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Old 02-03-2013, 11:18 PM   #1
Artin318ti
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Default DME M42 Repair:

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.

Last edited by Artin318ti; 02-14-2014 at 09:35 PM. Reason: Update miles driven since repair.
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Old 02-04-2013, 12:54 AM   #2
islandtime
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Good job. I haven't heard to much about people fixing the DME's but you did a good job with yours. As for aligning a new DME, all you have to do take it to the dealer or any other shop that has the tool, it takes just a few minutes after than.
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Old 02-05-2013, 12:12 PM   #3
Artin318ti
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Quote:
Originally Posted by islandtime View Post
Good job. I haven't heard to much about people fixing the DME's but you did a good job with yours. As for aligning a new DME, all you have to do take it to the dealer or any other shop that has the tool, it takes just a few minutes after than.
After being stranded for more than a week at northeast winter conditions - no fun by any standard - I'm planning on getting a spare DME. I will follow up on your lead and have it recoded for my car. Thank you!
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Old 02-14-2014, 08:14 PM   #4
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Did you put in all four BIP373's? I have the same problem; one bad transistor not firing #4 and I'd like to do this to repair the DME rather than buy another and mess around with the EWS. Did you need to put insulation behind them?
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Old 02-14-2014, 08:46 PM   #5
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a lot of the m42 engines, if you stip pin no 7 of the engine loom, ususally a green wire it bypasses teh ews anyway.
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Old 02-14-2014, 08:58 PM   #6
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Thanks, I've been looking into that. It seems like some people say if you keep your old chip, mine has a removable chip, that you can use a different DME. If I can fix this one with a $9 transistor all the better though. If it doesn't work I'll be on ebay looking for a silver 282.
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Old 02-14-2014, 09:31 PM   #7
Artin318ti
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mbradican -

No. Even though I bought 4 BIP373's I only replaced the damaged one, and it worked. It's been more than 9,000 miles since the repair and it's still holding.

As far as I remember I let the back of the transistor touch the heat sink - for the obvious reasons - and reassembled the metal clip over the entire cluster.

A.

Last edited by Artin318ti; 02-14-2014 at 09:43 PM.
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Old 02-14-2014, 09:37 PM   #8
mbradican
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Thanks for replying, I know this is an old thread. Glad to hear the repair was successful long-term.

Any advice for me? Did you use the resistors that come with the BIP373's? I think I'm going to need a more precise soldering iron than the old one I have. I took the board off the completely so I can get at both sides. I've never soldered on a circuit board before.
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Old 02-14-2014, 10:30 PM   #9
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Certainly

If you haven't soldered before I would suggest getting an old printed circuit board and practicing removal and installation, using cheap radio shack parts. YouTube videos will also help.

Removing the old transistor - When you remove the old transistor make sure that any solder residue is completely removed from the three connections, and that you start with clean holes for the new transistor. Use 'solder wick' to completely remove the solder from these connections. When done successfully the part should fall out. Don't use force.

Installing the new transistor - Orientation should match the original. Make sure you use solder that has a resin core (the resin helps clean the surfaces and spreads the iron's heat evenly on the components). When soldering transistors take care to not to apply heat for a prolonged time because excessive heat can actually damage the transistor.

Clean the connections to remove the resin from the terminals.

Good luck on your project.

A.
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