no spark on #2,swap coils around still #2!

[Fillbilly]

Thanks for the links and the specifics! I will let you know how it works out.

[Fillbilly]

Thanks a lot for the help Dave!!!
So, I reflowed all of the connections for the drivers. Unfortunately it didn't help (on either DME). It will run fine for a few minutes, 10-20 minutes idling or 2-5 minutes of actually driving. It seems to fit the M.O. of failing DME's, running when its cool and then failing once the DME is warm.  I did not see any obvious damage to the driver and did some testing with the  case off to make sure I was getting accurate readings and not a bad connection somewhere. SO I was testing directly on the pin (52) when cold it puts out 14v, once warmed up it drops to 7.5v. I then measured the driver and 3 of the 4 measured at 14v, the one in the "502" (bad #2 Coil)spot was only 7.5v.

So do you think I should take one of the drivers from the other DME and try to remove and replace the "502" driver? They also have different part #s listed on them so I don't know if that determines anything. Or should I just send my DME out somewhere for repair?
Sorry about dragging on the details, but I'm hoping the details might help someone else diagnose down the road.


Edit. I was thinking some more about it and I don't know how it works. Is the center post on the driver a power supply to the driver or a power out to the coil?

[DesktopDave]

I wonder if those #2 drivers are more vulnerable?

Having said that, I believe those drivers actually ground the coil packs as opposed to supplying them with +12VDC. It's a pretty normal Bosch way of doing things. They use a 'floating ground' in the coil pack wire harness to fire the coil pack. I couldn't find the early M42 ETM (Electrical Troubleshooting Manual) to be 100% sure, but each coil has three terminals (actually four, but I'm leaving out the coil secondary/spark plug wire). IIRC pin 1 is white (DME signal), pin 2 is brown (secondary ground), pin 3 is green (+12v switched).

I've always been tempted to swap some newer coil drivers into dead M42 DMEs to repair them, but I keep putting it off. I even bought a few of the silver-label EWS DMEs to make sure I had spares. I don't believe those part numbers are important, might just be date stamps or supplier codes. As long as you're staying withing the same Motronic family I'd suspect they're the same part. It's possible that the later Siemens DMEs could provide spares, but I have very little understanding of transistors and no good way of testing them.

[Fillbilly]

So you think that the driver is bad or is something else on the board bad that is supplying the driver with low power?

[DesktopDave]

I'd think it's a bad driver. It's a semiconductor normally...if internal damage like overheating has caused higher resistance the voltage has to drop. They're all supplied from the same output circuitry, so if any drivers are good the voltage supply is most likely good. These drivers run pretty high current, and they run pretty hot.

[Fillbilly]

I think I will try to swap some drivers around. Hopefully it will work and I wont screw more things up. I will let you know how it turns out.

[Fillbilly]

I'm not sure I didn't make a bigger problem but I'm still lost. 
I swapped drivers from the other DME that I had. SO I took out the driver from the other unit from the 503 spot (3rd cylinder?)  I replaced the driver from the original  502 spot which was the dead #2 cylinder.  To my excitement the car started after doing the Frankenstein switch. It ran good for 2 minutes and then same symptoms of a dead cylinder came back. Tested the drivers and the # 2 driver is seemingly working. 13v. now the 501 driver has low voltage (did not do anything with that one) Not only does it have low voltage, the case of the DME is too hot to touch. I got out my noncontact thermometer and the driver heat sink was 185 degrees.
I'M LOST NOW. here is a picture of the drivers https://photos.app.goo.gl/eJHvV9AnhHHB9Zz26 . The 30004 driver is the original. I assume the numbers were manufacture numbers and they were the same since they were both m42s and  they looked the same. Maybe that was a bad assumption on my part.

As I look for replacement DME'S Do only the numbers on the blue label have to match or do the bar code #'s on the white label have to match also. I found lots that end in 131 but mine ends in 060. Do you know the difference if there is one?

[Nick_318is]

For the DME you care about the big blue bosch sticker, I think most of ours end in 175. The #1 driver seems to be common weak point on the DMEs after several discussions I've had with people in the last few weeks.

[DesktopDave]

I checked to see what a common darlington driver would max out at, I'm told they can resist extremely high temps. Not sure if I got the right datasheet, but they claimed 500degF. Sounds WAY to hot to me.

+1 on what Nick said...the DME number for all early M42 ends in 175...they're are all the same as far as I know. They might have some different numbers on the bar codes, I've always ignored them. I'd bet some of them are part numbers and software revisions, others can be related to the original vehicle VINs.

Anyhow, I also found Timm's very interesting page. Looks like he's doing this on an E32 M60 but the DME repair is more or less the same as our cars. He states that the part is a TIP162, hard to find but available here for not much money. I''m considering picking a few up to see if it's the coil drivers that die, or if it's the signalling circuitry instead.

You did replace the coil pack, right? The coil pack can have a dead short internally and kill the coil driver. Even a new coil driver can't run a shorted coil for long, so they tend to die together...if you've put in a new driver I'd also replace the matching coil pack. If you don't have access to an original-style M42 coil, You can use early Bremi or Bosch stick coils in a pinch. They're the same design internally with the same three-wire plug. The short rubber extension is removable...I'm not sure if it will directly swap over, maybe the later Bosh square-type would work.

Here's a pic. Later Bosch square types on left, earlier Bremi roundies on the right.


I'd strongly suggest Rob-D's COP conversion. Really a nice upgrade, uses the Bremi round M52/M62 coil just like those pictured above. Here's the one I installed on mine:


I found a cheap eBay listing here with the early-style M42 coils I'm talking about, they're pretty common out there.

[Fillbilly]

I don't get it... I picked up a new to me DME from ebay that the seller says "works perfectly". I had a few issues at first with it not wanting to start or then idle. I started playing with the coil connections and I thought I had them connected correctly but I had moved them around so much trying to diagnose things its possible I mixed something up. I finally got them seamingly in the right spots and the car was idling smoothly and reving up fine.  I then let the car idle for about 30 min to warm up and charge the battery since I had not driven it in a long time. I went back outside and then the car was idling and reving like crap. (same symptoms as the lost cylinder from the beginning). I have not put an electrical tester on anything yet because it is now dark and 25 degrees, but I'm getting really frustrated. I don't know what else it could be. It seems really odd that 3 DME's are all bad in the same manner but I can't figure out what else could cause this. 

[bmwman91]

I don't think that the issue is in the DME if you have swapped it a couple of times. My bet would be on a cooked coil shorting out. It could be that if the primary winding is shorted such that it has really low resistance/inductance, it would damage the driver transistor. Inspect the coil body closely for any signs of the epoxy casing swelling or "oozing" since that is generally what it does when it cooks to death.

[Fillbilly]

I will have to find some info and test them again. I replaced 1 coil a while ago and it worked fine for about a year. Then i replaced all 4 with Bav Auto high performance coils which worked fine until  this debacle.  (same problem as why i replaced them in the first place) I don't know what I can be missing. Something is apparently frying the coils/computer. I'm lost.
Anybody have specs and process on testing the coils off the car?