Erratic Tachometer

[airshot]

Had my new to me boat out today for another short shake down cruise. Once underway I noticed the tach reading erratically. At half throttle it was showing 6000 rpm and went as high as 7500 rpm but the Merc 40 was not screaming like it should have been, so I know it is the tach not the actual engine running that fast!!! I played with it speeding up and slowing down, at one point the tach needle would not drop below 1500 rpm..even at dead idle...after about 15 minutes it began working properly and stayed that way for the rest of the ride.. Any suggestions ???

 

[alldodge]

Loose connection would be my guess, start with grounds

 

[Silvertip]

There is a calibration adjustment on the back of the tach. Note its current setting. Then carefully turn the adjustment back and forth several times to burnish the internal contacts. Set the adjustment back to the original setting. Make sure electrical connections are clean and tight.

 

[airshot]

Many thanks, I will try that

 

[airshot]

Did not find any loose connections, nor did I see any type of calibration adjustment. Just three wires going into a harness. Loosened, wiggled, retightened those connections. Tach reads 1500 rpm just sitting in my barn. As soon ascthe motor is started it drops down then back up to read current rpm. Will have to wait until these storms pass to ger her back in the water to see what happens now.