On most modern European vehicles, information on systems is shared throughout the entire vehicle electrical system. This means that an engine computer (normally responsible for engine electronics) can make more informed decisions if it has information on chassis system dynamics, ambient conditions and transmission control. This

Customer Complaint

In our current example, a guest brought their 2007 BMW X3 3.0si to the workshop with a concern of under heavy acceleration, all warning lights on the instrument cluster illuminate (DSC, 4×4, Service Engine Soon) and the vehicle “bucks” violently. They also stated this started happening after some work was performed at another shop.

Initial Inspection

Upon inspection, the work performed at another shop was: replacement of both rear tires, replacement of valve cover and replacement of transfer case. Given the extensive areas of work on the vehicle, the first step to be performed was to check for fault codes. During the code read, it appeared that the transmission control unit was not responding at all…

Initial Diagnosis

The first step to diagnosing a “no-comm” to a control module is to check the basic electrical support to that control unit. This includes checking for power/ground/CAN-signal at the component itself. In this case, the transmission control unit is integrated into the transmission valve body and a connection from the vehicle’s harness is made at the transmission case. This would mean we would have to disconnect that connector and check at the vehicle harness side if we had our basic electrical functionality to that control module. Source voltage was found on all appropriate connections along with proper chassis ground and CAN bus signals (2.3v and 2.6v on the can hi/low.).

Rationalize Diagnosis

This diagnosis result usually means that the component itself was faulty (transmission control unit), and most shops would have diagnosed this situation as such. However, given our experience with these vehicles, we just had a hunch that was potentially the wrong path and more extensive diagnosis would need to be performed prior to condemning a $2500 component and performing a $3000 repair. So – the next step was to try and reproduce this issue and get a better feel for what’s going on.

Using Customer’s Description of Symptoms

During normal driving conditions, the vehicle seemed just fine! It handled well, shifted properly and ran smooth. Several miles were put on the vehicle and basically nothing could be verified. A very strange result, considering that the transmission control unit was unresponsive to the diagnostic equipment. Usually if a transmission control unit doesn’t respond, the vehicle won’t even get out of 3rd gear and there will be other lights on associated with the issue itself. However, this wasn’t the case… we decided to drive it again, only this time, let’s put it under some harder acceleration and see what may transpire.

That’s when things got strange – under heavy acceleration and around 5k RPM, the vehicle would practically hit a brick wall – complete sudden deceleration and all dashboard lights would illuminate… within a second or two, the lights on the dash would shut off and acceleration would pick back up. All would return to normal.

Utilizing Vehicle Repair History

At this point, we thought it prudent to double check the work performed by the other shop. The transfer case replacement looked to have been done okay. Mounted properly, all connections tight etc.. the rear tires were replaced and on an all-wheel drive vehicle, it’s very important to make sure you have equal (or near-equal) rolling circumference front to rear. We did find that the difference front-to-rear was ~ 1″, which is too much and can cause the transfer case to wig out and do strange things. We decided to disconnect the transfer case and drive the vehicle again, just to rule out a possible transfer case intervention causing the strange shifting and bucking. The vehicle was driven again and the situation still occurred. Back to the drawing board..

Then we checked the valve cover replacement. Now, this may seem like a strange route to go, but what we do know about the valve cover is there’s a main harness for engine electronics that is shared with the engine computer *and* the transmission CAN network. If this harness isn’t properly secured, you could cause issues with the powertrain electronics system.. ..well lo and behold, it appeared that the ground straps for the ignition coil system were not tightened down all the way!

Verifying Repairs

We corrected the slight mistake made by the previous repair shop and road tested the vehicle again. All seemed fine.. further more, the transmission control unit was now responding to the diagnostic system! All components were re-installed, faults were cleared and the vehicle drove normally.


When it comes to diagnosis, man-made-issues can be the most difficult to pinpoint and resolve. It’s always a good idea to double-check work performed by yourself or others in an effort to speed up the diagnostic process. We all make mistakes, but much can be done to prevent them and prevent the customer being at the mercy of them!

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