Barrie,
Someone will have the specifics but here's the basics. The tach in a 'B
(or TR7 for that matter -- those guys face the same problem) is based
on a frequency to voltage convertor. This circuit takes the ignition pulses
and turns them in to a voltage. The movement on the tach is actually
a voltmeter.
The circuit is based on an IC. The IC is mounted on some
"thick film" circuitry on a ceramic substrate. The problem with
the V8, as you've seen, is that now you've got twice the frequency
(number of ignition pulses) for any given RPM and the tach reads high.
The fix involves adding a variable resistor and adjusting the circuit
so that the output voltage is corrected. Again, someone else
may have the info, or it may be on the web. I would suggest
that you get a "multi-turn" potentiometer for this, a $3 or so
Radio Shack (if they still sell such things) item. After setting
the "pot" so that you get the right reading, you'll want to glypt
it so that it doesn't rotate. The part you'll need is a "multi-turn
trimmer potentiometer", but I'm not sure what the value is.
I'm not at home right now so I don't have access to my info.
If no one else comes through I'll try and find it for you.
I've made it all sound a bit more complicatd than it is. Basically
you add the "pot", tweak it so the tach reads right, and then
glue the adjustment in place. If you're not comfortable putzing
around circuits and soldering things I'm sure someone on the
list can help.
-Keith Wheeler
Team Sanctuary
----- Original Message -----
From: Barrie Robinson <barrier@bconnex.net>
Date: Saturday, March 20, 2004 2:05 pm
Subject: The tachometer
> I presume all of the people who dropped a V8 into their MGB had
> the tach
> problem. I already have the description on how to change the
> original 4
> cylinder tach to 8 cylinder but I would appreciate words on any
> other
> solution. If you have done a conversion/replacement I would love
> to hear
> about it. It could be a subject to be covered in the British V8
> Newsletter !!
>
> I am currently trying an SD1 tach implant under the guidance of
> Martyn Harvey
>
> Regards
> Barrie
>
> Barrie Robinson
> barrier@bconnex.net
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