2005 325i cylinder 6 misfire originating from DME. Need help.

[Arog502]

First I want to say hello and introduce myself. My name is Austin and I'm an Infiniti owner and BMW fan. My girlfriend is the BMW owner in our relationship. I live in Kentucky near Louisville. She's a nurse and I'm a maintenance manager for an alcohol (specialty and beverage) company. I got my mechanical start working in car shops and even owned my own garage for several years. I'm very capable of diagnosing and fixing most things but this one has me stumped guys.

Problem: Misfire on cylinder 6
I've ruled out coil, plug, injector and compression and have discovered it to be an electrical problem. I used a Fluke meter and checked each ignition coil plug harness. Each coil has 3 wires. Brown for ground, Red with a white stripe for power and Black with various colored stripes for signal I presume. I got the following results with the engine running.

Cyl 1 through 5:


Brown 0 volt

Red/White 13 volts

Black/various stripes 13 volts


But then there's cylinder 6. I got these results:


Brown 0 volts
Red/white 13 volts
Black/violet stripe .20 volts <----This is the problem obviously.


I traced the Black/violet striped wire from the coil, across the engine bay and straight into the DME. When I probed this wire right at the DME I still only get .20 volts so it's not a break in the wire in the loom/harness. Looking at the schematics of the service manual by Bentley I'm unable to determine where to go next. It looks like it's Pin #7 at X60005 but that's to the coil. I'm looking to discover where this signal is generated from prior to X60005. Is there another plug going into A6000 DME that sends this signal or is this signal simply generated by the DME and I have a bad DME in front of me? Help is needed please. Thank you.

[MarcoZandrini]

The coils get 12VDC as long as the ignition switch is in the run or start position. The DME provides a ground to the coil until it’s time to fire the coil. The loss of the ground causes the magnetic field to collapse, thus inducing a massive voltage in the coil’s secondary winding: spark. The ground is provided via a metal oxide field effect transistor (MOSFET) in the DME. You can remove the DME cover and trace the black/violet wire back to a MOSFET. It’s likely bad. Check for signs of overheating or burning. If you’re handy with a low wattage soldering iron you can remove the DME and replace the MOSFET. Go to www.mouser.com to buy a replacement MOSFET. Mouser sells almost everything electronic.

[Arog502]

The coils get 12VDC as long as the ignition switch is in the run or start position. The DME provides a ground to the coil until it’s time to fire the coil. The loss of the ground causes the magnetic field to collapse, thus inducing a massive voltage in the coil’s secondary winding: spark. The ground is provided via a metal oxide field effect transistor (MOSFET) in the DME. You can remove the DME cover and trace the black/violet wire back to a MOSFET. It’s likely bad. Check for signs of overheating or burning. If you’re handy with a low wattage soldering iron you can remove the DME and replace the MOSFET. Go to www.mouser.com to buy a replacement MOSFET. Mouser sells almost everything electronic.

Great information there and thank you! I believe I'll pull it apart some time this week and see if I can repair it. I've soldered a lot of components in laptop circuit boards and audio amplifiers before and had good luck.

Next question. In the event I'm unable to repair this, what's the typical cost of a new DME and does it have to be programmed to her VIN or anything special like that?

[CorvidX6M]

If you are getting no voltage at the black/violet wire at the DME, you have an open coil. Check the resistance between the red/white and black/violet wire terminals on the coil. The primary resistance for a coil is usually between 5-15 ohms. I'll bet yours is open. Compare it to the other coils.

BEFORE tearing apart your DME! Heh

[Arog502]

If you are getting no voltage at the black/violet wire at the DME, you have an open coil. Check the resistance between the red/white and black/violet wire terminals on the coil. The primary resistance for a coil is usually between 5-15 ohms. I'll bet yours is open. Compare it to the other coils.

BEFORE tearing apart your DME! Heh

I'll do that. However, this problem didn't change if I moved coils around (known good ones that didn't miss on other cylinders) or even when I tried a new coil. The miss remained at cylinder 6 always. I'll probably look more closely at the coil harness to see if the prong hasn't been pushed back in the connector or other issues like that.

[MarcoZandrini]

Check the engine harness connector for the misfiring cylinder. Make sure the sockets (I think the coil connector has pins) are clean and tight.

[CorvidX6M]

Oh ok, copy that. So you have power to the red/white terminal at the coil, but it's not coming through the primary of the coil and into the black/voilet wire from the DME? I'd be checking the plug and wires then. Maybe check continuity of that black/violet wire.

The other thing could be the red/white wire has one strand hanging on somewhere, but once under a load it's not enough to charge up the coil. To check that, do a voltage drop test - (after checking the plug and black/violet). You'll need to backprobe the coil plug red/white wire with a paperclip and check voltage there while the coil is plugged in and the car is running. The other lead of your meter should be on good Battery positive. It should read close to 0 volts, indicating no voltage is being lost while it's trying to run the coil.

I'll do that. However, this problem didn't change if I moved coils around (known good ones that didn't miss on other cylinders) or even when I tried a new coil. The miss remained at cylinder 6 always. I'll probably look more closely at the coil harness to see if the prong hasn't been pushed back in the connector or other issues like that.

