[Farmall] Farmall not starting---still!

[Stephen Offiler]

On Mon, Aug 3, 2009 at 12:11 PM, <szabelsk at gdls.com> wrote:
> Assuming that you are supposed to have 6v at the points and not 12v (???),
> the only way you would have 12v is if the resistor is shorted out.

Disagree with the above.

If you have voltage at the points, that means the points are OPEN.
And when the points are open, no current flows, and when no current
flows, the resistor doesn't drop any voltage.  Therefore, in this
case, 12V at the points just means there's a 12V battery and a good
connection all the way back to it.

Now, turn the engine by hand until the points are closed.  You should
see essentially 0 volts at the points, because now, the points are
completing a path to ground.  You should have 12V into the resistor
and something less than that, ideally 6V but it might vary, coming out
of the resistor and going into the coil.

> Have you checked to make sure the points are not shorted? You could be
> getting voltage to the points going right to ground instead of through the
> points.

It is conceivable that a set of points could be shorted out.  That
would mean they never open electrically, regardless of mechanically
open vs. closed.  If the points are shorted, you will never see 12V on
them.  Measuring 12V at the points requires them to be open.  See
above.  Points are not shorted out.

> The condenser is actually a capacitor rated at a higher value than what it
> is intended to be used for, however they can short out internally without
> any visual sign.

Very, very true.  Bad condenser generally has no visual sign.

>... The condenser's main job is to build up a charge and
> release that charge at the right time to provide the higher power you need
> for the ignition system. The release is triggered by the opening/closing
> of the points. Think of it as another power source for your ignition
> system.

The condenser's main job is to absorb the inductive voltage spike from
from the coil primary that occurs the instant the points open.  The
condenser then discharges itself back thru the coil primary, with some
losses, and then the primary discharges itself back thru the
condenser, with some loss, and after a dozen or so of these incredibly
rapid events, there's no energy left in the coil primary and it's
ready for the next ignition cycle.  (All that happens REALLY fast).
If you completely remove the condenser, the engine runs badly if at
all because the coil can't properly charge for the next ignition
cycle.  That would be the same thing as a condenser failed open.  If
the condenser fails closed, it is the same as shorted points, point
that never open.  No spark.


Steve O.