[DEA12]

I have a 2003 40 Express with Cummins engines.  The port tach fluctuates intermittently only when warm and usually at cruise speeds, at times within the entire range.  I also have digital floscan instruments and the port side also fluctuates and mirrors the analog instrument (the information is picked from the tach input at the indicator).  So far here is what has been done:

1. Swapped indicators side to side...no change, problem stayed on port indicator

2. Checked instrument grounds....all appear good

I am in search of the tach pickups, the signal does not come from the alternator (no grey wire) so I assume it is getting the pulse from a flywheel pickup and I can't seem to locate it on these engines.  The discrepancy presents as a thermal issue (only when warm) and my thinking is that the sensor is breaking down when things heat up in the engine room.

Any suggestions are appreciated.