Diagnosing issues in modern vehicles often relies heavily on the On-Board Diagnostics II (OBD2) system. For BMW E39 5 Series models, particularly the 528i manufactured in 1999, understanding the OBD2 system is crucial for effective troubleshooting and repair. This guide delves into diagnosing a no-communication issue with the engine control module (DME) in an E39 528i, providing insights and steps for owners and enthusiasts.
The BMW E39, a popular generation known for its blend of performance and luxury, utilizes both a standard 16-pin OBD2 connector and a BMW-specific 20-pin diagnostic connector. The 16-pin OBD2 port, typically located under the dashboard on the driver’s side, is designed for generic scan tools to access basic emissions-related data and fault codes. However, for more in-depth diagnostics, especially on systems beyond emissions, the 20-pin connector found under the hood near the engine bay often becomes necessary.
Encountering a situation where your scan tool fails to communicate with the DME through either the 16-pin OBD2 or the 20-pin connector can be frustrating. This lack of communication hinders your ability to read fault codes, monitor live data, and perform necessary diagnostic procedures. Let’s explore a real-world scenario and the troubleshooting steps involved when facing this problem on a 1999 BMW 528i.
A 1999 BMW 528i, equipped with a 2.8L DOHC engine (M52 TU), presented with a peculiar issue: while the engine started, idled, and drove without major problems, it exhibited a misfire, and more importantly, no diagnostic tool could establish communication with any control module. This communication failure persisted through both the standard 16-pin OBD2 port and the 20-pin BMW diagnostic connector.
Initial troubleshooting steps are critical in such situations. The first actions taken on this E39 528i were logical and systematic:
- Fuse Inspection: A thorough check of all fuses in the glove box and the trunk’s rear power distribution box was performed. Fuses are designed to protect electrical circuits, and a blown fuse in the diagnostic system’s power supply could easily cause a no-communication issue. In this case, all fuses were found to be intact.
- Scan Tool Verification: To rule out any issues with the diagnostic tools themselves, three different OBD2 scan tools were tested on the 16-pin OBD2 connector. None could establish communication with the DME or BMW’s D-bus (diagnostic bus). Furthermore, a Mac Mentor (generation 3) was used on the 20-pin connector, attempting both generic OBD2 and enhanced (manufacturer-specific) protocols. Again, communication failed.
Further investigation involved examining the power supply to the diagnostic system. Pin 2 of the 20-pin connector is known to provide power to the diagnostic system during the initial key-on phase. Measurements revealed 12V at pin 2 upon starting the car, but this voltage dropped to 0V after approximately 10 seconds. This voltage drop, while potentially indicative of an issue, didn’t immediately explain the no-communication problem.
To understand the communication bus systems, the technician proceeded to scope the Controller Area Network (CAN) bus, which is used for communication between the transmission control module (TCM) and the DME. Pins 3 and 4 of DME connector X60002 are designated for CAN bus communication. Using an oscilloscope, the CAN High (CAN H) and CAN Low (CAN L) signals were examined. The readings showed CAN H voltage fluctuating between 2.5V and 3.5V, and CAN L voltage between 2.5V and 1.5V. The CAN H and CAN L waveforms were observed to be mirror images of each other, suggesting the CAN bus itself was likely functioning.
Next, the focus shifted to BMW’s proprietary diagnostic bus (D-bus). Pin 7 of the 16-pin OBD2 connector is the designated D-bus line. A graphing multimeter was connected to pin 7. At idle, 12V was present on this line. However, when a scan tool was connected to the 16-pin connector and attempted to initiate communication, the voltage on pin 7 was observed to fluctuate between 12V and 0V intermittently before returning to a steady 12V. The scan tool then displayed the common error message: “make sure ignition is on and scan tool is properly connected.” This voltage fluctuation on the D-bus during communication attempts suggested a potential issue within the diagnostic communication pathway.
As a troubleshooting step to isolate potential interference from other modules on the network, the instrument cluster (IKE), which acts as a gateway module in the E39, was disconnected. Despite disconnecting the IKE, the no-communication issue persisted on pin 7 of the OBD2 connector, indicating the problem was likely isolated to the DME or its immediate connections.
Based on the systematic elimination of other potential causes and the persistence of the no-communication issue across both diagnostic ports, the primary suspect became the DME itself. While further testing and verification might be recommended in a real-world diagnostic scenario, in this case, the evidence strongly pointed towards a faulty DME as the root cause of the OBD2 communication failure.
In conclusion, troubleshooting OBD2 communication problems on an E39 BMW 528i requires a methodical approach. Starting with basic checks like fuses and scan tool verification, and progressing to voltage and bus signal analysis, allows for a logical diagnostic process. When faced with a no-communication scenario on an E39 Obd2 system, systematically checking power, grounds, bus signals, and module connections is crucial before considering component replacement.
