What sort of crank problems have you? The CPS will usually cause a crank but no start condition (it generates signals used to control fuel injection and ignition timing) and a failed starter solenoid will cause a "no crank" condition (the solenoid, controlled by the ignition switch, controls the flow of power into the starter. Think of it as a big switch, and you are on the right track in this case.) There are several other solenoids present, but - as mentioned - the starter solenoid is the most likely to cause trouble. If you have a Diesel XJ, you likely do not have a CPS in the control loop for your engine - I'd have to check - but there would be something similar to run your tach.
If you can supply us with year/engine/&c, and give us a better description of your ailment, we can help you better! You may have an easier time finding Diesel parts than we do as well, since the Diesel was made by an European company and you are simply closer to it...
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