Tachometer Flutter

A

albaris

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What would cause a tachometer to flutter. I have 1984 PT 38 Europa style trawler with Perkins twin diesels 135hp 6.354M engines with about 2900 hours on them. There are two tachometers for each engine, (pilot house and flybridge) and all 4 of them flutter....meaning the needle on the gauge goes between 1 and 2 thousand RPM back and forth quite fast. They will not hold steady.
I don't know how to tell if the sending units are located in the alternator or on the transmissions.

Any advice is appreciated.
 
Check the groud wire, it may be loose or corroded. It sounds like your ground loops through all four tachs.

I had the same problem with one of my tachs on my older gas boat, however each were grounded seperately.
 
Al, my analog tachs don't flutter but are rarely accurate to my digital syncronizer tach. Bought boat new & they have never worked right. Faria even sent new gauges to me but it didn't change anything. I live with it & rely on my Glendinning automatic synchronizer for engine synchronization. Don't know why digital tach reads correctly as, if I recall, wiring originates from same place. Just my 2¢.
 
If all four of your tach's flutter the same amount, I'd bet your getting some AC input somewhere.
The ground is probably OK, but some where in the system a Filter capacitor has gone shorted and the AC its supposed to take off, is showing up in the tach, probably else where too but the Tach is the only guage sensative enough to move with the AC.

Way back, I built a Knight Kit Tach.
The accuracy test was to build a small cicuit with a Resister and stick the probe into a wall socket.
The 60 cycle AC out of the wall would make the tach show 600 Rpm or if I put the other resistor on it would show 6000 Rpm. The needle had a Shudder to it with that test. Becuase it just couldnt follow the AC from Zero to 110 and back at the speed the AC was cycling.
Your tach seems to be nearly following the cycles.

Is your tach needle fluttering at a very high speed?
Does the flutter speed change as the RPM's increase/decrease?
 
I experienced this on a gas engine. Rebuilding the alternator (bad diodes) fixed it.
 
Yup, the alternator is a very likely source of AC on board.
 
Before you start replacing diodes...

(1) Make sure the battery switch is not on both. Sometimes when alternator are connected together....both.... this can cause the tachs to jump around.

(2) Make sure any and all external battery chargers are off.

(3) Inspect and remove and clean all connections including those at the tach.

(4) Start the port engine alone. Does it jump around? Shut down the port engine and start Starboard. Does it jump around? If only one side does it then rebuild that alternator. If it only does it when BOTH are running then see above.

Dave
 
Or you can just get a DVOM, put settings on A/C volts run leads across the positive post and neg post and anything under .500 A/C volts is fair, I would be looking for huge A/C spikes such as 1-3 volts or more, that would warrant diodes replacements.

Eric
 
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