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Tacho over reading

PostPosted: Sat Aug 03, 2019 3:14 pm
by MikeB
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.

Re: Tacho over reading

PostPosted: Sun Aug 04, 2019 7:28 am
by rgh0
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

cheers
Rohan

Re: Tacho over reading

PostPosted: Sun Aug 04, 2019 7:59 am
by MikeB
Thanks, Rohan, will check

Re: Tacho over reading

PostPosted: Sun Aug 04, 2019 4:04 pm
by Bigbaldybloke
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.

Re: Tacho over reading

PostPosted: Sun Aug 04, 2019 8:13 pm
by Evante
Where should the tach be powered from?

Re: Tacho over reading

PostPosted: Sun Aug 04, 2019 8:39 pm
by MikeB
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?

Re: Tacho over reading

PostPosted: Sun Aug 04, 2019 10:12 pm
by rgh0
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

cheers
Rohan

Re: Tacho over reading

PostPosted: Mon Aug 05, 2019 9:45 am
by MikeB
Cheers Rohan, will sort that out :)