TCM and PCM not communicating?

dapartsman

Hello fellow Dodge enthusiasts!! I have a 2006 Dodge Dakota R/T with the High Output V8 and automatic transmission (only 90,000 miles). I have a recurring problem - the Check Engine light occasionally comes on when I start it. I always carry my code scanner with me, and every time this happens, I'm getting codes for transmission issues (P0700, P0846, P0871, P0876, P0988, P0882). Sometimes, it will show 12 codes, but it's just the same six codes twice. Once in a while, it's only three or four of those codes. If I drive it this way, it's locked in third gear. I always read the codes then try to clear them; sometimes I can immediately. Sometimes, I get an ERROR message and it recommends I check my connection. I unplug the scanner, plug it back in and start over. When I finally DO get the codes cleared, the Check Engine light goes out and the truck runs/drives just fine. Local Dodge Dealer said they would do a quick 30 minute check of my truck for free (I know them well.) It was running fine when they checked it but they told me the TCM and PCM were not communicating at all. If that's the case, how does the truck run at all? And, how can I clear tranny codes if they're not communicating? Any fellow Dodge truck owners ever have a similar issue??? Thanks in advance for any and all suggestions.

thecardoc3

Free checks are over priced. The P0882 means that the transmission supply voltage is low. The P0846, P0871, P0876, and P0988 are all pressure switch rationality codes. They use the same power source to report the switch conditions as the transmission supply power, so if that is missing that's what the computer should respond with. The questions then become why is that power missing, or why is the PCM/TCM unable to recognize it. BTW, the PCM and TCM are both inside the PCM assembly and are serviced together even though they act like separate components.

dapartsman

thecardoc3 - thank you for the comment. I apologize for the tardiness in acknowledging your response. My in-laws health, especially my father-in-law, has been pretty bad the last 5 weeks or so, and I've not had a good opportunity to log in and see any responses.

Anyway, I do appreciate your comments. I'm with you on "why is that power missing?" We just drove the truck to the grocery. I started the truck, pulled it into Drive and the check engine light came on. I pulled around to the side of the store, pulled out my hand-held scanner, and plugged it in. Just 4 of the codes were there. I shut the truck off, turned the key ON, then hit "Clear Codes" and got an ERROR on the screen. I unplugged the scanner, plugged it back in, and again read the codes. Same 4. I again hit "Clear Codes"; this time it recognized the command and asked for confirmation that I wanted the codes cleared. I hit YES, and it wiped them out. Turned the key OFF, then restarted the truck. She was fine all the way home. Very puzzling. The question becomes - Where am I losing power to the tranny? But yet, why is it restored so quickly/easily? Very odd..............

thecardoc3

Time to grab a schematic, multi-trace digital oscilloscope and be ready to confirm powers, grounds and communication data during a failure event.

The service advisor at the dealer probably didn't say that the PCM and TCM weren't communicating at all unless he/she misinterpreted the information from the technician. The historical code for the loss of communication can only be retrieved after communication has been restored. All of the diagnostics have to be performed while the problem is actively occurring.

dapartsman

It happened again today, except two of the codes indicated a "communication problem" - I cannot find the paper I wrote the trouble codes on. I had two tranny sensor codes, and two different communication problems. I wrote the codes down, and on the third attempt, was able to clear them and drove home. I have this terrible feeling that I'll need to wait until it completely quits so it will be MUCH easier to find the problem. The way it is now, it's so sporadic that it would be nearly impossible to find it - meaning this: if I can clear the codes, then the "problem" is gone - circuits are restored and communication has resumed. Just a real pain..............

thecardoc3

Problems like these are tough, but not impossible. It does take above average training and experience to handle them when they are still random in nature. The codes identify circuits that the module is detecting failures with, but they don't tell you what is wrong with them.

Some things that can happen that can cause a loss of communication include a loss of power or ground, and a circuit fault that is putting a load on the 5v reference output from the module. Have you tried to pull codes from all of the modules in the car? (ABS, Airbag, FCM/BCM, Instrument cluster, radio, HVAC as applicable ?)

dapartsman

No, I haven't. I do not have the knowledge of how to do that. I simply have a handheld OBDII scanner - purchased at AutoZone about 10 years ago. So I don't have a ton of technology either. I love the truck - the way it drives, the way it rides, etc. I won't even consider trading it, but this is a bit frustrating. As long as I can keep clearing the codes, that's what I'll do. But I have the feeling the day is coming that I won't be able to clear them. At that point, hopefully it will be at a point where a qualified technician can actually find what is wrong, where it's not communicating, and get 'er fixed once and for all. Thanks for all the suggestions. I do appreciate your efforts - thecardoc3.

thecardoc3

See if you can identify if there is a pattern to the failure such as how long the truck has to be driven. Or if it has to be run, then parked for a period of time and attempt to restart. Weather conditions, accessories that were being used prior to the event etc.

The most important piece of information is how likely is this to occur in any one trip? Or once a day, once a week out of how many times does the truck get used, or????

Reading back just now, there is a common theme. All of the codes listed previously are for low voltage to those sensors/solenoids combine that with the loss of communication and the focus is already on specific circuits for the first step in the pinpoint testing. You reported that the codes set when starting. I'd be measuring system power (battery), switched input powers and grounds, communication circuits, and 5v reference with the DSO (digital storage oscilloscope) during start-up.

Right now the battery state of charge and cable connections cannot be ruled out.