How Mercedes' CAN WorksPosted 5/15/2007
By Craig Zuidema
The out-of-breath tech had just spent five minutes reciting the code list spat out of the 2001 Mercedes-Benz E320 by his new, fully functional Mercedes-Benz compatible scan tool. Many of the codes in the description contained three words technicians are quickly learning to dread: controller area network (CAN) communication fault.
Where do I start?" This was a justifiable question. There were more than 25 diagnostic trouble codes (DTCs) from the powertrain control module (PCM), the electronic transmission control (ETC) module, the traction systems control module and several other body modules that receive or send information over the CAN bus to these modules. The problem seemed to be centered around the traction systems' control module, an amalgamation of chassis control systems including brake assist system (BAS), anti-lock braking (ABS), electronic stability protection (ESP) and acceleration slip reduction (ASR) homogenized into the "traction systems control module" by Mercedes-Benz. Combined, these systems serve to make braking and handling on any late-model Mercedes nearly idiot-proof.
Since 1992, Mercedes-Benz has used a two-wire redundant CAN communication network to link information transfer between an ever-increasing list of computers. In theory, if one CAN wire is damaged, the modules will still be able to communicate on the other wire. In practice, it seldom works that way. The data signal on the CAN wires is a digital signal pulsed by the control unit in a set pattern, which is decoded inside the receiving control module into data. Even with a scope fast enough to capture the signal, which runs at 125Kbits/second, a decoding algorithm would have to be used. This is why a scan tool is required.
I chose to focus on the basic fault codes and work back to the CAN fault codes. First was the C1200, stop lamp switch/plausibility. Brake light switch failure on these cars is a known pattern fault. A quick - but not foolproof way to test - is with the scan tool reading the data stream. The brake light switch is a dual-contact switch with a normally open contact and a normally closed contact. Both contacts have to switch in a set pattern when the brake pedal is applied and released, within a specific learned voltage range measured on the brake pedal travel sensor. The switch is continuously monitored, so just one glitch can set the code. But we ran up against the limits of the scan tool, where the baud rate of the scanner is vastly exceeded by the speed of the CAN bus. Just because the data stream of the scanner doesn't show the switch fault doesn't mean it hasn't glitched - only that the scanner didn't see it. After replacing the brake light switch, the C1200 code stayed erased.
Next code was C1103, the right rear axle wheel speed sensor. An ohm check at the sensor connector showed it open. Something passing under the vehicle had cut the wire, also the outer CV boot on the axle, which would have to be fixed later. We weren't the only ones being run over by the CAN bus! With the new sensor in place, however, we still had CAN fault codes returning: C1025, CAN communication faulty to BAS control module and C1000, ESP control module N47/5 faulty.
Because I really didn't suspect a faulty control module at this point, several wiring tests proved the continuity of the CAN wiring to the PCM and to the ETC module. I wondered if there was a relearn procedure for the traction systems module, even though there is no hint of such a procedure in the Mercedes-Benz service information.
I had the tech start the car, move it backward, and drive it forward to relearn the wheel speed sensor inputs. All to no effect. I still thought we were on the right track and remembered a technical service bulletin (TSB) on an older model's steering angle sensor initialization. After driving the car around the parking lot, it was now setting code C1140, steering angle sensor/initialization.
Bingo! The tech cleared the codes again, started the vehicle, stepped on the brake several times, then initialized the steering angle sensor by cranking the steering all the way to left lock for two seconds, all the way to right lock for two seconds, then back to center. After a lap around the parking lot, the BAS light went off and the scan tool showed no more CAN fault codes. What happened?
Since the CAN communication network covers so many modules and signals, it has to organize by priority. The vehicle speed signal is high priority, generated inside the traction systems control module by reading the wheel speed sensor signals. Until the traction systems control module can compute plausible data requested by other control modules, it doesn't communicate "no data." It simply doesn't communicate. These switches and sensors are, in effect, "prove-out circuits" for the traction systems control module, with the highest priority being to the brake light switch, then the steering angle sensor, then the wheel speed sensors. Once these tests are fulfilled in the traction systems control module, communication of the data on the CAN bus starts.
Lack of information in function description from the vehicle manufacturer and parts manufacturer is the biggest hurdle in trying to analyze faults with these multi-computer linked chassis and body control modules. Unlike powertrain systems, there is no legal obligation to provide repair information to the aftermarket.
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