Intermittent No Bus Stall on 2000 Dodge Ram

2000 Dodge Ram 1500 Van 5.2L V8 -- 200k miles -- conversion body

Hi Everyone,

I'm fighting PCM issues with a Gen 2 Dodge Ram Van. With the original factory PCM, it was throwing a P0505 code and stalling regularly while idling. Van would start right back up again immediately, but stalled if I let off the gas. A new Idle Air Control motor did not resolve the P0505 code or the stalling issue. There was good continuity between the IAC pigtail and PCM connectors, and the 5V signals were present, so I thought the harness was okay. Mechanic recommended a new PCM.

I bought a remanufactured, preprogrammed unit from SoloPCM. The first one they sent was a dud. Crank no start and the fuel pump never stopped priming after you turned the key to Run. Fuel pump would just run and run. But no CEL or P0505 code. I sent that unit back and received another (or maybe the same) unit a few weeks later.

This one starts fine and throws no CEL or error codes. But once the van warms up to operating temperature, whether driving or idling under a tree, it will die suddenly without warning or any mechanical symptoms. Engine shuts off, power steering and brakes go, and I have a mild heart attack. After 20 seconds, it will display No Bus on the odometer. While in this No Bus mode, the scanner can't pick up any codes or make connection to the PCM, all dashboard gauges are buried at zero, and fuel warning light is on. All other electrical systems (power windows, lights, wipers, horn) work fine.

Sometimes fiddling with the three PCM connectors (specifically the right-most black one) will wake the van back up. You hear the relay click and the fuel pump prime, after which the gauges are all functional and the van will start right up. Other times, just waiting half an hour will resolve the issue. Relay clicks, fuel pump primes, gauges start working again and van starts up. I have de-pinned, inspected, and cleaned all three connectors. Issue persists.

While in No Bus mode, both wires delivering power to the PCM (pins A2 and A22) show 12V when back-probed. Both grounds (pins A31 and A32) have good continuity to ground. Both 5V reference supply wires (pins A17 and B31) show 0.4V, same as at the Throttle Position Sensor, until either time or wiggling the PCM connectors brings the van back to life and the 5V reference returns. I've unplugged the IAC, MAP, TPS, Camshaft, Crankshaft, Governor Pressure, and Rear Wheel Speed sensors while in No Bus mode, and none of them change the 0.4V reference voltage at the TPS.

Any advice on next steps? SoloPCM recommended a new Mopar Crank Position Sensor, but it's $120 and I'm trying not to be a parts changer 

If anyone read this far, you're a saint. Thanks so much for any help to can provide!

Check the 5 volt ref, when you have the "no  bus" message.  This era of dodge is known to lose communication and display the "no bus" message if the 5 volt ref gets shorted. A bad sensor can pull it down, or a short in the wiring harness can pull it down.

 

Thanks so much, Chad. And for the wiring diagram. I've got the Chysler service manual but the wiring diagrams are much less legible.

How would you recommend testing the 5V reference circuit, beyond removing the sensors?

Sensors - I've disconnected the MAP, TPS, Transmission Solenoid, Crankshaft Position, and Camshaft Position sensors while measuring reference voltage, and none of them caused the voltage to increase above 0.4V.

Wiring Harness - what's a smart way to find issues in the wiring harness? I'm afraid I'll damage it trying to remove it, and don't have an amp-meter.

Thanks again for the help!

While in No Bus mode, both wires delivering power to the PCM (pins A2 and A22) show 12V when back-probed. Both grounds (pins A31 and A32) have good continuity to ground. Both 5V reference supply wires (pins A17 and B31) show 0.4V, same as at the Throttle Position Sensor, until either time or wiggling the PCM connectors brings the van back to life and the 5V reference returns. I've unplugged the IAC, MAP, TPS, Camshaft, Crankshaft, Governor Pressure, and Rear Wheel Speed sensors while in No Bus mode, and none of them change the 0.4V reference voltage at the TPS.
Any advice on next steps? SoloPCM recommended a new Mopar Crank Position Sensor, but it's $120 and I'm trying not to be a parts changer 

If anyone read this far, you're a saint. Thanks so much for any help to can provide!

If you unplugged the crank position sensor and your 5V didn't return, then it's not that sensor that's responsible.

I don't recommend doing a continuity test on the ground pins A31 and A32. It sounds like you're using the ohm meter for that - don't do that. Just have the key on, and check for low voltage (50 millivolts or less, or .05 volts) there with your meter just like you did the test at the 2 power feeds.

Since you're dealing with low voltage on the 5V circuit, something is pulling it down. It can only be a bad computer, shorted (to ground) wire or shorted (to ground) sensor.
Eliminate the wiring next of course. Identify the wires going to the PCM at A17 and B31, then cut the 2 wires in an easily repairable location, then turn the key on and off again and check for 5V again where you made the cut.

If it's still not there then the computer's bad which is what I suspect when you wrote "Sometimes fiddling with the three PCM connectors (specifically the right-most black one) will wake the van back up."

but hey it's good to do the checks anyway cause you could have shorted wires and a bad computer it can happen.
 

then turn the key on and off again and check for 5V again where you made the cut.

This is very good point. Some times you may need to cycle the ignition to bring the reference voltage back, even if you have eliminated the short. 
I also really like the idea of cutting the red circuit near the PCM. That's a very effective way of deciding if you need to keep unplugging sensors, or start testing the computer.

Hi Everyone...

I tested grounds as described by Ferris, and they seem fine. I did not cut the two 5V reference wires coming out of the PCM yet, because another PCM arrived from SoloPCM. This is the original factory unit -the one that had a P0505 error originally- but now SoloPCM has fixed the P0505.

The van now starts up fine with no error codes or CEL. It idles happily until engine coolant temperature reaches around 80-90*C, then it dies as if the key's been turned off. No sputtering, just dead. As soon as it dies I hear the fuel pump re-prime and the gauges all stay alive. No error code or CEL, although the CEL flashes once right when it dies. After that, it will start right back up easily, idle for 2-3 minutes and die again. It will keep starting, but dies more and more quickly each time, down to about 30 seconds.

Does anyone have any more ideas or troubleshooting steps?? Thank you!

To know where to go next, you will need to identify what goes missing when the vehicle dies.
I would monitor the power feed to the coil, injectors and fuel pump relay and see if any or all drop out when the van dies.
I don't want to throw a bunch of hypothetical situations out there and complicate things for you, but the pump cycling after the stall makes me think the ASD relay may be opening for a moment (dropping power to the coil, injectors and computer) when it should not be and then coming right back on. I have seen this before on Chrysler vehicles where the PCM driver for the ASD relay just stops for a second. It's kind of a chicken/egg scenario because the ASD relay powers the computer, but the computer turns on the ASD relay. One step at a time though, see what is missing when the van dies and trace the fault from there.

Thanks Noah - diving back into this over the weekend. Appreciate all the help!