Mercedes-Benz S350 CAN-BUS Troubleshooting with Star Diagnosis

Mercedes-Benz S350, chassis W220.

Symptom: This car is the breakdown vehicle which had been removed the whole car parts. After the repair, every time the owner start the engine and then turn off the ignition switch, the engine does not flame out and the meter lights are immortal. If the engine intake pipe is blocked and force the engine to flame out, the ignition switch can not locate in the ON position, that is, it can not go to the ignition switch. After disconnecting the battery power, the ignition switch can be turned and the engine will start, but the ignition failure remains.

Troubleshooting: First, I check with V2015.05 MB SD Connect Compact 4 and get the fault codes of DTC EIS control module and I am told that: bad ignition switch circuit 15, and the fault code can not be cleared. When I check the left front SAM, right front and rear SAM SAM and other control module, I find that the diagnostic tool can not enter the system. But data streams of EIS control module shows: 15R, 15,50, which is normal.

Now there are two main problems: first, the power control module SAM system does not work,  MB SD Connect Compact 4 Star Diagnosis can not detect the car and the fault codes can not be cleared so that the engine does not stall. When EIS control module is plugged, we use test light detect the control module circuit 15 joint EIS and find that there is still 12V power, and 15R, 50th line are normal. In principle, the 15th line is provided by the EIS control module. If I turn off the ignition switch or disconnect the plug of the EIS control module, 15th line should be 0V. So why is there 12V power on the 15th line? Obviously, this has nothing to do with the EIS control module, even if the failure rate of EIS control module is high. We continue detecting wiring diagram and find the 15th line is connected to many vehicle control modules including two fast control modules— the left anterior SAM and the right front SAM. Work to be done now is to find out which one control module or linae error provides power for the 15th line in EIS control module. According to the principle from easy to hard, I first press the plug on the left front SAM and right front SAM control module to determine whether when 15 and 15R in these two control modules plug, the ignition switch becomes normal and the engine also flame out normally. After turn off the ignition switch, we measure the left front line within SAM by multimeter and find that SAM output 12V power to the 15th line. Thus, after turning off the ignition switch, the 15th line still with 12V power results from part of the line fault in left Front SAM control module (but this does not prove that SAM control module failed).

After we check left front line of SAM, we find that when we turn off the green CAN-BUS connector, the engine can BE normally turned off and the ignition switch is also normal. In the left SAM control module, there are two CAN-BUS line connector, a group connected to the X30 / 6CAN-BUS control module, and the other is connected to the control module X30 / 5CAN-BUS. X30 / 6 connectors are located beneath the steering wheel (the top of the brake pedal) which is connected with EIS control module, the steering column module, AAC air conditioning control module, instrument cluster control module, UCP upper part of the control module, control module and the right front SAM Left Front SAM control module. Then we try our best to find the connector. We unplug the connector X30 / 6 one by one, and detect voltage with a multimeter on the CAN-BUS (the normal voltage is 0 ~ 5V). But when we check another CAN-BUS connector, fingding that the voltage is 12V while the other line is 0V. Obviously, when this one plug in, the CAN-BUS system does not work properly. When unplug the connectors, the ignition switch operation is normal. According to the circuit diagram analysis and see another Mercedes S350′s CAN-BUS connector, we are surprised to find that: when the joint X30 / 6 and X30 / 4 has a surplus CAN-BUS connector. These two joints in the factory are redundant (backup). The line voltage is 12V. These plugs can not be plugged into the X30 / 6 or X30 / 4. if they plug, CAN-BUS system is not working properly.

Finally, the 12V power connector on X30 / 6 and S30 / 4 will be unplugged and re-detect EIS control module. The fault code is no longer appeared on MB SD Connect Compact 4 diagnostic tool, and body control module on the left front SAM, right front SAM, SAM, etc. can be detected. The engine flames out normally. The instrument can work properly.