Dodge Repair Questions? Ask a Mechanic for Answers ASAP
the speedometer signal originates at the rear speed sensor on the rear axle, it then goes to the abs controller and engine computer (PCM NOT The diesel ECM) where it then travels to the speedometer, since you already replaced the speed sensor and there are no lights coming on we can ASSUME (remember I am basing this on information you supplied and any info you supply in the future) the wiring is ok to the abs controller ( it would set a fault and turn on the abs light IF the sensor was dropping out) the abs controller then communicates the signal via a digital square wave to the PCM which then hardwires the signal to the cluster, no based on what info you provided about the cruise control dropping out when the speedo drops out we can ASSUME the wire from the PCM to the cluster is ok that leaves the PCM either not receiving the signal from the abs controller or an internal failure in the PCM itself, to get a BEST GUESS you will need to scan the PCM and see if it logged any faults for a loss of sped signal if it DID NOT then a good ASSUMPTION would be a failure in the pcm itself, this is assuming the connections have no corrosion and are tight
if there is a fault the PCM will store a code any dealer should know this the ONLY time a code is erased is if you erase it with a battery disconnect OR with a scan tool, it MAY self erase AFTER 255 starts with NO repeat of the fault.
you can always purchase a decent scan tool (code readers are very spotty on the OLDER diesels) for a few hundred bucks or less amazon has quite a few BUT I would contact the manufacturer and verify it will work on your truck, 01 is a changeover year and can have several versions of the cummins from 12 valve to 24 valve with the 24valve having a dedicated ECM for the engine.
we have not used dedicated speed sensors for some time and on the trucks with rear abs or 4 wheel abs the signal is from the rear sensor no others are needed or used.
if you have a scan tool that reads data you can see if the PCM is losing the signal BUT without a way to see if the digital square wave is actually being sent to the PCM you will need to rely on scan data and any faults in the PCM
remember talking to a service writer is NOT the same as talking to a tech on the phone many writers have little actual experience working on vehicles so their info can be spotty