Lively tacho

[kevin61]

Hi all,
My tacho seems to have a mind of it's own! i.e. it seem to jump all over the place flicking mostly under acceleration... can anyone advise as to with direction to take to fix this problem please?

[john.p.clegg]

Needs a good clean between the rotating magnet and needle disc...
That is , if it's not the pivots that are worn....

John

Edit Sorry I read speedometer .

[david.g.chapman]

I would check the above things first.

If they are OK, then it could be the capacitors in the rev counter circuit. They lose their value over time, and might need replacing with new ones.

Dave Chapman.

[ricarbo]

You have lots of options. They depend on how happy you are to take it to bits. Personally, I enjoy that. Assuming it's the original style 'current operated' tacho (RVI on the face), these have either one or two transistor circuits. Early ones are two transistors. Early ones have a loop of the wire feeding the ignition coil on the back. The later type have the ignition wire plugged in by bullet connectors.
The usual cause of erratic behaviour is said to be a faulty electrolytic capacitor, which are only a few pence. Over the years the electrolyte inside them dries up, so they then go open circuit. If you want to replace these, you may have to settle for near values, e.g. 2.2uF instead of 2.5uF, which should be fine. I would also replace the other electrolytics, given their age. You can find more information by searching ?RVI revcounter? or ?RVI tachometer circuit?. Given that many people (including me) report problems running these circuits with electronic ignition systems, you may want to change to one of the less problematic ?voltage operated? circuits. I see these advertised on ebay for ?43. You could make one for less. I think the one I use costs less than ?10, based on a 555 integrated circuit. Firms like Speedy Cables offer conversions. I think whatever you do, if you do it yourself you will have to consider recalibration. Lots of ways to do that. Perhaps easiest is by temporarily attaching a second known good tacho or using a handheld non contact digital tacho (paint half only of the crankshaft pulley with white paint or even toothpaste if you don?t want permanent paint). I see you can buy these from ebay, with a claimed accuracy of 0,05% for ?6.99. If you change to a voltage operated circuit you will have to run an extra cable from the low voltage side of the ignition system to the tacho. That is to say from the contact breaker to the tacho. If the transistors fail in these early tachos, you are unlikely to find a direct replacement as they are germanium types, no longer made. You can however still find substitute germanium transistors. I don?t think that is your problem, though.
Good luck
Richard

[ecamiel]

There are now free strobe light tack apps for you phone. Worked for my IPhone
And an acoustic one as well which I haven't tried. Just google it

Eric
64 S1 Hart

[Chrispy]

I had trouble with mine after replacing quite a few components. Turned out it was the rheostat which was loose. Whenever going over a bump it would jump around like crazy. Have a look on the 2nd page of this thread:
lotus-electrical-f38/tacho-repair-t41110-15.html

[kevin61]

Thank you all, I shall try the various suggestions.

[bob_rich]

Hi kevin

Quite a lot on this forum about tacho and a recent one (link attached) on the RVI type with the internal current loop shows a number of interesting posts including some possible methods of calibrating

hope it proves useful

best of luck

Bob

lotus-electrical-f38/testing-tacho-t41369.html