Perkins 4108 Tachometer Reading High

[jlogan]

I have a strange problem that has cropped up. I've owned this boat for several years so I am very familiar with the engine and systems. I went for an overnight cruise the other day, tied up at the end of the day, and hooked up to a shoreside power provide by a town on the water. Tachometer was working normally at this point.

The next morning, I started the boat and the tachometer is reading high - I mean really high, by about 900-1000 rpm. You could tell by the sound, but as I said I know this boat and the rpm it takes to make my cruising speed of 7 knots under power, and it was reading about 1000 rpm high.

The tach drops to zero when the engine is off, and I have not worked on anything or fiddled with the alternator to cause this. Nothing is slipping and the RPM is really not as high as it is reading - I've verified this with a phototach.

Anyone got any ideas as to what could cause this, and more importantly, what could fix it? The boat is a 1979 Gulfstar 37 - engine a 1991 replacement, no problems prior to this, as far as I can tell the gauges are the orginal that came with the boat....

I've checked the forum archives, and don't seem to see anyone that has had this problem....

Old boats, what we buy when we think we have too much time on our hands......

[Bill S]

I would check out the alternator and voltage regulator first to see if it is ok. Since the tach gets its info from the alternator.

I have a 4108 and the original tach went bad on mine. I replaced it with a generic tach, but had trouble calibrating it. I put on a high output alternator and the tach was never right after that. I switched to a Tiny Tach that does not run off alternator.

[perchance]

You might try checking the voltage/current at the tach. to see if it agrees with the manufacturers spec.