Tachometer trouble

[Harlan]

Tachometer gives crazy readings for first 15 minutes or so of use. Starts and idles at a correct reading but after a minute or 2 the tach jumps up to a much higher reading then down - up - down, etc. Bring the engine up to cruising speed of 2000 rpm and tach continues to jump up and down - mostly above the actual rpm. Goes up to 3000, down to 2000, up to 2500, etc. Change is instantaneous jump - this is not a mechanical problem of engine actually changing rpm or slipping belt, etc. If used for a long run at set speed (usually about 2000 rpm) the tach will settle in after maybe 15 or 20 minutes and hold at the correct speed. Boat is OD34 with Universal M-25 engine, standard Motorola alternator (don't have P/N - sorry) connected to a Teleflex tachometer using the a/c output from the alternator.
Any thoughts on cause and cure are appreciated.

 

[LeslieTroyer]

Either the connections/wires are corroded or your line is ringing for some reason. You might try a 1k resister across the signal and ground to see if it stops the ring.

 

[capta]

Since most tachs are connected to the alternator, this can indicate a blown diode. The alternator may still function, but not as well as before.

 

[LeslieTroyer]

Since most tachs are connected to the alternator, this can indicate a blown diode. The alternator may still function, but not as well as before.

I doubt a diode would do this. The tax signal is measuring the AC frequency, so is measured before the diode pack.

 

[jviss]

If your motor is a diesel (and most gas), then more than likely there is a blown rectifier diode in the alternator. The Tach counts the number of alternating current (AC) cycles from the alternator.

The 6 diodes are arranged in a three phased full wave configuration. If one or 2 diodes are dead, the AC pulse will be missing at least 1/3rd. The needle usually fluctuates a tiny bit but the weight of the dial arm smooth it out. That's why at high speed, the needle seems to be steady.

Bring your alternator to any automotive electric shop and have them check it out. It is not too expensive to replace the diodes.

It's possible you switched the battery selector while the engine is running. That action usually blow the diodes."​

from:
https://forums.sailboatowners.com/index.php?threads/tachometer-needle-bouncing.138102/#post-891251[/quote]

 

[capta]

I doubt a diode would do this. The tax signal is measuring the AC frequency, so is measured before the diode pack.

I lived with this problem for several years and new diodes fixed it.

 

[LeslieTroyer]

I guess I’m confused why would they measure the ripple on the DC side for frequency ?
This is the only place where a pulse is lost with blown diode - and it would be consistent, not blown for a couple minutes then working again.

 

[Stu Jackson]

It's possible you switched the battery selector while the engine is running. That action usually blow the diodes.

Only if you still have the AO going to the switch instead of to the house bank.