cummins 6b5.9 tach fluctuations

[no1fishhead]

My tach on my 1995 220HP 6B5.9 running at idle fluctuates between 100 and 1000 RPM but the engine appears to idle smoothly...What could be the problem.

[In Memory Walter K]

Sounds like a loose wire somewhere.

[In Memory of Vicroy]

Ditto Walter's advice - look for a loose connection. What kind of tach drive? Alternator or magnetic off the flywheel? And does it indicate properly at higher rpms?

UV

[no1fishhead]

Ditto Walter's advice - look for a loose connection. What kind of tach drive? Alternator or magnetic off the flywheel? And does it indicate properly at higher rpms?

UV

I'm not really sure what type of drive it is, whatever cummins stock item. It only appears to fluctuate at low RPMs after it heats up, otherwise it appears to be fine.

[In Memory of Vicroy]

Its probably an alternator drive, which counts the electrical frequency of the a.c. output of the alternator (vs. a magnetic drive, which counts the number of flywheel teeth that pass a bell housing port over a short period of time). As the alternator spins faster, the electrical frequency increases and the tach converts that to an analog readout. If its just doing it at idle its either a loose connection or a fault in the alternator output. A sure way to find out would be to switch tachs and see what you get.

If you can live with the idle bounce but want to know your exact idle to syncro with the other engine, buy a photo tach. The cheap ones are less than $40 nowdays. I bought one a couple of years ago and have used it on several different things.....one of those handy-dandy gizmos you don't just HAVE to have, but slick as owl doo-doo when you do.

I converted the alternator tach pick-ups on my 6BTAs to magnetic years ago and never regretted it - much more reliable and accurate system.

UV

[MikeD@Lightningshack]

I had a similiar problem one time. I tapped the bezel with the back of a screwdriver and it stopped the fluctuations and never had the problem again.

Of course, I did't use it for more than a season afterward as I replaced all of the gauges as part of a winter project.

Worth trying, I guess.

good luck,
mike dolan
www.lightningshack.com