What has an error code regarding the ignition in the second cylinder to do with the ABS system?

We have previously reported how error codes can be misleading and cause mechanics to falsely question the quality of a replaced part. Recently, our Product coordinator Søren Nielsen experienced himself how the error code reading on his BMW 320i (E90) pointed in the wrong direction.

After a few times having experienced uneven engine running, he visited his local free car repair shop and asked them to take a look at the problem. An error code readout clearly showed that there was an error on the ignition in cylinder 2. The coil which is of the type “plug top coil” was replaced, but shortly after, he once again experienced uneven motor running. 

Once again, the error code readout showed that there was ignition failure on cylinder 2 and it was therefore quite reasonable to question the quality of the replaced ignition coil. However, replacing the ignition coil for a second time did not solve the problem of the unevenly running engine either. 

Søren Nielsen then contacted Frank Donslund, from Elektro Partner, who provides hotline and technical solutions for car repair shops in Denmark, Norway and Sweden (Autodata, TEXA, Delphi and Nextech) to hear if they were aware of similar episodes.
 


       
Pictures of drive shaft with a defect and a functional ABS-ring (Autodata)
 

Frank Donslund was able to explain that these models may experience sporadic dropouts or ESP errors in conjunction with a false signal from the rear ABS sensors. "Often the reason for the incorrect signal is that the ABS-ring and the ABS sensor touch each other. This typically occurs when rust over time has developed between the ABS ring and the drive shaft joint, causing the ABS-ring (made of a thin metal plate) to grow", he  explained and continued: "The ESP system interprets the erroneous signal as wheelspinn and reacts by reducing engine power, which to the driver may feel as if the engine cuts out while driving. The problem is especially noticeable when the cruise control is activated and can also result in increased fuel consumption equivalent to about 2 km less per. liter gasoline".

Back at the repair shop, the rear axle ABS-rings were replaced and since then the car has run perfectly. The ignition coil which at first was proclaimed to be the culprit had nothing to do with the problem.
       

Plug Top Coil # 8860 11013 

ABS-ring # 8540 10403

 

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