[Mgs] Mode of failure for ignition coil

[Paul Hunt]

Ordinarily I'd ask what the tach and ignition warning light did when this
happened, but if you were in a driveway you probably weren't going that fast
and the engine and car stopped pretty-well instantly.  Normally on the open
road if you look at the tach while the forward momentum of the car is still
spinning the engine, if the tach is still registering the problem is HT or
fuel.  If the tach has dropped to zero then look at the ignition warning
light, and if that is still dark (and it normally works as it should) that
indicates the problem is in the ignition LT somewhere i.e. coil, points,
distributor ground wire or connections.  If the warning light is glowing then
the ignition has lost its feed from the ignition switch.

If flicking the points open shows no spark then either the condenser *is*
shorted, or there is no 12v feed through the coil.  But if you can see pulsed
12v going into the coil (how?) as you flick open the points that means there
*is* a 12v feed through the coil, and the condenser *isn't* short, which begs
the question why aren't the points sparking?  If you really did measure the
coil primary as a dead short, and not simply too low a resistance for the
meter, then if a chrome bumper i.e. 12v coil I'd expect much arcing at the
points and a harness getting very hot as there would effectively be a
full-short on the white circuit.  But if a rubber bumper i.e. 6v coil the
harness ballast will limit the current to a safe-ish level (which would still
cause that part of the harness to get hot eventually) but because there would
be no inductive effect from the coil there would be little or no sparking from
the points.

Many things do up and die, and a coil can be among them, but they more often
fail open-circuit.  That would result in no arcing at the points or HT spark,
but no pulsed 12v going into the coil either, just a steady 12v, and that is
on both 12v and 6v systems.

PaulH.

----- Original Message -----


  I was heading out for an MG club event this morning
  (beautiful indian summer here in the Pacific Northwest)
  and I got about 200 feet up the driveway and the engine
  just died.  No warning, just stopped like you'd turned
  off the key.  No spark.