control module, and steering column control module, Additionally, the central gateway module contained multiple fault codes indicating communication failures on the Chassis Controller Area Network (CAN E) bus—including bus shutdown, short to ground, and short to positive—leading to the conclusion that the vehicle’s CAN E bus communication had failed.
Locate the CAN E bus voltage distributor X30/30 at the left front door sill (Fig. 1). It has a total of 7 wire connectors, numbered sequentially from 1 to 7; the connector with 3 wires is connector No. 1.
Using the Hongke Pico automotive oscilloscope to measure the CAN E bus signal waveform at connector No. 1 (Figure 2), we found that the signal waveform was noisy, with the CAN H and CAN L signals essentially overlapping. The latent voltage was approximately 0.7 V, which is too low (the normal value is approximately 2.5 V). This led to the suspicion that the CAN L and CAN H circuits were shorted to ground (with resistance).
Disconnect the wire connectors numbered 2 through 7 in sequence; when the connector numbered 7 is disconnected, the CAN E bus signal waveform returns to normal (Figure 3), The CAN H signal and the CAN L signal are mirror images of each other. The low-level voltage of both the CAN H and CAN L signals is approximately 2.5 V, while the high-level voltage of the CAN H signal is approximately 3.4 V and that of the CAN L signal is approximately 1.6 V.
A review of the circuit diagram revealed that connector No. 7 is connected to the collision avoidance assist control module (A90). The A90 was located in the front bumper, and an inspection revealed that a large amount of water had drained out of it, indicating that water had entered the A90.
After replacing the A90, we conducted repeated test runs, and the engine started and ran normally again, resolving the issue.
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