BMW Z4 E89 PT CAN Bus Issues

Wiring rubbing on sharp edge of DSC unit block near top center of picture

Today I got all happy because when I asked my team leader for work, he told me he had another difficult car. They had scanned it earlier and found about 100 CAN bus related communication faults. Most techs hate this kind of problem because it's really hard to fix. A CAN bus is how some of the control units in a car talk to each other and share information. For example, the DSC control unit will take the signal from the wheel speed sensors and send it over the CAN bus to the transmission so it know what gear to be in, and will also send the information to the instrument cluster to display the vehicle speed in the speedometer. The bus itself is usually two intertwined wires that connect to a certain group of control units. There’s a PT CAN, which lets all the power train control units talk to each other, K CAN bus for the control units of the vehicle’s interior, and other buses for sub systems. It’s hard to find these communication issues because they are usually intermittent and everything is working fine when you go to check it. I was wondering what the hell to do with all the faults. I started looking at all of them and the amount of times they had happened. I found they were mostly related to the DSC control unit, which controls the stability and ABS of the car, among other things. I thought to start with the control unit itself, but I decided to inspect the big wiring connector in case there was anything stupid going on, or maybe mouse damage (when rats make nests inside the engine bay and chew through wires, causing problems). When I was looking at the connector I noticed the wiring harness was rubbing up against the sharp metal edge of the DSC unit. When I looked more closely I found it had cut through the harness and was slightly cutting through the PT CAN wire, causing it to short and mess up communication between the control units. I fixed it and cleared the faults. Drove the car and rechecked faults. Car was fixed. I couldn’t believe how lucky I was to have found that so quickly. It made me look like a bad ass.

After unplugging the wiring and inspecting closer, you can see the small cut through the harness in the middle

I cut the harness open to investigate further. The red/ blue wire with the small pin hole in the middle is the PT CAN wire that was slightly shorting.