Sounds to me like there is more resistance than there should be between the
battery and the engine. With little or no current being drawn, you see the
same voltage at the coil as the battery is giving out - 12V. When you
engage the starter, it tries to draw a humungous current, so there will be a
massive drop in voltage across any resistance between the battery and the
starter. Even when things are working fine you can see voltage drop to
around 8V when the starter is engaged, just due to the internal resistance
of the battery itself.
Likely causes could be:
Poor or corroded contacts on the battery
Partially damaged or disconnected cable between the battery and the engine
Partially damaged or disconnected earth strap from the engine back to the
battery
Dead battery
Since this car has only just been acquired, and we don't know what the DPO
has been up to, if I was a betting man my money would be on a missing earth
strap, with dodgy battery terminal questions coming a close second.
Richard Gosling
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