Troubleshooting The CAN Bus
The failure of communication on the CAN bus can be caused by several sources
- Failure of the CAN bus cables.
- Failure of one of the control units attached to the CAN.
- Failure of the voltage supply or ground to individual modules.
- Interference in the CAN bus cables.
Troubleshooting The D-Bus
The failure of communication with one or several control units via the D-bus can be caused by
- Failure of the D-bus cable or its individual connections.
- Failure of the IKE/KOMBI control unit.
- Failure of the I/K or P-bus or its individual connections.
- Failure of the voltage supply or ground to individual modules.
- Interference in the D-bus cable.
Troubleshooting The I/K Bus
The failure of communication on the I/K bus can be caused by several sources
- Failure of the bus cable.
- Failure of one of the control units attached to the bus.
- Failure of the voltage supply to individual modules.
- Interference in the bus cables.
The I/K bus is active when KL R is switched on, it remains active until 60 seconds after the last message. If the key is switched off (KL30) the bus may be activated for a time by individual users via a "wake-up" message.
Unlike the CAN bus where each control unit (subscriber) provides voltage for communication, the I/K-busses use only determined Master or Stand-by Controllers to supply B + for communication. The voltage level on the I/K bus must be above 7V. The nominal value should be close to the system voltage of the vehicle.
Just like the CAN bus, the fact that voltage is present does not mean that the bus is fault free, it just means that the voltage level is sufficient to support communication.
Identifying Multimeter Display. Scheme 36
Control units that provide operating voltage to the I/K bus are
Troubleshooting The P-Bus
The failure of communication on the P-bus can be caused by several sources
- Failure of the bus cable.
- Failure of one of the control units attached to the bus.
- Failure of the voltage or ground supply to individual modules.
- Interference in the bus cables.
The P-bus may be active at any time following a wake up call. The GM provides the voltage necessary to support communication. The voltage level of the P-bus is 12V.
The Diagnosis of the central body electronics is carried out via the K-bus. The GM converts diagnosis request from the DISplus into diagnostic mode messages and transmits them the peripheral modules over the P-bus.
Automatic testing of the P-bus connection is carried out every time the GM communicates with the diagnosis program (not during a short test).
Identifying P-Bus Display. Scheme 37
Checking the bus line is carried out just like any other wiring. Perform continuity tests between the connections of different modules (all modules disconnected) without forgetting to make sure that the bus has not shorted to ground or another wire. It is recommended to use the "Wire Test" in "Preset Measurements" which is more sensitive than just a resistance check.
Troubleshooting of the P-bus network is carried out the same as the I/K bus.
M-Bus Troubleshooting
The failure of communication on the M-bus can be caused by several sources
- Failure of the bus ribbon, e.g. open or shorted.
- Failure of one of the stepper motors attached to the bus, e.g. shorted to B + or B - .
- Failure of the voltage or ground supply to the IHK control unit.
The M-bus is active at any time following KLR on. The IHK module provides the voltage necessary to support communication. The voltage level of the M-bus is 5V, but because status communication occurs at an average 50% duty cycle the observed voltage is approximately 2.5V. The presence of 2.5V means that communication is occurring.
Identifying Multimeter Display. Scheme 38
Checking the M-bus ribbon is carried out just like any other wiring. Perform continuity tests between the connections of the stepper motors (all motors disconnected) and the control unit without forgetting to make sure that the data line has not shorted to ground or power.
It is recommended to use the "Wire Test" in "Preset Measurements" which is more sensitive than just a resistance check.
If Voltage level and the wire test are O.K, then looking at the communication signal may be useful.
The following is an example of a scope pattern that may be observed when checking the M-bus. Notice the very high frequency of the signal at approximately 20 kHz.