[bmwdirtracer]

Personally, I'd guess that you've got a bad coil driver in the DME. If it were my own car, I'd ask Abel to source a used DME, mail him my DME with enough money to cover the part and his time and expertise, and wait for the new clone to arrive.... (All the drivers and solder joints in your DME have lived through the same conditions)

Not only has he done this for my personal M5, but he's accomplished this miracle for quite a few customer cars for me. If you decide that's the way you want to go, it's likely he will pop into this thread and offer help.

[MarcoZandrini]

Chris, I remember that! I think you were pulling your hair out. But Abel saved the day.

[CorvidX6M]

If the driver was stuck closed to ground it could cause that too. OP could check that terminal with key on engine off to see if it shows ground, compare it to the other coil drivers. Should be open to ground when the car isn't running.

Personally, I'd guess that you've got a bad coil driver in the DME. If it were my own car, I'd ask Abel to source a used DME, mail him my DME with enough money to cover the part and his time and expertise, and wait for the new clone to arrive.... (All the drivers and solder joints in your DME have lived through the same conditions)

Not only has he done this for my personal M5, but he's accomplished this miracle for quite a few customer cars for me. If you decide that's the way you want to go, it's likely he will pop into this thread and offer help.

[Arog502]

Check the engine harness connector for the misfiring cylinder. Make sure the sockets (I think the coil connector has pins) are clean and tight.

That's definitely on my list of things to check when she's free this weekend to let me look.

- - - Updated - - -

Oh ok, copy that. So you have power to the red/white terminal at the coil, but it's not coming through the primary of the coil and into the black/voilet wire from the DME? I'd be checking the plug and wires then. Maybe check continuity of that black/violet wire.

The other thing could be the red/white wire has one strand hanging on somewhere, but once under a load it's not enough to charge up the coil. To check that, do a voltage drop test - (after checking the plug and black/violet). You'll need to backprobe the coil plug red/white wire with a paperclip and check voltage there while the coil is plugged in and the car is running. The other lead of your meter should be on good Battery positive. It should read close to 0 volts, indicating no voltage is being lost while it's trying to run the coil.

Copy that. I'll add that to my list. I think I'm getting a crack at it tomorrow or Saturday anyway.

- - - Updated - - -

Personally, I'd guess that you've got a bad coil driver in the DME. If it were my own car, I'd ask Abel to source a used DME, mail him my DME with enough money to cover the part and his time and expertise, and wait for the new clone to arrive.... (All the drivers and solder joints in your DME have lived through the same conditions)

Not only has he done this for my personal M5, but he's accomplished this miracle for quite a few customer cars for me. If you decide that's the way you want to go, it's likely he will pop into this thread and offer help.

If all else fails in the above recommendations I'll be looking forward to getting some help from Abel.

[Arog502]

Guys I solved it and I made the discovery while executing the test methods suggested above. My girlfriend had some time tonight to let me look at the car, instead of waiting for the weekend. First I'll share the results I was getting and how it lead to the solution:

1. Following MarcoZandrini's advice I checked the harness connector. The pins were good and shiny/clean.

2. Following CorvidX6M's advice, I began testing coil ohm load as suggested. I tested two different ways, plugged in and unplugged.
With the harnesses plugged in, I back probed each coil harness.

A. Cylinder #1 was 0.9 ohm
B. Cylinder #2 was 1 ohm
C. Cylinder #3 was 1 ohm
D. Cylinder #4 was 0.9 ohm
E. Cylinder #5 was 0.9 ohm
F. Cylinder #6 was 2.44 M ohms!!!

I then re-tested each igntion coil while unplugged across the male blades.

A. Cylinder #1 was 2.2 ohms
B. Cylinder #2 was 2.1 ohms
C. Cylinder #3 was 2.9 ohms
D. Cylinder #4 was 2.3 ohms
E. Cylinder #5 was 2.2 ohms
F. Cylinder #6was 2.0 ohms

Clearly the coil itself was good or at least likely good. So I then tested continuity of the Black/Violet wire from coil harness to the DME. This yielded 0.4 ohms so the wire was good.

3. I then moved on to CorvidX6M's suggestion to test for voltage drop across the red/white wire. I used my Fluke insulation piercing probes that screw on and penetrate insulation. My Fluke meter showed -.41 volts but it suddenly dawned on me that the misfire was gone while I was running this test. I unhooked my probe and began to wiggle the red/white wire. The misfire came and went with each bend of the wire. I'll be damned. Temporarily the miss is gone for now after pushing the red wire into the coil plug firmly but I'm going to completely replace it with a new pigtail harness ASAP.

I can't tell you how much I appreciate you all for taking the time out of your day to help with this! It means a lot and we're both very grateful!

[MarcoZandrini]

You’re welcome.

[CorvidX6M]

Hey that's great! I'm glad we could help!

[shadowpuck]

good advice, good testing, good work everyone!

now, we can talk to the op about our fee - we do accept specialty beverages....

[Arog502]

good advice, good testing, good work everyone!

now, we can talk to the op about our fee - we do accept specialty beverages....

Unfortunately we are TTB regulated so I can't do anything with our beverage alcohol but I can send out hand sanitizer as a token of my appreciation!

[Breeze1]

good advice, good testing, good work everyone!

now, we can talk to the op about our fee - we do accept specialty beverages....

I was going to comment to say the same thing. Really appreciate the "flow" on this post. I'll keep that one archived just in case!

[Baylor]

Hi, my name is Baylor and I like in Frankfort KY. I have an extra 325i BMW that has all the parts you need if you want I’ve been selling off parts for people who are in real need for them if you’re interested...