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Re: The tachometer

To: kmwheeler@ualr.edu
Subject: Re: The tachometer
From: "James J." <m1garand@speakeasy.net>
Date: Sat, 20 Mar 2004 18:45:54 -0500
Cc: Barrie Robinson <barrier@bconnex.net>, mgb-v8@autox.team.net
In-reply-to: <10737a2106eb6c.106eb6c10737a2@ualr.edu>
References: <10737a2106eb6c.106eb6c10737a2@ualr.edu>
Reply-to: "James J." <m1garand@speakeasy.net>
Sender: owner-mgb-v8@autox.team.net
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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
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