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06 Thomas C2 school bus no start
Posted to HD/Fleet Forum on 4/11/2013 3 Replies

We have a no start on this 06 Thomas C2 bus. It was towed in. Drove fine the day before, driver parked it at home, then wouldn't start the next day. It has the Mercedes Benz MBE906 engine. We have a constant active code of 248-02, Communication problem between the VCU in the dash and the PLD-MR on the engine. Easy enough, I thought. Found a pinout and diagram in the Motor manual, located my pins for the J1587 datalink that these two communicate on, and tested them. Found 6 ohms resistance on each wire. Load tested for a volt drop of almost a full volt at 4 amps. I thought I had it. Replaced the twisted pair for the J1587, and no change. No start, code still active. Disconnected the batteries, cleaned all the grounds I could find, and relocated the ground from the inner framerail by the air compressor. Welded a bolt to the frame on the outside this time, and cleaned all the terminals and reinstalled. No change. Checked the J1939 just for fun, 60 ohms on it and I can communicate with all the modules. I do not have the Mercedes engine software, so using a ProLink hand held I got the 248-02 code.

We checked CMP and CKP signals, found no CKP signal. Replaced the sensor, now have signal, but still no start. Swapped the VCU with a sister bus, and the other bus starts and runs great. Same test, PLD-MR on the sister bus as a control, and it starts and runs fine. No codes while in the other bus. Put back in my bus, no start.

Here's an interesting note - If I turn the key on and wait a few seconds, for the intake heater for example, when I try and turn the key to the start position, I have nothing. If I turn the key off, then immediately begin cranking the engine, it will crank fine. Another tech noticed today if we watch the guages sweep, by the time they are done and they park, we have no crank. So if we turn the key on, wait a second or two, and begin cranking as the needles rise, by the time they park at 0 again, cranking is inhibited by something. But if we crank immediately, not waiting for the dash, we seem to be able to crank as long as we want, regardless of guage sweep. Cannot figure out what is telling it to not crank. Another tech found that if you keep bumping the key to the start position, it will eventually begin cranking. What is going on here?

We are not very familiar with this engine at our shop, and this has been in here for the whole week now. Has anyone seen this, or does anyone have any suggestions? We have proven the modules work on another bus, so it seems to have to be in the wiring. But the code calls out the twisted pair I specifically replaced. Pins are good at each end, and we did not use the firewall pass through, but ran it through a rubber bushing in the firewall, eliminating that connection. Still have the active 248-02 code, nothing has changed. Ideas anyone?

Tyler Coressel
Rosencrans Truck Repair
Columbia City, Indiana, USA

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