[MC1600]

You have CATS or Cummins?

Not sure about the Cummins, but the CATS connections start right in the front of the engine, centered, above the belt cage. Directly connected to the block is the Aetna sending unit (P/N 8901). There are two wires from that but also a mechanical tach cable that goes to your synchronizer. The location is horrible because the cable gets kinked making a very tight turn between the sending unit and the belt cage. If I were you, I would do the following:

1. Have Glendinning make you a 12" extension (I'd just do both engines). The extension pushes the sending unit outside the belt cage and gets rid of any pressure on the cable.
2. If #1 doesn't fix it, I'd replace the sending unit. Super easy swap-out. They run about $135
3. If #1 and #2 fail, I'd send the tach to Lauderdale Speedo for a rebuild. They're awesome.
4. Last thing I'd try is replacing the tach cable between the sending unit and the synchronizer.

If you have a Cummins, I suspect it's a similar trouble-shooting process, but without the need for #1 above.