I have what I hope is a quick question. I have a 99 Ram 2500. When it gets warmed up I sometimes get a no bus failure and it won't start until it cools down. I've replaced most of the usual suspects. Camshaft position sensor, Crankshaft position sensor, ignition coil and wires before replacing the PCM. The wiring harnesses look "OK" for a 20+ year old truck. It does not seem to shift into 1st gear when I put it in drive, it doesn't downshift on most hills, and I have really poor gas mileage. Could heat cause the hall effect (I assume) sensor in the transmission to short, causing a no bus condition? It's a relatively cheap part to replace, but I don't wanna waste any more money on the MFKER if I don't have to.
Any advice would be appreciated.

 

"no bus" failures are notoriously difficult to troubleshoot so there's no quick answer, sorry. As a background, the bus in question is the CCD bus which connects PCM, instrument cluster, ABS, airbag and timer module (on some models). The message in the instrument cluster indicates that the cluster has no communication with the PCM, it is not a message from the PCM itself. Three possibilities: There is another module on the bus misbehaving, there is a wiring fault (unlikely as it is temperature related) or the PCM itself stops talking. If you have an OBDII code reader see if the PCM still responds on the OBDII connection which is separate from the CCD bus. (If you have access to a shop level scan tool you could also check if the other CCD bus controllers still talk but I realize those tools are not common for DIY).
I'm not clear on if you actually replaced the PCM already ? In any case I've not heard of defective sensors taking down the PCM completely (sensors are not on the CCD bus) so I'd start with the basics, power and ground. Pins 2 and 22 in connector 1 on the PCM supply power, 2 is switched coming from the inside fuse panel and 22 is permanent from the PDC. The usual song & dance, look for green crusties, evidence of excessive heat at fuses and such. Pin 31 and 32 are ground.
If that checks out you can try to drive with the transmission connector disconnected which would eliminate anything inside the transmission. You'd have the manually shift 1-2-3 and back as there is no automatic shifting.

 

Thank you for your response.
I did replace the PCM. I swapped the ignition coil and wires beforehand, as I had heard of high resistance causing voltage to backfeed through the winding of the transformer to the bus (somehow, I'm not clear on the path the voltage would take).
I will keep my obd with me the next time I drive long enough to get it to shut down.
If I am experiencing the no bus fault, and happen to unplug the faulty sensor, should I expect the bus to return?

 

Yes it will

 

I checked with an OBD and didn't come up with any active or pending codes. Next, I ran an onboard monitor test, I came up with $11 and $13 failing. I am not sure if the info I found is correct. I have found sources that say $11 is the throttle position sensor and $13 is an o2 sensor. Anyone know if this is correct? If these sensors fail, can they intermittently bring down my PCM bus when the truck is hot?
Is there some way to check the function of the output of the transmission speed sensor?

 

Got a cable to connect to my computer. In the onboard monitoring, Mode $06, I'm seeing $11 and $13 fail. Does anyone know where I can go to look up manufacturer definitions for these problems? I'm getting a lot of conflicting information. Any help would be appreciated.

 

Digging around I found the chart below. $11 and $13 are not pass/fail items but rather counters which exact purpose eludes me at this point. They are also not listed in the SAE standard so I'm guessing manufacturer specific.

Edit: Smarter through Google... these tests actually do have pass/fail criteria depending on if the counters reach certain numbers.
Everything you ever wanted to know:
- -

 

Thank you for the information. I really appreciate it. Can O2 sensors cause the pcm bus to drop?
I want to drive the truck to the point of PCM bus failure and record the data, but I'm hesitant to spend the money to license and get insurance if the problem too expensive to fix right now.

 

The two monitors are Chrysler specific and do not necessarily indicate a problem. If there was something serious wrong with the O2 sensors it would cause a DTC/check engine light to be set. FWIW, my truck fails $11 as well although I can see the O2 sensor switching between lean and rich when graphing the voltage.
As I stated before, I've never heard of the whole PCM failing because of a problem with the O2 sensors. O2 sensors generate voltage and do not have a sensor supply power. They do have a heater though which obviously uses 12V. When there is a short between the two circuits it typically causes a DTC, P0138 O2 Sensor Circuit High Voltage or similar.
The transmission issues could indicate a problem with the solenoid assembly or the wiring inside the transmission. That actually can cause all sorts of weird symptoms, hence the suggestion to temporarily disconnect the connector at the transmission and see if it makes a difference.

 

understood. thank you