2002 Ram Van (5.9).
Today, I got the dreaded "no bus" code.

The scanner can't communicate with the PCM, and the self-diagnostic procedure revealed the 920, 921, & 999 code (these are PROPRIETARY codes, NOT OBD2 codes).

This failure was completely spontaneous and occurred at start-up.

Before I spend all sorts of money on a new PCM, how can I *definitively* tell if it's causing this? If there is a wire to the PCM that has 5 V going to it (as I've been told), which pin is it, what color is the wire (or wires), and which connector has them?

 

Actually, there are a couple wires coming OUT of the PCM for the 5 volt feed to the sensors. A short in any one of those circuits can cause the no bus error. Rear O2 sensors are a common culprit. Easy to test though... Verify you are still getting the no bus error, then turn off the ignition, unplug the rear O2 sensor, (if you have more than one, just do one at a time) turn the ignition back on, see if you still have the error. Repeat that process for every sensor on the 5 volt bus.

 

FYI, this issue can also be caused by a bad cluster.
Have you checked for any blown fuses?

 

Yes. All fuses are good.

 

This is very good information.

Can you tell me what sensors are on the 5 volt bus besides the O2 sensors? Once I know what's included, I can start unplugging things one at a time to see if the error message disappears.

 

MAP, TPS, IAT, ECT, maybe the oil pressure sensor..... and probably a couple others I have missed.

 

Could a fault possibly exist with the oil pressure sending unit if the gauge cluster self-check was passed? I am asking because I am unsure.

So far, the transmission range switch (PRND21) and both oxygen sensors were disconnected, but there was no change.

Would the change be immediate, or would "no bus" disappear only after some kind of re-set?

 

I would turn the van off, and then back on for each test. Not sure it would recover from the no bus when you unplugged the offending sensor...

 

If it doesn't recover after the bad sensor is unplugged, how will I ever know where the problem is?

 

That's why you cycle the key. If it is related to a bad sensor, when you have it unplugged, the no bus should go away, and the cluster should work as intended.