Lotus Elan

Wildly swinging tach

PostPost by: Terry L » Mon Jun 27, 2011 2:49 am

The electronic tach on my '72 Sprint swings wildly under acceleration. I assume it's a bad ground. Anyone know where I should look first?
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PostPost by: reb53 » Mon Jun 27, 2011 3:38 am

This is one of, if not the most, discussed topic on this forum.

There is a huge amount of info if you go searching.
I have literally just come in from the garage having put the tacho back into my Sprint.
Even after changing to electronic ignition years ago it carried on working ok until relatively recently. It would bounce around, flicking full scale sometimes.
I couldn't see why I should have to change out the internals, as everyone seems to do, as it worked fine in the past.

Somewhere in here there's a post about checking a 2.2 mike capacitor on the board inside the tacho as it can loose capacitance over time.
I did that and all appears to work again. I won't be certain until I've had a bit of time back on the road but it looks good at the moment.

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PostPost by: memini55 » Mon Jun 27, 2011 7:11 pm

I would completely agree with Ralph on this one as the search will bring up many options for a solution. His sugestion of the capacitor drying up and changing value is also a very common issue with old caps. I would replace all the caps at one time.

Now I have also been working to bring the old into the new technoligy and I found a great circut design on the web that works great with about any type of input. The mod was designed by Steve Maas of Long Beach Cal. http://www.nonlintec.com/sprite/Sprite_ ... c_Tach.pdf

I have built three of these to date and they work very well, one running points and the other two run on a Pertronix. The nice thing is he also provides and easy circut for calibration if you wish.

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PostPost by: billwill » Wed Jun 29, 2011 11:57 am

>The nice thing is he also provides and easy circut for calibration if you wish.

UK & Other European members need to note that we use 50 hertz electricity supplies here not 60 hertz so the calibration circuit will provide pulses corresponding to 1500 RPM not 1800.
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