Posted: Wed May 26, 2010 5:56pm

Tony, I have what appears to be a failed tachometer. Before I shell out for a replacement, could you suggest a few simple tests to determine if anything else is wrong. I have checked all obvious wire connections to the back of the panel and from the alternator. The engine is a Yanmar Barrus Shire 1200 type 3TNE78A supplied in 2003 on a steel 45ft narrowboat. Perhaps a clue: The tacho needle has periodically stuck on 1000rpm when turned off. Hope you have enough info to help. Thanks Tony

Posted: Wed May 26, 2010 7:28pm

Dear Tony, Mine, a VDO, always sticks at about the idle reading when I turn the engine off. I think the tachometer has its power turned off before the engine has stopped although the alternator is still producing pulses. The result is the needle stays in the position it was in when the power was turned off. Of course much depends upon the internal circuitry and I have no knowledge of that. Once you are sure the meter has a good supply (pos) and negative connections plus the alternator lead is attached and in good condition there is only one thing left to check and that is the alternator. The rev-counter uses pulses from just one of the six main diodes in the alternator and if that diode or it's pair has failed the Tacho would stop working but the alternator would still give a charge of sorts. If you have a multimeter that reads frequency (Hzs) set it to a low Hzs range and connect it between the alternator's W (tacho) terminal and negative. When you rev the engine the meter should go up and down in time with the engine. Repeat at the instrument end of the wire. You should get a similar result with the meter set to a "more than 20 volts" AC range. If you have an ammeter and upon first start up it gets close to the design output of the alternator its probably OK. Also at the end of a days cruising the charging volatge should be above 14 volts DC with the engine revving - probably 14.2 to 14.4. If its lower make sure the drive belt is tight and then suspect a diode fault. That's about all I can say apart from see how it behaves under cooler conditions. Tony Brooks

Posted: Thu May 27, 2010 10:44am

Thanks for your prompt and as usual, most helpful reply. When next aboard, i shall try to observe your suggestions. I did forget to mention that the hourmeter, set into the tacho dial, is still working, but this probably just works from "ignition" current when the key is turned on. Many thanks again.

Posted: Thu May 27, 2010 11:04am

Not a lot of help or comfort but its usually the LCD hour counters that pack up while the rev-counter still works.