No signal to 1st coil back from DME (please help me diagnose wiring or ECU)

[patina]

Bought a 1993 300CE recently that has been maintained very well throughout the years.

After purchasing, I noticed an occasional hesitation/skip/stutter at idle while the car was in gear. Shifting to N would essentially cure it. I never experienced hesitation under acceleration.

That being said, I drove the car to breakfast on Saturday and about 2 miles down the road I noticed a loss in power. Pulling off the exit the car seemed like it was not running on all cylinders.

I towed the car to my shop and started primitive diagnostics. I determined that the coil pack nearest the firewall that sends signal to cylinders 1 and 6 was not functioning as neither cylinder was getting spark. I then swapped that coil pack with a new unit thinking it was just a bad coil pack and the problem persisted. I then plugged the original coil pack into the second bank and both plugs fired.

Therefore I have determined that I have an issue with either the DME/ECU itself or the wiring harness. I am wondering if there is a wiring diagram available that would help me determine which connectors at the DME send signal to the affected coil pack connector. I am trying to determine whether I have a bad ECU or a bad wiring harness while being as efficient as possible. For what it's worth the wiring harness looks to be in good condition physically, I do not see any corroded/rotted wires or connectors.

Thanks in advance for any help.

[nulu]

Did you swap coil pack with assoc connector(the part that connects to plug) or just coil pack , those sparkplug connectors can cause your issue if bad, the rubber looking boot below coil pack , if thats not it , usually bad wiring harness causes issues as well

[patina]

As an update, I checked conductivity in the harness from each pin (8,9,21 I believe) that leads from the ECM to each of their respective connectors at the coils and they were all perfect. This leads me to believe it is the ECM that is responsible for my problems.

Ordered another ECM that was said to be 100% working from eBay, received it and plugged it in. Started the car up and the problem persisted. Checked the originally affected coil (closest to the fire wall, fires cyl 1&6) and now it's sparking! But the next coil down the line (fires 2&5) isn't firing. WTF.

Swapping back to the old ECU reproduces the original result, new ECU shifts the problem to a new coil.

Is it possible that I received another dysfunctional ECU with the exact same problem as mine but on a different pin? I don't see any other possibility.

[patina]

Well I'll update this again just in case anyone has the frustrated experience I did. It seems a faulty coil can actually damage the ECU which is what happened in my case. If for whatever reason this happens to you, REPLACE ALL 3 COILS BEFORE INSTALLING A REPLACEMENT ECU.

ECU Repair Centre, Motor ECU Repair, Motorsteuergert Reparatur, 412.229....

Problems with the Motor ECU-module are quite common for Mercedes, Audi, Volvo, BMW and many other car type’s. Most common problems are loss of spark on 2 cylinders. Intermittent poor/non start or poor idle/running.

We are specialized in repairing car electronics and can offer you a repair service for the Motor ECU. In the table below you can find some of the Motor systems we repair.

Please don't hesitate to contact us for further information, first fill in our repair request (free of any obligations!) so that we have al the information we need to answer your questions.

Common failures:
Engine misfire on two cylinders
Blown ignition coil/s

Please Note:
The common fault with the VDO ECU originates often from the ignition coil packs, these coils must be then be replaced before the remanufactured ECU is fitted to the vehicle. Otherwise the faulty ignition coils can blow the ECU again!