tach problem/question

[offshore31]

ok, here's an interesting one. i've got a pair of 1998 cummins 330hp cpl 1975. yesterday, i was running at 2000 rpm and everything was fine. so i pushed them up to 2200 rpm, but after a couple of minutes the stbd tach started moving all over the place. 2000, 1800, 2300, 1500, etc. Engine sound didn't change, no pull to one side or the other, all guages read normal, so i figure it's the tach and not the engine. it behaved like that all day. below 2000 rpm the tachs were in sync. above 2000, the stbd would be all over the map. i noticed if i ran my finger over the tach guage it moved, almost like static electricity.

does anybody have any ideas what could cause this? i'm probably going to start by swapping port and stbd guages to see what that does. Any other ideas?

Thanks
Joe

[MikeD@Lightningshack]

Did you try the mechanical tap!

A couple light blows with the back of a screw driver on the bezel of the gauge has fixed problems I had before.

Of, coures if it works, I immediatly replace the gauge, but it is a quick way with out all kinds of wiring work to determine if there is a mechanical issue inside the gauge.

mike

[In Memory Walter K]

Check to see if you have a loose or bad connection on your alternator wires. Walter

[Harry Babb]

I'll bet a dollar to a doughnut that the tachometer does not have a good ground. The same thing happened to me on one of my gas engines.

A good test would be to make up a piece of wire with alligator clips on both ends.........hook one end to the tach ground and the other to a good known ground and see if the problem goes away.

Harry

[offshore31]

Mike,

Yup, tapping it doesn't do anything.

Walter,

Everything felt ok, but i'll double check.

Harry

First thing I'll try next time i'm down on the boat.

Thanks for the suggestions guys.

Joe

[Mikey]

Having just been through this with my port 6BTA I'd bet on the alternator. Take it to a reputable shop and have it tested ,including the tach connection. The stator which controls the tach as well as the alternator output may be kaput.
Rebuild was just South of $100.
How is the output of the alternator? Another clue.

[offshore31]

Mikey

Output is right in spec, but it could be the alternator. The engines are 1998's and the starter and water pump went last year. I expect with 10yr old engines that the accessories are starting to go. I'll pull it and get it tested.

thanks