[willisodhiambo]

I am trying to figure out what could be happening with a faulty bus on an E90 before opening up things. This is my first foray into a bus system.

Looking at the wiring diagram of the K-bus on this car (only available options) we have

1. Junction box directly wired to IHKA, CID, KOMBI and MRS
2. Branch 1 has the following all in parallel configuration, Tilt alarm, Seat module driver, CIC, Radio
3. Branch 2 has CAS, FRM, Seat module Front passenger
4. Branch 3 has comfort access module

The fault is logged on the following branch wise
a) Junction box - Errors in JBE itself, IHKA, CID (MRS is OK!!)
b) Branch 1 - Errors occur in Radio but CIC and seat module driver seems ok
c) Branch 2 - Error logged in CAS, FRM, SMFA basically the whole branch
d) Branch 3 has comfort access, error logged

The first error by mileage occurred simultaneously on the IHKA and FRM. IHKA is wired direct to JBE, FRM to an external branch. So my questions are,

1. Wouldn't a short in the bus take out the whole bus or at least ECUs after the point that has shorted? In this case we seem to have an illogical order of faults, not what I would expect of an ECU had an internal fault or if the line had a break or short to ground.

2. Based on the above observation and purely from wiring diagram analysis, would it be safe to assume the JBE is most likely faulty? Thus the illogical faulty vs good communication of ECUs on the same circuitry?

I have looked under the carpets, dry. In any case none of these ECUs have wires away from the under dashboard area except the seat modules.

I have an oscilloscope, how would one go about looking for the problem here if it is not obvious its the JBE?