Re: The tachometer

[Barrie Robinson <barrier@bconnex.net>]

James,

Just been talking to Martyn Harvey.  He told me about an SD1 tach swap - so 
I got myself an SD1 tach - and this is what you do!

First, one must remember we are talking 100mm gauges (77-80 dash)

Take the SD1 casing off, take off pointer and dial face (two small 
screws).  Take the MGB tach and remove pointer and dial (two small 
screws)   Put MGB dial (the screw holes match the SD1gauge perfectly) and 
pointer on SD1 tach and mount back into dash !!   You got to admit this is 
easy - but you have to have the 100mm gauges!!


At 03:45 PM 3/20/04, James J. wrote:
> To add to Keith's words, I'd use as high of a value as I can find (many 
> mega-ohms) since you want as little current passing through it as 
> possible.  These types of pots are often called "trimmer pots"  or "cermet 
> pots" because the resistive element is made from a plastic called 
> "cermet".  You can find them in 20 or 30-turn units.  Unlike conventional 
> "volume" pots that you find in radios, etc., these are often long and 
> rectangular with 3 little terminals coming out.  We're talking about 
> something that looks like a 1" french-fry (or, ahem, 
> 'freedom-fry').  RatShack might have these, but your best bet is Allied or 
> Mouser Electronics on-line.
>
> Now I don't know if there is a procedure for callibrating these, but if 
> the Tach is just a voltmeter, then all you have to do is measure the 
> resistance across the entire potentiometer, and then adjust the tap until 
> you read exactly 1/2 that value.  A 20-turn pot gives you alot of 
> resolution to get to exactly the right value.  This will give you exactly 
> 1/2 the input voltage, but if the pulse-to-volt converter is off, this 
> will also be off.  If you have a DMM that has a pulse-counter (or a 
> timing-gun with a tach) you can tweak the potentiometer to compensate for 
> any linear errors in the p-to-v converter.
>
> Best of luck!!!
> JJJ
>
>
> kmwheeler@ualr.edu wrote:
>
> > 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
Barrie Robinson
barrier@bconnex.net 
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