Mercedes Diesel Engine Wiring Problem

imported_John Whelan - May 13, 2014

The diesel engine in the video is a 2007 Mercedes MBE 900 and it was shutting down on the road unexpectedly. The operator would be driving under several different conditions when it happened. We found the harness where the fault wire was located. Of course the engine did not die for us but you can hear it cutting out when the harness is moved up and down.

Troubleshooting Wiring 101

Manipulating the harness in this manner is troubleshooting 101 when there is a suspect wiring problem in that could be anywhere on a highway vehicle. We have had luck using this method several times. One occasion on a C2 Thomas conventional there was an intermittent problem with various lights and accessories coming on by themselves.

Mystery Wiring Problem on a Thomas C2 School Bus

The driver was mystified by this and tried to pin point the exact scenario when all this would develop. He even took a video to prove to us that he wasn't making all of this up. Of course when we checked it out in the shop nothing would fail. Eventually we used the “grab the harness, tug and push method” to try and locate the problem area.

What happened was the bus was in a normal state with out any faults until we grabbed a part of the harness half way down the frame that was an 8 pin connector. Once it was aggravated using this method everything went haywire. One headlight, the stop sign and half the loading lights started working on their own with all of the power shut down on the dash.

The connector was badly corroded and had to be serviced as needed to establish a good connection again. The wiring on these buses is a multiplex system and extremely sensitive to poor connections and corrosion. It was humorous to watch the lights go nuts when we finally found out the cause.

Required Repair for Engine Wiring Defect

With this original problem concerning the engine shutting down there was one #14 guage wire at the engine PDM (power distribution module) that supplied battery power to the MCM (motor control module). This was an elementary repair and we were lucky to find it this fast. Sometimes it's a needle in a haystack situation but in this case the mechanic gods were on our side :)

Conclusion – Check The Simple Things First

Wiring on any vehicle is a challenge when it starts to act up and as you saw in the video we found the problem with the engine shutting down by tugging on the harness. This was the first place we checked since we knew that was the supply power to the engine control module. If we had not had this kind of luck we would have been looking for the wiring schematic that controlled the engine run system.
Thank you for reading this article and I hope you have success with your next wiring problem. If you have a similar story or have any questions please enter your comments below. You may help someone out who is fighting a similar problem. Check out the can join for free and interact with other members with your questions and feedback.

Diesel Engines