Trojan Boat Forum

Coming home from a weekend on the water last Sunday and cruising at my normal 3600 rpm, I noticed the tach slowly creep up to 4000 rpm then slowly back to 3600 rpm. This condition repeated itself another 4 times before getting back to the marina. Although the tach was indicating a 400 rpm change, there was no noticeable change at the engine and no change in speed.

The tach is a Faria analog gauge and was new in 2001 along with all of the rest of the gauges. The distributor is the original Mallory with a Sierra electronic conversion kit that I installed about 6 years ago.

Any ideas on what may cause this?

I have the same gauges and about the same vintage. My starboard tach on the flybridge will do what sounds like the same thing. I give it a light knuckle rap on the glass face and it settles down.....not the best fix but better (and cheaper) than replacing the gauge.

Paul, look at the back of the tach and if it has a switch that you can adjust for 4,6 8 cylinder.......sometimes simply moving the switch around resolves this with some tachs( worked with mine)....maybe some kind of corrosion??....

Thanks gents, I'll check it out.

mikeandanne wrote:Paul, look at the back of the tach and if it has a switch that you can adjust for 4,6 8 cylinder.......sometimes simply moving the switch around resolves this with some tachs( worked with mine)....maybe some kind of corrosion??....

Agreed - crank the adjustment back and forth a few times to (hopefully) scrape away any corrosion that may be causing you trouble, then set it to calibrate it as needed.

Not to hijack the thread, but on the same note as instruments not reading properly, had something weird happening to me on my vacation trip.

Stbd engine hours meter read exactly what it should, but for our last 2 runs port hour meter read 1 hour less than we actually did. Is this an electrical issue, or toasted gauge?

Capt M you could be losing power or ground to your gauge I assume its a stand alone gauge ?

prowlersfish wrote:Capt M you could be losing power or ground to your gauge I assume its a stand alone gauge ?

That's what I was thinking and will check it out, but power and ground are daisy-chained across all gauges (separate feeds for port and starboard of course) and the rest seemed fine.

captainmaniac wrote:

prowlersfish wrote:Capt M you could be losing power or ground to your gauge I assume its a stand alone gauge ?

That's what I was thinking and will check it out, but power and ground are daisy-chained across all gauges (separate feeds for port and starboard of course) and the rest seemed fine.

Could be wiring or gage. I just replaced a gage that only recorded 3 hours on a 10 hour bench test.

captainmaniac wrote:

mikeandanne wrote:Paul, look at the back of the tach and if it has a switch that you can adjust for 4,6 8 cylinder.......sometimes simply moving the switch around resolves this with some tachs( worked with mine)....maybe some kind of corrosion??....

Agreed - crank the adjustment back and forth a few times to (hopefully) scrape away any corrosion that may be causing you trouble, then set it to calibrate it as needed.

Not to hijack the thread, but on the same note as instruments not reading properly, had something weird happening to me on my vacation trip.

Stbd engine hours meter read exactly what it should, but for our last 2 runs port hour meter read 1 hour less than we actually did. Is this an electrical issue, or toasted gauge?

Yep, pretty common symptom and fix. If that doesn't do it, look for a poor connection or if twins, try swaping units and see if the symptom follows the gage. Distributor issues usually show up as an erratic/fluctuating reading.

mikeandanne wrote:Paul, look at the back of the tach and if it has a switch that you can adjust for 4,6 8 cylinder.......sometimes simply moving the switch around resolves this with some tachs( worked with mine)....maybe some kind of corrosion??....

I didn't have the opportunity to try this fix last year since the next time that I took the boat out the tach worked fine, and did up until this weekend. When I started the boat Friday morning to head out for the weekend, the tach was floating around at about 400-600 rpm higher than it should. When we came home Sunday, I rotated the switch back & forth between settings and the problem went away.