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Fluxuating Tachometer
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 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.
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Tach pickup should be on the flywheel housing left side. Unless flowscan is tied into tach signal probably not sender. If ground checks out possible you have fluxuating +vdc from key switch going bad. Install a jumper between +12v terminal (red wire) and run terminal (usually purple wire) will tell you if this is the problem. Just take care not to jump the start wire (maybe yellow and red wire) and also take note that engine will not stop with jumper in place.
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Hmm, I didn't think of the floscan angle....does the floscan system replace the original tach sensor when installed? I thought it picked the RPM information from the sensor input terminal on the tach indicator itself. I will have to look into that diagram.
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I guess I wasn't clear. What I meant was depending on which Floscan instrument you have it may not be connected to the tach sender. If you have the tach option then yes sender is a possibility. If the entire floscan gauge is flickering it may be power related; if it is just the tach reading that is bouncing around it is more than likely the sender.

As far as I know floscan uses the engine tach sender and does not have a separate signal generator. Engine tach sender should be on left side of flywheel housing. It senses the number of teeth on the flywheel.
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Thanks for clarifying, the floscan indicators do have digital tach readouts. The RPM is the only indication that is bouncing and in sync with the analog tach indicator. I will look for the sensor when at the boat next week.
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