Tachometer fluxuation

[Roger Patterson]

The tachometer reading on Blue Magic, our 34 Mk II, will intermittently increasing by up to a thousand rpm without a change in the throttle or the observed (sound) engine speed. The rpm will eventually return to its previous reading then jump again. The initial readings of the alternator showed normal output. Same for the back of the tachometer. I am stumped as to the next step. Any ideas would be most welcome. Thanks Roger

[Ken Juul]

The alternator must be charging the batteries to get an accurate rpm reading.  If your batteries were full with little load that might be the cause.  Perhaps the fridge cycling was causing the jumps.  Turn on a big load (engine fan?) and see if that helps.  Also check your fan belt for proper tension.

[Ron Hill]

Roger : You might also check the tac wire at the alternator - that's where it gets its signal.

[Ron Hill]

Roger : Might be a simple thing, but make sure that your drive belt is tensioned and in good condition.  A 1/2" deflection on the longest length or a twist between two fingers of not much more than 90 degrees.

At the cost of engines etc., it's cheap insurance to change to a new belt every other year (depending on the hours you put on per season).

A few thoughts

[Bobg]

There was a time my guages would all act up, tach jumping around included, all erratic problems went away when I changed my ignition switch.