> From: (sfisher@Pa.dec.com) Scott Fisher:
>
> Well, the adventures of my sporadically failing MGB continue. [...]
>
> One failure seemed to be due to a loose HT lead from the coil to the
> distributor; I shoved that in and the car started up again.
One thing to keep in mind whenever this happens, if the loose secondary
lead from the coil creates a significant (>= 1/4") gap that the spark has
to jump, the secondary voltage increases *tremendously* (50,000 volts/inch)
which puts a strain on the secondary insulation. The coil will typically
either fail while the secondary lead is loose, or shortly after the loose
lead is tightened back down.
> The next failure was a complete mystery, as was the following failure.
>
> That one occurred on the way to Andy's house to work on the race car,
> so after I finally showed up we thought about it for a few minutes and
> determined that the coil was HOT to the touch. I swapped the coil for
> a known good, newer coil and the car started and ran smoothly.
So, this (probably) rules out a bad coil...
> [speculation on what might cause a hot coil, and whether a shorted
> capacitor could cause it...]
> (It also seems to me that this might result in a hot coil as current
> would keep soaking into it without ever discharging, but that's pure
> speculation on my part.)
Yes, the coil, being an inductor, would saturate. All of the current
flowing into it would get dissipated in the resistance of the primary.
Try this experiment: Turn the ignition on, after making sure that the
points are closed, but *don't start the engine.* After about an hour or
so, reach in and feel the coil. You will definitely find it warm, but it
should by no means be hot, at least not as hot as your description.
It sounds to me that, instead of a problem with the primary circuit, there
is an unusual load from somewhere on the secondary of the coil.
> I pulled the distributor cap and found some interesting things. I think
> I'll pick up a new cap and rotor tomorrow, the existing cap had verdigris
> on the plug lead contacts and the rotor was scored from the carbon brush.
A light patina on the contacts on the inside of the distributor cap is
usual; it alone shouldn't interfere with ignition.
> More to the point, the ground wire from the points base to the outside
> of the distributor is missing almost all its insulation, and the LT lead
> from the connector on the outside of the distributor was touching it.
I'm not sure of the significance of all that, but I would believe that if
the primary lead from the coil were intermittently shorting to ground, or
if the capacitor were shorted, you simply wouldn't get any spark. The car
wouldn't run poorly - it wouldn't want to run at all!
> Am I missing something? Does this all make sense? Well, even if it
> doesn't, it shouldn't cost much to try it out...
Maybe, and maybe. By the time you read this, you may already know the
answer. Concentrate on the secondary wiring, including the wires, the
distributor cap, and the rotor. I would bet that your hot coil and poor
running are caused by some of the coil secondary voltage arcing to
inappropriate places before it gets to the spark plugs.
Good luck.
Pat Vilbrandt John Fluke Mfg. Co., Inc. Everett, Washington USA
UUCP: pwv@tc.fluke.COM or: { uunet, uw-beaver, sun, microsoft }!fluke!pwv
ARPA: fluke!pwv@uw-beaver.ARPA
P.S. In a mechanical breaker point ignition system, the ONLY function of
the capacitor is to provide a place for the coil primary current to
go when the points open, which minimizes point arcing, and keeps the
points from being prematurely burned up. An open or "weak" capacitor
will NOT cause the engine to run poorly, but it will cause the points
to be burn and pit more than usual. The only time a "bad" capacitor
will cause running problems is if it goes short, which, as I said,
will kill the spark. (BTW - I have never seen an intermittently
short capacitor - either it is or it ain't - once the dieletric
breaks down, it seems to short for good.)
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