In the end
The ECM on this truck most likely took the crank sensor signal input and internally reduced the frequency of the signal delivered to the PCM for some type of strategy reason. There was an internal circuit board failure and the only choice was to have the ECM sent out for repair because there was not one available from the dealer or aftermarket due to the age of the vehicle. I just feel bad for the owner of the vehicle because this was a work truck and the shop owner never knew what a nightmare it would turn out to be and how long he would have it in his possession.
Okay so about another week goes by and the rebuilt ECM finally showed up and the shop installed the ECM which was a "plug and play" unit. The vehicle started up and the good news is that everything worked like a charm. I had to go back to just get a scope pattern out of this diesel monster and to validate the repairs before the shop buttoned it all up. I quickly hooked my scope up again to view the patterns and you could now see the input and output RPM signals were working as designed with different frequencies (Figure 10).
This vehicle really proves that a trouble code does not always lead you down the right path. It is vitally important to know how to read data PIDs. The code in the PCM only validated that the alternator field circuits had a power feed or switching ground issue that was pointing you to a wiring fault or a possible bad PCM with an internal alternator field driver failure. There was no cam signal reference to the PCM like there would be in a gas engine so there was no way for the PCM to fail the crank sensor for a lack of input. The strategy of this system would be for the PCM to provide a voltage output and field circuit toggling once a specific RPM threshold was met but this never took place. In the end it took some out of the box thinking to unmask the true culprit. My only hopes are that this story will enhance what you know or don’t know about Chrysler diesel controllers working in tandem.