BMW 520Li

Posted by Tony Brown on

Make: BMW 520Li
Model: F18.
Driving mileage: 49,710 miles
Trouble phenomenon: The user reported that the instrument steering system failure warning light was on when the vehicle was driving, and the central information display screen prompts "Steering system, please drive carefully!" When turning the steering wheel, the turning becomes very heavy.

Fault diagnosis: Upon inspection, the issue is confirmed, the EPS fault alarm lamp on the instrument panel continues to light up to give an alarm. When turning the steering wheel, the feeling is very heavy, and not assisted.

Connect ISID for diagnostic testing, the EPS displayed in the diagnostic tree is yellow, indicating that the control unit is not communicating. The fault code of the steering system as follows: S0399 cannot communicate with the following control units: electromechanical power steering system. Select the fault content to execute the test plan and run the automatically generated associated FlexRay test module according to the test plan: the bus system analysis indicates that the FlexRay bus connected to the central gateway is normal. Call up the circuit diagram of EPS, as shown in Figure 1. The power supply voltage between the No. 1 pin and No. 3 pin of the EPS plug A67*2B and the No. 2 pin of the
grounded 2A67*2B has been checked, and it is the normal on-board voltage.

Since the power supply harness has been measured, no abnormalities have been found. The test plan also prompts that there is no abnormality in the FlexRay bus. Therefore, the analysis believes that the steering gear control unit EPS may be faulty, so it is recommended to replace the steering gear assembly. The failure phenomenon still exists after replacing the steering gear assembly. Go back and perform basic analysis, inspection, and exclusion again. The EPS control unit is connected to the vehicle network via 2 plugs. The steering resistance torque sensor is connected to the EPS control unit through another plug. According to the model series, this plug connection is 6-pin (occupying 2 pins) or 5-pin (occupying 5 pins or 3 pins). The EPS control unit is a bus component on FlexRay. According to different equipment series, different terminal KL.30 is provided for EPS unit. According to the model series, the distributor in the junction box (such as F10, F25) or the front distributor (such as F20) supplies power to the EPS control unit through the terminal KL.15N. FlexRay is a brand-new bus system and communication system. The requirement for FlexRay is to provide reliable, real-time, and efficient data transmission between electrical and mechanical and electronic components. FlexRay is used for networking innovative functions in todays and future vehicles. FlexRay includes powerful records, suitable for real-time data transmission in accordance with the distributed system in the vehicle. The maximum data transmission of each channel is 10Mbit/s, so FlexRay is a very high-speed system. FlexRay is significantly faster than the previous data bus used by vehicles in the body, drive unit and chassis area. In addition to supporting higher bandwidth, FlexRay also supports deterministic data transmission and can be configured for fault tolerance. In other words, even after individual components fail, the remaining communication system can still be allowed to operate. The Central Gateway Module (ZGM) establishes the connection between different bus systems and FlexRay.

Because the maximum data transfer rate provided by the 4th pin of the bus system A67*1 can reach 10Mbit/s per channel. This value is equivalent to 20 times the data transmission rate of PT-CAN2 (drive train CAN) or D-CAN (CAN-based diagnostics), so IMIB cannot be used to measure waveform signals. Therefore, the bus voltage of Flex Ray can only be measured through the universal function. The measurement results are as follows:

A67*1B Pin4FR_BP voltage to ground: 2.45V

A67*1B Pin1 FR_BM voltage to ground: 0

Compared with the normal working vehicle, the bus signal voltage of FlexRay is measured. The measurement result shows that the voltage of FR_BP to ground is about 2.45V, and the voltage of FR_BM to ground is 2.35V. This shows that the FR_BM of the FlexRay bus has a short circuit or open circuit to the ground.

Directly disconnect the FlexRay bus connection plug A51*1B on the side of the central gateway module and measure the open circuit between the No. 34 pin of A51*1B and the No. 1 pin of EPSA67*1B. The inspection found that the line is disconnected in the wiring harness under the hinge of the cover, as shown in Figure 2.

Repair this harness in accordance with ISTA's requirements for FlexRay bus wire maintenance, then restore the disassembled parts, re-diagnose and test, the EPS control unit can read normally, delete the fault memory, start the vehicle, turn the steering wheel lightly and eliminate the troubleshooting of the test drive


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