You are going to likely need a scan tool the can interpret Chrysler Collision Detection (CCD) bus signals; (where the name came from I don’t know?) so that you can observe the communications between the different modules.
Most generic scan tools do not have this ability; which means you will need a Chrysler’s Diagnostic Readout Box (DRB) scanner.
The vehicle speed sensor signal goes directly to the Powertrain Control Module (PCM). The PCM converts this signal, so that it can be sent in a form that is (for the lack of a better phrase) in computer language to the different modules that require these signals.
Their reasoning for doing this is that they can reduce the size of the wiring harness because they can send several different signals down the same wire. Each signal has its own code attached to it, which the different modules (computers) can recognize as their required signal and ignores the others.
What you need to do with the scan tool, is see if the speed sensor signal is getting to the PCM; is the PCM sending out the required signal to the instrument cluster and then see if the cluster can interpret that signal.
I know this doesn’t help you find out why your speedo is not working but maybe give you an understanding of what a tech is going to need and do to find the problem.
Dan.
Canadian "EH"
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