Getting the MULF out of the MOST

There are several advantages in using polymer optical fibre instead of copper wiring in your car. Visibility of a signal with the naked eye, immunity to electromagnetic interference, weight reduction and maybe also cost reduction…

…but on the other hand you can’t “tap” POF with an insulation-displacement contact branch connector. So the MOST-Bus has a ring structure – and if a transmitter in one of the devices fails the whole ring fails.

This happened to the audio system of my BMW E61 – and it is not uncommon for the multifunktionale Lade- und Freisprechelektronik, or MULF, to contain the faulty transmitter.

The MULF is behind the rear seats and relatively easy to access. You just have to remove the “hard carpet” (trim panel) in the boot and the storage troug underneath – I did a bit too much, beacause I wanted to see more of the wiring…


P1050068 P1050078P1050074 P1050071

Testing, if it really is the MULF that shuts down your Top HiFi System (“logic 7 by Harman/Kardon”), can be achieve by bypassing the device. And that can be done by disassembling the connector.

I had a piece of bamboo at hand to point input directly towards output:

P1050081 P1050085 P1050086 P1050087 P1050090 P1050091

=> That was it: no Bluetooth connection anymore (was gone before BTW), but the audio system was working again!

Afterwards you can disconnect the MULF entirely and buy a “proper” connector (1,- €) from your local Bimmer dealer…


…or you can search for a way to replace that control unit. I did that just before selling the car.

After convincing eBay that I really meant MULF, results appeared and repair was an option: 99,- € or something. This also worked. No need to buy a new unit for a ten year old car – the guy also claimed that he compensated the design flaw leading to the failure in the first place and that the ECU will live longer now. But I won’t be able to validate that…


(Original post:, November 18th 2013 and, November 27th 2013)

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