DME M42 Repair:

[Artin318ti]

Sorry for the long-winded post, but I'm hoping to be thorough.

It started on a very cold morning [0-F]. I still don't know if that was a factor...

So, after the car warmed up normally I started driving, but at less than a street block away it suddenly started idling very roughly, and had almost no acceleration. I drove the car back, and upon inspection I discovered that the coil pack had a hairline crack all along its side, and melted plastic at the Cylinder #1 coil. To be sure I tested for spark on Cyl#1 and there was none.

I replaced the coil pack - bavauto.com ($169)- and started the engine, but spark was still absent from Cyl#1.

Thinking I don't have any time to get deeper into it, I took it to a shop & after a while they determined it was a faulty DME. They said that the compartment had a lot of water in it when they pulled the unit out, and that the unit was smelling of burnt electronics. They tried to replace the DME with a salvaged unit - same year, same configuration, then they realized that the on-board CMOS had to be coded to the car's VIN so, it had to be sent away, might take weeks, etc., etc., - so long story short, I took the car back.

I knew at this point that the critical parts of the DME - the processor, the memory module, etc. were still good because the engine could be started and driven. There had to be something else.

I opened the DME and visually inspected it. I noticed that there was a group of 4 transistors leaning on a heat sink (markings on the transistor: Bosch Logo, then Top Line: 24612 1695, Bottom Line: A 30051) and upon closer look I noticed that one of the transistors was burnt. I traced the transistor's output to terminal 25 of the DME which corresponded to Cyl#1.

With some background research and I learned that the Bosch M42 DMEs used type VB921 transistors for driving the coils, and that they have been discontinued for a long while. A Google search returned a possible replacement transistor - the Bosch BIP373 - that is supposed to be a direct replacement, albeit with slightly modified characteristics. I ordered four BIP373s from DIYAutotune.com ($9 ea.).

When the parts arrived I was quite skeptical because compared with the original transistors the BIP373 was tiny and the leads had to be reshaped to fit the hole spacings on the DME. I was anxious to try it so I tack-soldered the transistor and plugged the DME to the harness.

To my surprise and relief the engine started right up and revved as if the past week had never happened (!!!).

I have driven the car for about 50 miles so far, and everything is good to this point. I will update.

** 2013-02-07 Update - 150 miles.
** 2013-03-01 Update - 350 miles.
** 2013-03-26 Update - 675 miles
** 2014-02-14 Update - 9,000+ miles since the repair & still holding.

Artin.

1995 318ti 5-Speed Manual M42 4-Cyl.