[Mgs] Voltmeter bounces when I shut off the engine

Peter Schauss rpschauss at gmail.com
Wed Jul 17 07:23:55 MDT 2013


The voltmeter is a mechanical gauge which I installed in the hole where the
clock was when I bought the car 20 years ago.  I never noticed it doing
this before I installed the Pertronix.  The positive terminal is wired to
the fused side of the circuit which is hot when the ignition switch is on.
The other terminal is grounded.  The behavior does not seem to be related
to the cooling fans which are not normally on when I shut off the engine.
The engine does not run on after I turn off the ignition.

My first guess is that the stray voltage has something to  do with the
collapsing magnetic field in the coil feeding voltage back into the
ignition circuit after power is turned off.  Whatever the case, I have put
more than 100 miles on the car since I installed the Pertronix system and
everything else works properly.

Peter Schauss
1980 MGB


On Wed, Jul 17, 2013 at 3:29 AM, PaulHunt73 <paulhunt73 at virginmedia.com>wrote:

> What type of voltmeter is it?  Digital?  Slow-acting thermal like the fuel
> and temp gauges?  Or something else?  Where is it connected?  Has it only
> started doing it since you fitted the Pertronix or did it do it before?
> Does it only do it while the engine is spinning down or after it has
> stopped?
>
> If only since you fitted the Pertronix then it almost certainly has to be
> associated with that, either directly because of how you have wired it or
> because you have disturbed something else.  If a digital or fast-acting
> gauge, and it's only doing it as the engine is spinning down, then it's
> possible the Pertronix is feeding pulses back into the system.  This can
> happen with late-model MGBs because of a wiring design error, that can
> result in the engine continuing to run after the ignition is switched off
> if there is anything wrong with the emissions plumbing or anti-runon
> circuit. This is because from the factory those cars stopped the engine not
> by disconnecting power from the ignition as you would expect, but by the
> anti-runon system starving the engine of fuel.  Hence the ignition is still
> firing after switch-off and while the engine is still spinning down, hence
> possibly feeding pulses back to the rest of the cars electrics.
>
> If it's a slow-acting gauge doing it while the engine is spinning down
> then it sounds like a physical problem with the instrument and some kind of
> vibration.  In which case it should still do it with the wiring to it
> disconnected.
>
> Highly unlikely to be the cooling or heater fans still spinning, they
> would only supply a gradually reducing voltage as they slowed, it would
> have always done it, and easy to eliminate by turning the engine off when
> they are not running to see what happens.
>
> PaulH.
>
> ----- Original Message -----
>
>> The car runs fine, but I noticed that my voltmeter bounces around for a
>> few
>> seconds when I shut off the ignition switch.


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