TL;DR: The short answer to many BMW electronics problems is re-flowing the solder on high-current components.
BMW loves ‘solid-state’ electronics. That’s a really awesome thing: it’s given us high-reliability fuel injection with programmable ignition & cam timing, ‘electronically-controlled’ automatic transmissions and hundreds of other automotive refinements even down to mundane stuff like memory seats and automated HVAC systems. I’m totally on board with this technology. All of it depends upon a single revolutionary development…high-current transistors. Sure, they’re basically a switch, but the abilities they bring to the automotive world are astounding (small size, low current losses, near-instantaneous speeds, ability to switch hundreds of times per second, etc.).
As remarkable as this technology is, it’s decidedly un-wonderful when they stop working. I’ve come to the conclusion that in more than a few ways, Bosch is the heir apparent to Lucas. Most of you kids likely don’t recall Lucas. They were (are?) a British electronics company that was infamous for baked-in unreliability. Sound familiar? I’m not sure where the fault might lie, but BMW cuts a lot of corners where long-term reliability is concerned. Most American and Asian makes have very little in the way of electronic troubles. OTOH European makes have earned a reputation over the last half-century for being eccentric, expensive & unreliable. I’m not going to debate the relative merits nor the accuracy of that statement. One of the major factors in the poor resale value of BMW automobiles, IMHO, is this expectation of unreliability and prohibitive repair costs. Whether it’s a perceived or actual fault, the end result is a lot of really cheap second-hand cars. That’s something I can appreciate!
The basic process is to find whatever controller manages the system, pull it out of the case, and carefully add a little solder to fix “cold” solder joints. Occasionally, you’ll need to do a little component-level repair. Capacitors are infamous for a long slow death, especially in the DME (Bosch’s name for an ECU), gauge cluster and stereos. As for the DMEs, several generations of Bosch units tended to cook coil driver transistors, particularly with those early ’90s coil-on-plug ignition systems. These drivers are Bosch-specific parts (thus difficult to find published specs or cross-references), but I’d presume they’re just a standard high-current transistor called a Darlington pair.
Let’s start with the DME I’m personally the most familiar with,. Here we have a selection of three BMW M42B18 ECUs. The first and second units are early versions, from the ’91 318i (E30 3-series, Motronic 1.7). The third unit is a later model, with Bosch’s infamously-unreliable EWS system. Note the damaged coil driver on the first unit. It’s in the third picture from the left:
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- E30 M42 “175” DME. Odd that it’s been painted, must have been repaired once before.
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- Remove the case, this is what you’ll see.
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- Can you see the damaged coil driver transistor?
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- Here’s a stock 175 M42 DME
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- This is what a good set of coil drivers looks like.
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- Here’s the E36 M42 version, with DME.
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- Broadly similar to the earlier versions, note updates like the EWS ‘daughter card’ on the right.
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- Similar coil drivers, different part number.
Stay tuned…I’ll be attempting a repair on these DMEs! Don’t worry, I’ll post all the gory details and links for some decent tools to get you started. Here’s a few useful links that describe a repair on these vintage ’90s DME units: Timm’s page ~ M42Club post