[ROVERNET - UK] Dist cap and other HT failure
Fletcher
gofanu at usachoice.net
Mon Mar 12 17:33:13 GMT 2007
I posted this to another board recently, in response to questions about
rotor failure. Note that many failures are the result of bad moves on
the part of those working on the car. The use of a ballast required type
coil on a non ballasted system is the overkill version of a coil that
will put out excessive high voltage sparks, but the developed voltage is
still a function of whatever takes the most voltage to fire - either the
plug gap or a disconnected wire. A special note on excessive plug gaps:
Some platinum plugs, Bosch in my experience, have very skinny center
electrodes which burn down inside the insulator, giving gaps much
greater than they appear. I lost quite a few caps on my Dodge truck
before I figured that one out.
Rotor failures are due to two main reasons:
Faulty molding - all the OE Lucas ones (maybe 3)I ever had bad, had
bubbles in the plastic when broken in half. There may also be issues
with repro material since it's the same sort of thing as the rubber bits
they now make that go away on the shelf in a couple of months. No excuse
for that with modern materials science.
Overloading of an old tech design - The fundamental limitation is the
thickness of plastic between the center rotor contact and the dizzy
shaft. OK for the Max old time 40KV of a Sports coil, which was rarely
if ever actually developed with .025 plug gaps. Modern systems can put
out in excess of 60KV, and the use of wide gaps will actually result in
that kind of voltage. So will broken conductors in plug wires, or
amateur testing by removing plug wires to see "how far the spark will
jump", or cranking the engine with IGN ON and plug leads disconnected.
Solid copper wires rarely had broken conductors, and most coils have
internal voltage limiting "sparkover gaps"; if the coil is designed for
50 or 60KV, the internal gaps don't work until well over the design
voltage. These voltages WILL puncture the plastic at the weakest point
of the rotor, cap or coil, and will damage plug/coil leads. If you find
a rotor that will stand this abuse, then the spark will simply track
across the cap to the nearest ground, either externally or at a
neighboring plug contact; that's why modern distributors are larger in
diameter, as in the change from 25D to 45D Lucas dizzies.
For good results, use standard or moderate high performance coils, don't
use excessively wide (over .035 on 25D dizzy by my preference) plug
gaps, never disconnect a plug or coil lead with IGN active unless it is
held no more than 1/4" from ground, and ALWAYS carry spare rotors
previously tested as good.
FRM
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