Graham,
I have a vageu recollection of reading somewhere a tip for calibrating
electronic tachos. Since they work by counting 12V pulses off your
coil, all you need is a source of 12V pulses at a well-defined
frequency. This you already have at home... your battery charger! This
takes your 240V mains supply at 50 Hz alternating current, reduces it to
+- 12V alternating current (still at 50 Hz), and then turnes the
negative 12V peaks upside down to make them positive peaks. Result -
100 pulses per second, peak a little above 12V. For battery charging
purposes there is no need to smooth the supply (as you may get with some
transformers), so this is your output.
Since you get 2 pulses per rev off a 4-pot engine, 100 Hz represents 50
cycles per second, or 3000 rpm. For a 6-pot, 100 Hz gives you 2000 rpm.
I haven't needed to fiddle with my tacho, so I haven't investigated the
wiring on the Spitfire, but the general supply needed as far as I can
remember is a regulated voltage from your voltage stabiliser, the signal
supply which can vary in voltage (after all it will vary depending on
your alternator output 12-15V), and a common earth. Since you have a
through feed plus live and earth, I would suggest +ve from the charger
to +ve on the through feed, regulated voltage to the live feed, connect
the -ve from the through feed to the earth on the case, connect this to
both -ve on your charger and to your car's earth.
By the way, will putting an electronic tacho from a 4-pot car work in a
6-pot car? Since they work on counting pulses from the coil, surely it
would end up reading 50% too high? Or am I missing something here?
Richard Gosling and Daffy
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