BMW No-Start Shows Need For Vehicle-Specific ScannerPosted 4/1/2008
By David Tidaback
We realize that you techs out there must get tired of hearing the dreaded question from us, "Do you have a (car/make) - compatible scan tool?" Often there are work-arounds that will get the job done without vehicle-specific scan equipment, but more and more often, the correct scanner is an absolute necessity.
A case in point is a 2003 BMW 745i brought into a shop as a no-start. It was quickly determined that there was no fuel pressure and the pump was not running. The rear seat cushion was pulled out, the pump was accessed, and there was no power to it on the black/violet wire. Applying power made the pump run. At this point, the tech did the sensible thing and went to his computer to find the pump wiring schematic to see if he might have a bad fuel pump relay or if the relay was missing an input. It did not take long for him to discover that this car was not equipped with a fuel pump relay.
The fuel pump is actuated from the 'Satellite module, right B-pillar,' which is getting power from fuse 73 and has a connection to chassis ground. It is connected to two other modules, the Car Access Control Module and the Safety Information Module (SIM). OK, but what are these modules, and how does the 'Satellite module, right B-pillar' know to actuate the fuel pump?
Looking further at the wiring diagram, the SIM is shown with a single wire connecting it to the Central Gateway Module (ZGM), and the ZGM in turn is connected via the controller area network (CAN) bus to the Engine Control Module (ECM). Now we seem to be getting somewhere - evidently the fuel pump request goes out on the CAN bus to the ZGM, which distributes CAN information to other modules throughout the car. However, the single wire between the ZGM and the SIM is labeled BYTEFL. What does that mean, and what is the SIM, anyway? The diagram also shows that single BYTEFL wire between the SIM and the 'Satellite module, right B-pillar.' The SIM is the control module for the restraint system, controlling the seat belt tensioners and air bags. BYTEFL is short for byteflight, which is BMW's high-speed fiber-optic bus connecting the various modules in what BMW calls the Intelligent Safety Integration System (ISIS).
So, to get back to our original problem, what does this all have to do with the fuel pump? Two things, actually: 1) In the event of a crash, the system wants to be able to shut off the fuel pump. The 'Satellite module, right B-pillar' is the controller for the right front seat belt tensioner, and also controls the battery safety terminal and the fuel pump, so they both can be disconnected instantly if necessary, and 2) The wiring distance from the Satellite to the pump is short, so this saves on wiring between the two.
How can a lack of power to the fuel pump in this system be diagnosed without a BMW-compatible scan tool? Is the request being sent from the ECM to the ZGM via CAN bus? Is it going from the ZGM to the SIM, and from the SIM to the 'Satellite, right B-pillar' via byteflight? Without the proper scan tool, all that the technician can do is see that the Satellite is getting power and ground, and that the wiring (for both power and ground) between the Satellite and the pump is intact. Obviously, it would be helpful to know if this vehicle has had a fault warning for the restraint system. But even if it has, a scan tool is necessary to read codes and data stream for the system, and any module that is replaced, such as the Satellite, has to be coded to the vehicle.
The fact is, on more and more late-model cars (not just BMW or other high-end Europeans), all vehicle systems are networked and coordinated through CAN bus and optical networks that can only be diagnosed and tested using a scan tool. Another important factor is that more programming updates are available for vehicle systems. Each change in programming for one system sends ripples through the rest of the car, often necessitating more or less complete vehicle updates, without which the car may actually be in worse condition overall than it was before the original programming change.
Finally, just so you know how the story ends, this particular car had a restraint system warning, and the BMW dealer's scan tool showed that there was a fault in the Satellite Module itself. After replacement and reprogramming, the car started and ran as it should.
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