Experiencing OBD2 communication issues with your 1999 BMW 528i (E39) while the engine runs perfectly fine can be frustrating. This guide breaks down a diagnostic journey on a 1999 BMW 528i equipped with the 2.8L DOHC M52 TU engine and a base instrument cluster (without IKE), where everything seems to work, except for establishing communication via the OBD2 port.
Before diving in, let’s clarify some key components mentioned:
Definitions:
- A) OBDII Plug: The standard 16-pin diagnostic connector located at the driver’s side kick panel, above the dead pedal.
- B) 20-pin Data Link Connector: A circular, 20-pin BMW-specific diagnostic connector situated under the hood, near the right front strut tower.
- C) Connector X60004 & D) Connector X60002: Two of the five connectors on the DME (Digital Motor Electronics), which is the engine control computer.
- E) DME: Digital Motor Electronics – the engine control module.
- F) IKE: Instrument Cluster Electronics – the instrument cluster module (not present in the base model of this 528i).
Background of the Communication Problem
The vehicle in question, a 1999 BMW 528i, starts, idles, runs, and drives without any major issues. The primary symptom reported is an engine misfire. However, the crucial challenge is the inability to communicate with any control module on the vehicle’s diagnostic bus system. This communication failure is persistent through both the standard 16-pin OBDII connector inside the car and the 20-pin BMW diagnostic connector in the engine bay.
Troubleshooting Steps Taken
To pinpoint the cause of this communication breakdown, several diagnostic steps have been performed:
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Fuse Inspection: All fuses in the glove box and the trunk’s rear power distribution box were meticulously checked. The fuse integrity was confirmed to be in good order, ruling out a simple power supply issue to the diagnostic system.
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OBDII Scan Tool Tests (16-pin Connector): Three different OBDII scan tools were connected to the 16-pin OBDII port. None of these tools could establish communication with the DME or any other module on BMW’s D-bus network. This indicates a potential issue upstream of the individual modules, possibly in the communication lines or the DME itself.
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Mac Mentor Scan Tool Test (20-pin Connector): A Mac Mentor (generation 3) diagnostic tool was utilized on the 20-pin under-hood connector. Attempts were made to communicate using both generic OBDII protocols and enhanced, manufacturer-specific BMW protocols. Regrettably, communication failure persisted even with this more advanced tool, suggesting the problem is not isolated to the OBDII port itself.
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Voltage Check at 20-pin Connector: Pin 2 of the 20-pin connector, which should provide power for diagnostic functions, was tested. Upon starting the car, 12V was present for approximately 10 seconds, which then dropped to 0V. This behavior is unusual and could indicate a power supply issue that is time-sensitive or triggered by a fault condition.
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CAN Bus System Examination: The Controller Area Network (CAN) bus, responsible for communication between the Transmission Control Module (TCM) and DME, was examined at the DME (pins 3 and 4 of connector X60002). Oscilloscope readings showed typical CAN bus signal characteristics: CAN High voltage fluctuating between 2.5V and 3.5V, and CAN Low voltage between 2.5V and 1.5V, with CAN H and CAN L signals mirroring each other. This suggests the CAN bus itself is likely functional at the DME level.
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D-bus Voltage Monitoring at OBDII Connector: Pin 7 of the OBDII connector, dedicated to BMW’s diagnostic bus (D-bus), was monitored using a graphing multimeter. A constant 12V was observed at idle. When a scan tool was connected to the 16-pin connector and attempted communication, the voltage on pin 7 fluctuated between 12V and 0V several times before returning to a steady 12V. Subsequently, the scan tool displayed a “make sure ignition is on and scan tool is properly connected” message, reinforcing the communication failure.
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IKE Module Disconnection: Given that the Instrument Cluster Electronics (IKE) module often acts as a gateway in BMW communication systems, it was disconnected to rule out its interference. Despite disconnecting the IKE, communication with the DME via pin 7 of the OBDII connector remained unsuccessful.
Probable Cause: Suspect DME Failure
Based on the systematic elimination of potential causes, the most probable culprit for the OBD2 communication failure in this 1999 BMW 528i 2.8L is a faulty DME (Engine Control Module). While the engine operates, the DME’s communication circuitry may be compromised, preventing external diagnostic tools from accessing the vehicle’s data. The fluctuating voltage on pin 7 during scan attempts could be indicative of the DME trying and failing to initiate communication.
Seeking Expert Opinions
While DME replacement appears to be the likely solution, further insights and alternative diagnostic approaches are always appreciated. If you have encountered a similar “99 Bmw 528i 2.8 Everything Work But Communication Obd2” scenario or have suggestions for additional troubleshooting steps, your input would be highly valuable.
