does anybody have 1991 M42 DME for sale

[Fillbilly]

Hi everybody.
I am looking for a good DME for my 1991 318is. I have purchased 2 of them from ebay and neither solved my problem. I even sent one out to "get repaired" to G7 computers and they told me the damage was in the chip and could not be repaired.  It blows my mind that my original had a problem that one coil was getting low voltage hence only running on 3 cylinders. So i got a replacement, (with no warranty) and the same problem So I tried to frankenstien the two and replace the coil driver but know luck.  It baffled me that it would have the same problem so I tried testing everything I could and found no problems elsewhere. I bought another one and had the same results. Does anybody have one that they know is good that they will sell or lend to figure out my problem. Any other suggestions for repairs are welcome as well.
My original Part #s are  1734060 001 078
Blue Label 0 261 200 175

[bmwman91]

At this point, it sounds like there might be some sort of coil or wiring fault in your car which is killing DME units. What do you mean when you say that one coil was getting "low voltage"?

[Fillbilly]

When I tested the voltage on the coil it was only getting  8 volts. The other 3 were getting 13ish. I tested it at the DME pin and was also only getting 8 volts at the pin.  That is why I tried swapping the driver from another DME that had the same problem. (using the #4 coil instead of 3) and that didn't help. I'm just lost now and I cant find anything else wrong.

[bmwman91]

The coil primary windings get their 12V supply from the ignition switch, and the DME drivers switch the other end of the primary winding to ground through the T501-T504 power transistors. Low voltage on one coil could mean an internal fault in the coil (and you swapped #3 / #4, and it stayed with cylinder #3 it sounds like), a fault in the driver (which you replaced) or a fault in the S500 IC which drives the drivers (and you tried a different DME so it is not that). That leaves the wire harness. Disconnect the 4 coils and test the resistance between one of the 12V terminals and a ground point.

[Fillbilly]

I will have to wait till I get my coils with me at work since my meter is a junky harbour freight one which does not seem to want to measure resistance for me.
While trying to make sure I had everything hooked up right I ran into this dilemma. Let me know if this logic is on track.
to make sure it was connected in the right order I tested continuity from the computer harness to the Coil plug.
Continuity from
pin 24  is coil 3
25 Coil 1
52 Coil 4
51 Coil 2
All plugs off the coils  and testing from the dme harness to the coil plug. But when I connect the corresponding coil plug to a coil I also have continuity on the other 3 coil plugs brown and green wires. Except for one of them.

Pin 24(coil 3)I have good Continuity from the wire harness to the coil plug(#1) with no continuity on 4a(brown wire) or 15(green wire) and when that coil plug is plugged into a coil, I also have continuity on 4a and 15 on the other 3 coil plugs but not on the other three #1pin on the coil plug.

Pin 25 (coil 1)I have good Continuity from the wire harness to the coil plug (#1) with no continuity on 4a(brown wire) or 15(green wire) and when that coil plug is plugged into a coil, I also have continuity on 4a and 15 on the other 3 coil plugs and not on teh #1pin on the coil plug.

Pin 52(coil 4) Same as above

51 (Coil 2) THIS ONE IS DIFFERENT ,I have good continuity from computer harness to #1 pin on coil plug, but when I plug that one into a coil(different coils too) I do not get the continuity on the other 3 coil plugs brown and green wires. 

I assume the one outlier is the one that is wrong but I dont know. Why is there continuity between the ground and the computer harness of the other 3 coils when the 4th is plugged into a coil? Is that normal? or is the one coil that does not have any shared continuity the right one and I have three problems. I can see some logic that there may be reciprocal continuity since it is an interference system, but I really dont know enough about it to know or understand.

The other odd thing is that I'm sure I was very careful in labeling the plugs when I took them off to change the coils. But when I test continuity from the computer harness to the coil plug, they do not line up the same. According to the continuity test, they dont even line up logically the way the harness is assembled. By continuity the 3 and 4 coil plugs are across from each other not next to each other. Which is not physically possible the way the wire harness is assembled. It just isn't long enough. SO I had to move plug wires (I know its not logical but I'm grasping at this point) to correlate and big surprise it runs like the firing order is screwed up. I hope that my descriptions make sense.

[bmwman91]

You have coils #2 & #4 mixed up, at least in the post's text.
http://www.armchair.mb.ca/~dave/BMW/e30/e30_92.pdf

Get a good look at page 1360-3.


Just to be clear, do you mean that when you have one of the coil connectors plugged into a coil, you get continuity between its pin 1 and all three of the other plugs' pins 2 & 3 (4a & 15)? Is the harness plugged into the ECU when you are testing this, or no?

[Fillbilly]

Thanks for the link, i will look that over.

You are correct, I had those coils wrong, that's what I get for not cross referencing the chart I found first on Revlimited. I will have to check the layout when I get home on that one.  While that is an issue, it doesn't clear up the continuity question.

The harness is not plugged into the ECU when checking the continuity,  I have one lead from the meter plugged into the ECU Harness and the other lead checking for continuity on pin 1 in the coil connector to identify them. Then when I plug that coil connector into a coil, leaving the meter lead in the same spot on the ECU harness, I have continuity on pins 2 & 3 on the three other coil connectors (unless I am plugged into #2coil, then there is no continuity on pin 2 or 3 on the other coils).

[Fillbilly]

The coil primary windings get their 12V supply from the ignition switch, and the DME drivers switch the other end of the primary winding to ground through the T501-T504 power transistors. Low voltage on one coil could mean an internal fault in the coil (and you swapped #3 / #4, and it stayed with cylinder #3 it sounds like), a fault in the driver (which you replaced) or a fault in the S500 IC which drives the drivers (and you tried a different DME so it is not that). That leaves the wire harness. Disconnect the 4 coils and test the resistance between one of the 12V terminals and a ground point.

So I tested the resistance on all of the coils that I have had on the car at some point (7 of them). Most of them were between .5 and .9 ohms.  one was 1.4 (not on the car but taken off because I thought it was bad) and one was 110.2 which was on the car but not on the problem cylinder. I will replace that one with one that has a normal reading and keep searching for other problems.

[Fillbilly]

Just an update for anybody that may find this same problem. SO I was messing around with the car again just trying to fix something since I couldn't figure out the idle. So I was working on heat shrinking some of the vacuum hoses to resolve a rough idle. I fixed the cracked hoses that were easily accessible and while cleaning out some leaves that fell  under the hood I noticed a bare wire. Turns out I found 3 bare wires and 2 more just cut off and left. The PO replaced the headlights with HIDs and did a mighty poor job at splicing them in. So I redid all of his connections and capped the extra bare wires(why there are extra I don't know) and Voila! She Runs. The heat shrink fixed the rough idle and not having random extra wires grounding/shorting out the car runs great. ( I also had an early experience with a similar problem early on when I got the car years ago. When I put the left turn signal on, the car would stall. What do you know I found a bad wire contacting the body. fixed and problem solved.) I've put about 1000 miles and its been running great! So short story long, if you have any electrical gremlins, check for loose wires anywhere.

[bmwman91]

Wow, awesome! It is great that you were attentive and found those wires...I never would have guessed that shorted lighting connections could cause the car to misbehave like that. I'd have expected a fuse to blow or something, not drag down the whole system voltage lol.