Re: How to test diesel rpm sender and gauge?
Lots of engine RPM meters work off the impulses from the alternator.
The sorts of questions that you need to be asking are:
Is the alternator still charging? (Check the battery volts when the engine is running - should be 13.8 - 14.5 volts of thereabouts)
Are the connections on the back of the alternator to the tacho still ok. (Wires fall off, dirty connections etc etc)
Has the tacho still got its 12v supply? (Check with your multimeter on the wires going into the tacho etc etc)
My tacho stops working when the alternator stops charging - its happened twice. Once when the bolt holding the alternator onto the block sheared, and once when a wire broke on the back of the alternator.
Hope that this helps.
Wishing things away is not effective.