Subject: Re: [Mgs] mga 1500 with early 5 main mgb engine

From: dave northrup <[email-masked]>
Date: Tue, 8 Aug 2023 19:35:58 +0000

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From: Paul Hunt <[email-masked]>
Date: Wed, 9 Aug 2023 08:21:45 +0100

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Have you disconnected the tach with the engine running and confirmed it 
stops?

You can check the calibration using an old-fashioned trickle charger but 
if it's not working with coil current passing through it then it won't 
work on that either.  The only thing you can do (apart from sending it 
away to someone) is to open it up and look for any broken connections in 
wiring and an the circuit board.  If you are familiar with electronics 
there are certain tests you can do inside.  This circuit 
https://www.classictiger.com/techtips/motach.html is of the early tach 
with the external pickup and this one the later internal 
http://www.mgb-stuff.org.uk/harvit.htm.  It can also be converted to the 
later voltage pulse type with a module as here 
http://www.mgb-stuff.org.uk/rvctach.htm

PaulH.

On 08/08/2023 20:35, dave northrup wrote:
>
> I do not know the origins of the tach.  I have no idea what is inside 
> it and I have not taken it apart. The car came to me in kit form.  I 
> don’t really even know if the tach worked.  Any way to test it?
>
> I do know that with the tach hooked up the engine runs, so power is at 
> least going through it.   this is the only source of power for the 
> coil.  Anyone have an mgb electic tach to sell?  I’d like to hook up 
> something that is a known quantity.
>
>
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    <p>Have you disconnected the tach with the engine running and
      confirmed it stops?</p>
    <p>You can check the calibration using an old-fashioned trickle
      charger but if it's not working with coil current passing through
      it then it won't work on that either.  The only thing you can do
      (apart from sending it away to someone) is to open it up and look
      for any broken connections in wiring and an the circuit board.  If
      you are familiar with electronics there are certain tests you can
      do inside.  This circuit
      <a class="moz-txt-link-freetext" href="https://www.classictiger.com/techtips/motach.html">https://www.classictiger.com/techtips/motach.html</a> is of the early
      tach with the external pickup and this one the later internal
      <a class="moz-txt-link-freetext" href="http://www.mgb-stuff.org.uk/harvit.htm">http://www.mgb-stuff.org.uk/harvit.htm</a>.  It can also be converted
      to the later voltage pulse type with a module as here
      <a class="moz-txt-link-freetext" href="http://www.mgb-stuff.org.uk/rvctach.htm">http://www.mgb-stuff.org.uk/rvctach.htm</a></p>
    <p>PaulH.</p>
    <p><br>
    </p>
    <div class="moz-cite-prefix">On 08/08/2023 20:35, dave northrup
      wrote:<br>
    </div>
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      cite="mid:[email-masked]">
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        <p class="MsoNormal">I do not know the origins of the tach.  I
          have no idea what is inside it and I have not taken it apart. 
          The car came to me in kit form.  I don’t really even know if
          the tach worked.  Any way to test it?<o:p></o:p></p>
        <p class="MsoNormal"><o:p> </o:p></p>
        <p class="MsoNormal">I do know that with the tach hooked up the
          engine runs, so power is at least going through it.   this is
          the only source of power for the coil.  Anyone have an mgb
          electic tach to sell?  I’d like to hook up something that is a
          known quantity. 
          <o:p></o:p></p>
        <p class="MsoNormal"><o:p> </o:p></p>
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      <br>
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From: Paul Hunt <[email-masked]>
Date: Thu, 10 Aug 2023 08:56:55 +0100

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It can be surprising how often you can be absolutely positive about 
something, then find it wasn't as you expected.  Nothing like proving it.

There will be no harm swapping the pickup connections at the tach, and 
it's worth trying as someone may have swapped them internally for some 
reason.

Swapping the 12v and earth is a little more tricky.  With electronics 
reverse connection can sometimes blow the semi-conductors, so it's a risk.

PaulH.

On 09/08/2023 17:00, dave northrup wrote:
>
> There is only 1 wire going to the coil. That wire comes directly from 
> the tach; I wired that myself as well as the one wire going from the 
> fusebox to the tach.  So I can just about guarantee that if I break 
> that circuit the engine won’t run.  I have not yet tried “rapping” the 
> tach.
>
> I was not certain of your previous answer – can I do any harm by 
> swapping the connections on the tach?  Of course, if its not working 
> anyway . . .
>
>
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    <p>It can be surprising how often you can be absolutely positive
      about something, then find it wasn't as you expected.  Nothing
      like proving it.</p>
    <p>There will be no harm swapping the pickup connections at the
      tach, and it's worth trying as someone may have swapped them
      internally for some reason.</p>
    <p>Swapping the 12v and earth is a little more tricky.  With
      electronics reverse connection can sometimes blow the
      semi-conductors, so it's a risk.</p>
    <p>PaulH.<br>
    </p>
    <div class="moz-cite-prefix">On 09/08/2023 17:00, dave northrup
      wrote:<br>
    </div>
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      cite="mid:[email-masked]">
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        <p class="MsoNormal">There is only 1 wire going to the coil. 
          That wire comes directly from the tach; I wired that myself as
          well as the one wire going from the fusebox to the tach.  So I
          can just about guarantee that if I break that circuit the
          engine won’t run.  I have not yet tried “rapping” the tach.<o:p></o:p></p>
        <p class="MsoNormal"><o:p> </o:p></p>
        <p class="MsoNormal">I was not certain of your previous answer –
          can I do any harm by swapping the connections on the tach?  Of
          course, if its not working anyway . . .  </p>
        <br>
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