Re: H john deere question

Posted by Bob on December 11, 2015 at 17:45:56 from (64.255.159.192):

In Reply to: H john deere question posted by Leonard Dacus on December 11, 2015 at 17:20:48:

It will have NO effect on the coil, it simply reverses the polarity of the spark at the plugs, which is desired to be a negative-going pulse.

If the spark is marginal to begin with, this may cause some misfire, but with a good spark, either polarity works.

Modern cars that fire 2 plugs with one coil fire one with a (+) pulse, and the other with a (-) pulse successfully because the spark is "hot" enough to both.

Also, it's been a long time since I've been inside one of those distributors, and I can't remember, but if both primary leads... the one that is grounded, and the one that goes to the points/condenser are accessible swapping their positions would make the output spark the same polarity as it was originally.

The following is QUOTED from an old IH truck service manual...

"What difference does it make whether positive or negative voltage is supplied to the spark plug terminals?. It directly affects the amount of voltage required to fire the spark plugs. When polarity at the spark plug terminals is positive, it's harder for the voltage to jump across the air gap than when polarity at the plug terminal is negative. Just why this is so is related to a pair of electrical theories--the electron theory and the theory of thermionic emission. According to the electron theory, all current flows from negative to positive. The theory of thermionic emission states essentially it's easier for electrons to leave a hot surface than a cold surface. Combining the two theories, one finds that electrons will always leave a negative charged surface for a positive charged surface, and they will leave the negatively charged surface with more ease when the surface is heated. Spark plug design is such that the center electrode almost always operates at a higher temperature than the ground electrode. Since it's easier for electrons to leave a hot surface, it is preferred to have the electrons "jump" from the hotter center electrode to the cooler ground electrode. When the center electrode is negatively charged (negative voltage at the spark plug terminals), this is what happens. Stated another way, putting the negative charge on the hotter center electrode causes the gap to be ionized at lower voltage. (Ionization is necessary to permit passage of the spark through the high resistance of the gases in the cylinder.) When positive voltage is supplied to the plug terminals, which happens when coil polarity is accidentally reversed, the hotter center electrode becomes positive charged. Consequently, electrons must leave the negative charged ground electrode and move to the positive charged center electrode. But, since the ground electrode is cooler than the center electrode (and remember, it's easier for the electrons to leave a hotter surface), it takes more voltage to make the current jump the gap-in fact, up to 45 percent more."

Clear as MUD, eh?

Replies:

• Re: H john deere question Kent Petersen 05:49:19 12/12/15 (0)

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