How to bench test speedometer on prefacelift 1991 XJS V12

Have read through archives and did find info about speedometer problem, but no info on bench testing - I checked for broken wiring to interface unit in boot and to sensor at differential - thanks, Tex.

unless you have the proper electronic equipment - sounds like an auto elect job

What test equipment are you referring to - I do have an oscilliscope (that I have’t used for years) but no signal generator and of course a VOM, and a low volt source, if these are the equipment you are talking about - thanks, Tex.