Lotus Elan

Tacho over reading

PostPost by: MikeB » Sat Aug 03, 2019 3:14 pm

Folks,

Have fired up the engine in my 1971 Sprint for the first time and the motor is running fine. As a precaution I fitted a new 23D type distributor and then put in a Lumenition Classic system. The tacho at idle appears to be reading twice the amount of revs, this was confirmed when we plugged in a rev /dwell angle reader, which showed 1000 rpm, when the tacho said 2000.

Has anyone had this occur before? I did a quick search, but seemed to find more about under reading than over reading.
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Mike
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PostPost by: rgh0 » Sun Aug 04, 2019 7:28 am

where have you powered your lumenition from ?. If its from the coil positive you may double the pulse count and thus double the tacho reading

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PostPost by: MikeB » Sun Aug 04, 2019 7:59 am

Thanks, Rohan, will check
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Mike
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PostPost by: Bigbaldybloke » Sun Aug 04, 2019 4:04 pm

Mine also reads approx double with a Pertronix system, I?ve added a recommended interface box, but it?s still not right. Have abandoned it for a while due to fitting new kitchen for wife, but must get back on to it in the next week or so.
Change is inevitable, except from a vending machine!
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PostPost by: Evante » Sun Aug 04, 2019 8:13 pm

Where should the tach be powered from?
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PostPost by: MikeB » Sun Aug 04, 2019 8:39 pm

I assume the Positive to the coil feeds the signal to the tach, afterall if you are running points it?s the only wire connected to the loom ( the Neg only goes from the coil to the distributor).

So I suppose the question is how remote from the Coil positive does the Lumenition positive have to be, do you need to fit it via a relay with a permanent positive feed turned on by the ignition switch?
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Mike
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PostPost by: rgh0 » Sun Aug 04, 2019 10:12 pm

Yes you need to take it from a supply turned on by the ignition switch that is separate from the positive that goes through the tach to the coil. The lumention generates a current pulse in its supply every time it triggers the ignition and this can be read by the tach and added to the pulse the coil generates

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PostPost by: MikeB » Mon Aug 05, 2019 9:45 am

Cheers Rohan, will sort that out :)
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