The SH that would not Fire

2 weeks ago before I went on vacation I posted about SH that would fire. It had been on 29 mile tractor ride and in the next few days began to start very hard. then would not fire. In the last 40 hours of use I had converted to 12v. I had got a new point set and 12v coil. Before I started changing parts I checked the resistor that is before the distributor. It had 12v on both sides even going to the dist. I removed resistor and installed new 12V coil and She started up on the first bump. Question?? Is the porcelain resistor directional? I saw no markings on it. or is this the nature of a resistor. Looking back I know I should have checked output of resistor on installation.

(reply to post at 05:09:13 11/03/13)

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I do not believe they are directional, current just passes through and it is reduced.

> I had got a new point set and 12v coil.

If you put in a 12 volt coil (one that requires no resistor) you should have removed the resistor.

> Is the porcelain resistor directional?

No. However, when the points are open (as they usually are) no current flows and therefor there is no voltage drop across the resistor and therefor you will read 12 on both sides of it.

Might be you could check it. Just put a multi-meter on the resistor with polarity one way, then reverse it, try it the other way.

The coil should have 12V on both sides with points open, then about 6V on one side and 0V on the other with the points closed.

Resistors are not directional. The resistor is a piece of Nickle/Chromium wire and has no other features of electronic value.
Here is what is likely to have been the cause:
A coil that fails with an open primary circuit (in this case your first new 12 volt coil for use with an external resistor) can sputter and misfire as the broken wire makes and breaks contact. When it fails permanently it stays open (like a turned off switch)
New electronics and electrical devices fail most often in the first hours of operation. When it failed it was not conducting electricity. When a wire wound resistor is not under load (not connected in a live circuit) it will pass what appears to be full voltage to the other side of it. It will appear to have no resistance, and that is what you measured (12 on each side).

A bad coil can go bad in several other ways. One of them is shorted (internally layer of turns contacting another layer of turns, not to ground), and that causes excessive current draw and over heats the resistor as well as the coil, and burns points from excess current. Not your issue.

They can also make high voltage failures internally failing to produce enough voltage to fire the plugs. Not your issue.

They can also short to ground in the high voltage coils failing to get the spark out of the coil wire hole. Not your issue.


They can also carbon track on the outside and jump voltage to the primary terminals (closest path) or the case. This is usually caused by dirt buildup on the tower, or a bad set of wires that cause excessive output voltage. Not your issue.

I hope this helps. If you still have the resistor, an ohm meter should show only one or two Ohms across it.

Only one possible way that my diagnosis could be in error is if the tractor engine stopped with the points open. If they did, the same 12v on each side of the resistor findings would have happened. inline 4 cylinder engines almost always stop when turned off at on of two places.
and that is usually with the pistons part way down the cylinder (due to compression stroke bounce). Thus the points are closed.
I hope this is understandable. A look at a picture of an ignition coil guts would help.
Jim

I got carried away explaining that very fact. your confirmation is nice. Jim

I never dispute you Jim, only clarify.

Thanks for the confidence builder, Yours was posted prior to mine! Jim