[Mgs] Fw: Re: Mode of failure for ignition coil

Paul Hunt paul.hunt1 at blueyonder.co.uk
Mon Sep 15 02:11:35 MDT 2008


In my experience you can both hear and see the arcing at the contacts on a
good system, but of course circumstances can vary.  When the condenser has
failed open-circuit the arcing is very obvious, if in doubt compare it with
the amount of arcing you get if you tap the distributor to coil wire on and
off the coil terminal (it should arc much more than the points).  When the
condenser is short, or indeed if the wire between coil and points is open or
shorting to ground, there will be no sparking at all.  You can diagnose many
ignition LT problems by connecting a voltmeter to the points terminal on the
coil.  With the points closed you should see 0v, and with them open you should
see 12v.  If you see 0v all the time remove the distributor wire from the coil
and measure again.  If you still see zero volts then the coil is probably
open-circuit, or there is no 12v feed to the coil.  If the distributor
terminal on the coil *now* shows 12v either the condenser or the wire to the
distributor are shorted.  If you see 12v all the time, points open or closed,
the wire to the distributor is open-circuit.

The *primary* (ho ho) purpose of the condenser is not to quench the contacts
but to provide a tuned circuit for the coil to oscillate and give several
high-power pulses of HT in quick succession, without the condenser the spark
is very much weaker.  It is a by-product of this that because there has been
more energy directed into the HT, there is less left for arcing at the
points.

If you had said it was an MGA, then maybe I would have noticed it :o)

PaulH.
  ----- Original Message -----
  My understanding is that in a circuit like this, the condenser
  is there to prevent arcing at the contacts.  The condenser takes
  over the load from the air gap.  I was looking at this in
  bright daylight, so if there was a small spark I would not have
  seen it.


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