Honghong Diagnostics | 2008 BMW X5 3.0Si K-CAN Bus Troubleshooting and Repair

Failure phenomenon

A 2008 BMW X5 3.0Si with N52B30BF engine has accumulated mileage of approximately 190,000 kilometers. When the vehicle was inspected by Diagnostic Troubleshooter during factory maintenance, it was found that several control units, including the Suspension Height Control (EHC), Keyless Start Control (CAS), Park Distance Control (PDC), Center Touch Display (CID), and Reversing Camera Module (RFK), had stored the fault code of "K-CAN Bus Fault" as shown in Fig. 1 below, and could not be cleared. cannot be cleared.Initially, the K-CAN bus of this vehicle may have an abnormality.

Fig. 1｜K-CAN Error Codes Stored in the Control Unit

Fault Diagnostics

The K-CAN bus signal waveform is measured from the EHC module side using an Acer Pico automotive oscilloscope (see Figure 2):

K-CAN H line The signal is transmitted normally when the low level is about 0.1 V and the high level is about 3.9 V.

K-CAN L lineThe signal amplitude is normal, but the signal is lost frequently.

Note: The vehicle's K-CAN bus has a communication rate of 100 kbit/s, which is a low-speed CAN system that can maintain communication in a single-wire situation.

Based on the results of waveform analysis, it can be determined that the waveform characteristics are consistent withK-CAN L line openThis means that even if one line is down, the other line can still maintain communication. That is, even if one line is interrupted, the other line can still maintain communication, so it can be initially recognized as aK-CAN L line break fault。

Fig. 2｜Waveform of K-CAN bus signal during failure

Further inspection of the K-CAN bus circuit reveals that there is a total ofFour Docking Points：
1. X15001 and X15002
2. X15003 and X15004
3. X15005 and X15006
4. X15011 and X15012.

Since oxidized or loosened contacts are a common problem, the technician prioritized checking the contacts X15005 and X15006 on the right side of the rear row, and found that there was a problem inside the wiring harness. Water seepage marks(See Figure 3).

Fig. 3｜Water seepage marks inside wiring harnesses

After stripping the wiring harness, a contact was found. X15006 Obvious oxidized corrosion(See Figure 4). The K-CAN L wire breaks when gently pulled and tested; it is assumed that the fault is caused by this contact. And the contact X15005 also slightly oxidizedThe K-CAN H line can communicate normally because it is not in contact with the K-CAN H line, but still maintains contact.

Fig. 4｜Contact X15006 oxidized heavily

fault resolution

技師重新修復並焊接 X15005 與 X15006 處的導線，完成防水與絕緣處理後再次試車。 故障碼「K-CAN 匯流排故障」已成功清除，系統恢復正常通訊，代表故障排除完成。