[Mgs] Voltmeter bounces when I shut off the engine

PaulHunt73 paulhunt73 at virginmedia.com
Wed Jul 17 01:29:12 MDT 2013


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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