Richard,

Have you ruled out vibration through the floor of the garage? Car 
engines produce significant vibration at startup and shutdown, due to 
inertia of the flywheel and cylinder compression. When the engine is 
running at idle these forces still exist but are much higher frequency 
and mostly damped out inside the engine.

You might be able to test for this by removing the magnet from near the 
coil, and starting the car. It would then respond to magnetic/RF noise 
but not to vibration. If you don't see the noise, then the noise is 
likely from vibration of the garage floor.

Karl


On 01/07/2011 09:47 AM, Richard Schneider wrote:
> Geoff,
>
> That all makes sense, but what is the explanation for *shutting down* the
> engine and seeing a similar in amplitude spike occur at that very instant?
>   At that point, the starter is not involved.
> It appears to point to the alternator field collapsing on shutdown. BTW, the
> start-up spike and the shut-down spike's have opposite polarities.
>
>
> On Fri, Jan 7, 2011 at 7:38 AM, Geoffrey  wrote:
>
>> When you first start an AUTO like 50 or more amps
>> might flow through the starter motor creating a large
>> circuit for an electro magnet ??
>>
>

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