No bus message, engine won’t fire, or runs rough

1999 Dodge 3500 van. When trying to crank, engine turns over, but does not fire. No bus appears in the odometer window. Letting the vehicle sit for a few minutes resolves the problem sometimes. But the engine will rough run very roughly, backfire, and stall. No trouble codes. Planning on replacing the crankshaft position sensor. Might also replace the camshaft position sensor. Am hoping it is not the electronic control module (ECM).

You are on the right track with replacing the crankshaft position sensor.

If one of the engine sensors is shorted the PCM will shut down and “No Bus” will be displayed in the odometer. If the problem occurs again unplug the crank sensor ans see if the no bus message goes away or just replace the crank sensor, it is a likely cause of the problem.

Thanks for the response. I replaced the crankshaft sensor, and the problem remained. When the odometer displaying the “no bus“ indication, I unplugged the crankshaft sensor and then the camshaft sensor. Still had the “no bus.”
Pulled out the computer, and it doesn’t look or smell burnt or overheated. The pins look fine, no corrosion.
I suppose I could have it towed to the local dealership, but they are likely just to tell me to replace the ECM.
I’m not sure what else could be the problem.

If you can get a wiring diagram or pin out for the ecm and figure out which wires at the ecm are for your cam and crank sensors, I would unplug the connectors from the
ECM and then unplug the crank and camshaft sensors and OHM the harness wires with a multimeter to see if you have an open or a short in one of the wires. Be ginger with your test leads, you dont want to just go jamming them into the terminals on the connectors or you could create more problems by spreading out the female side of the terminal.

There’s likely multiple engine sensors that should be at least visually checked for broken or chaffed wiring near the connector. Besides the crank/cam sensor, take a look at the maf/map, engine coolant temp, throttle position sensors. Maybe you’ll get lucky and spot the problem. I wouldn’t expect a shorted sensor to cause a “bus” error, seems not very diagnostic friendly, but I guess that is how the electronics are configured on this vehicle.

fyi: Your car has multiple computers, and therefore has computer wires running here and there in order for the various computers to communicate with each other. These are called “bus” wires. If one computer tries to communicate with another, and doesn’t get the expected reply it decides there’s a problem with the bus wires and issues a bus error message. Basically its saying it has no idea what to do b/c the car’s inter-computer message system isn’t working.

If any of the engine sensors with a 5 volt feed has a short to ground the PCM will shut down. Check the TPS, MAP and I believe the governor pressure sensor in the transmission so also check the for damage on the wiring to the transmission.

Thanks for all the answers! I checked the MAP sensor connector, and with the ignition on, it has +5v on two pins, and the third pin is shorted to ground, which I believe is correct. Same thing with the crank sensor. It was too cold to go crawling around underneath the vehicle, so I gave up and took it to a local Dodge dealership that I do a lot of business with. Of course, it drove fine on the way there, though it DID throw a crankshaft sensor error. The dealership has initially indicated that they have experienced similar problems with after market crankshaft sensors. So they will work it in today when they are able, and we’ll see what they come up with.
I’ll post back the results!

After the technician spent several days trying to track down an intermittent problem, I ended up buying a rebuilt PCM from AutoZone and had the dealership install it. The van ran fine, but it was throwing several error codes. The dealership said it was because the new PCM still had an old vehicles Vin number on it, which they were not able to erase. So I took that one back to AutoZone and had the dealership order one from Dodge, also a remanufactured unit. After installing the new PCM, the van ran for 10 hours straight with no hiccups, and I was able to drive 400 miles home.
Hopefully the problem is solved, though I wish I knew for certain what was wrong with the old PCM, and what caused it to fail.

No more PCM problems after 3000 miles, so I think we can say it’s fixed.
New problems now, though. See my new topic on a squeaking front wheel!