Mercedes Diesel Engine Wiring Problem Part 2

Mechanics Hub - May 27, 2014

This is part 2 of a previous video I posted here that showed me wiggling the harness to make the engine quit. It didn't work but the engine did cut out a couple of times. This video shows you the actual wire that is at fault. It's a grey14 gauge wire that runs power to the MCM (motor control module).

The driver had a no start problem that threw us off at the beginning thinking it was the starter. That was first thoughts and soon it appeared that it was something to do with the wiring. The other bus which was a year older was quitting on the road randomly so the unpredictable nature of that type of occurrence immediately led us to the wiring system.

In this video I'm holding on to the PDM (power distribution module) which is supply wiring, fuses and relays strictly for the engine. It's mounted to the side of the fender and is open to the environment. Once I agitate the wiring you can see that it kills the engine. The only way to repair this problem is a complete clean up of corroded wiring connections running to the bottom of the PDM.

The MCM is a 120 pin module that controls the engine side of the operation. If you ever have a sensor or ignition problem you may have to remove the connector from the MCM and test a circuit at one on those pins. The wiring schematic will be available if you have the Detroit Diesel Diagnostic Software. It will be very specific when there is a code providing a schematic of the sensor and the pin description at the MCM.

The schematic from the sensor will show you the 5V reference or ground which ever it may be at the end of a dotted line signifying the motor control module. It will indicate 2 numbers with a forward slash in the middle like this 120/25 which means the 120 pin connector and pin number 25.

I have to warn you though the pin marking on the connector is very hard to see so you will need to get out a magnifying glass. This schematic information is like gold and troubleshooting is much easier. Getting back to the job at hand once we serviced all the connections at the Engine PDM for the bus in this video the engine has been starting perfectly since.

It's funny how the same problem happens consecutively and I'm waiting for the third wiring problem in this area to happen since all of these Thomas C2 buses are wired very similar. Just the other day someone called from a bus leasing business asking questions about the units we have coming up for sale. Our conversation led to the problem he's having with one of his buses (the same bus model) that's shutting down unexpectedly.

I mentioned the problem area we've been dealing with and that could be the same thing that's happening to his bus. I haven't talked to him since but I'm wondering in the back of my mind if he had the same wiring problem as us. I'm tempted to give him a call to satisfy my curiosity.

That's it for now. If you have any C2 buses running in your fleet I would take a look underneath the Engine PDM mounted on the side of the fender for corrosion. You may be able to prevent an engine shut down or no start condition.

Diesel Engines