I would like to share a screw up of mine. 1999 Dodge 3500 5.9 diesel AT. Concerns-No Bus message, no communication with ECM, alternator not charging, and trans in limp mode. Engine starts and runs. Verified no communication, unplugged the PCM and that restored communication with the ECM. Plugged it back in and tested powers and grounds to PCM, all checked out. So I called the PCM. Took almost 2 weeks for approval and to get the PCM. Installed the PCM and of course it didn't fix it. Started back over, retested powers and grounds, all good. Tested 5 volt reference out of the PCM, 0 volts. I don't remember testing the 5 volt reference on the first go round. Probably because I didn't, not sure why. Wiring diagram shows 5 volts only goes to the transmission. Went to the connector at the transmission, the ends of the wires at the connector were bare and touching. Spread them out and 5 volts is back, communication restored, and alternator is charging. Replaced the connector and drove it, transmission still doesn't shift. Scan data of the PCM shows the TPS at 5 volts, so it's open circuit. Found the broken wire at a previous repair about 12 inches from the PCM connector. Will test drive again in the morning to verify the transmission is shifting.
Owning our mistakes helps us learn, and even better when we share. Helped a guy here with similar a few years back, and YUP, 'twas a Dodge/Jeep product. And just a heads up to anyone who hasn't been there yet, you may have to cycle the key to bring the 5V Ref back "up" after removing a short.
Been there. Chalk it up to a bad day. The scenario you described is one of the toughest diags out there. No matter how good you are, calling a bad computer is tough. Never the less, you did a great job. Tomorrow is another day.
Sh_t happens....at least you caught an issue it before giving back to customer. I recall the first time I realized a “5VRef” fault could take down PCM comm...I learned the hard-way, but I will never forget it, and that’s the take-away 👍
It's nice to see someone tell it like it is. Great Job. I find it sad that out of the hundreds of lost communication trouble trees I have looked at over the years, not one of them lists or tests for a 5 volt reference shorted to ground. This is especially odd since I have not seen a manufacturer that does not use the 5 volt converter to power the logic chips. I have wondered if this is an oversight on the manufacturers part or deliberately left out due to some kind of trade secret fear?
Ford Workshop Manual loss of communication to the PCM diagnostics do not show to check for Vref, but when the diagnostics are followed in the PC/ED (powertrain control/emmisions diagnostics I think) It will typically have you check for Vref. Sometimes the information in there, just not where you would expect it. I like to use VREF to determine if the pcm is powering up or just not talking. If the VREF is present you know there are at least some of the powers and grounds present even if there are no comms.