Jumpy Tachometer

Tuck · Post by **Tuck** » Tue Jun 08, 2010 5:15 pm

i had the boat out yesterday, and at one point, i noticed that the port tac on the upper station was jumping a bit. it took several minutes for it to stop, and then it only did so after i increased the rpms a few hundred revs.

this is a new tac ( i just had all the upper guages replaced this winter/spring).

while this was happening, i didn't notice a miss or anything wrong with the 'feel' of the port engine.

thoughts?

bobg · Post by **bobg** » Tue Jun 08, 2010 6:39 pm

Possibly EMI noise getting introduced on the sending wire from the coil. Also possibly a bad connection. Was it jumping to higher RPM or lower RPM? The first would tend to indicate EMI while if it is the second I would lean towards bad connection. EMI would be introduced by other wires (with higher voltages) running close to the tach sending wire.

But you can't rule out bad tach even though it is new. A lot of tachs are easily damaged by strange things. I have a tach (on my little boat - came with the boat) where the user manual said to not jump start the engine from another engine because it could cause damage. Sounds like a bad design to me, but what do I know.

bobg · Post by **bobg** » Tue Jun 08, 2010 6:57 pm

Here is an idea to help isolate the issue. Swap the sending wire at the tachs between the port and starboard. If the other tach starts jumping you have an issue in the sending system. If it stay at the original tach it is probably a problem with the tach.

Tuck · Post by **Tuck** » Tue Jun 08, 2010 9:11 pm

tks for the input. i'll look at that this weekend.

alexander38 · Post by **alexander38** » Wed Jun 09, 2010 5:39 am

loose ground will cause it to.