1984 190D RPM gauge will not work

The rpm gauge on my 84 190d will not work. 84 and above do not have a tach amp so I can't do that quick easy fix. I made the assumption it was a bad RPM sensor so I replaced it. The rpm gauge still does not
register. I don't know where all the sensor wires go once they go to a relay station behind the battery. The only relays there are the kickdown switch and something else I forget at the moment. I suppose the rpm gauge could be bad. Is there any way to troubleshoot it? The car has 308,000 miles and that is pretty much the only thing wrong with it.
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Fuse? These things do need power.
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Fuse is good. Can I hook a multitester to the wires going to the RPM gauge behind the instrument cluster? What voltage and such should I look for?
Thanks
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I haven't a clue, sorry.
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