Points to HEI conversion. Transistor and resistor purpose?

In the following diagram, what is the purpose of the resistor and transistor?


In other words, why would the following diagram not work?


Here is the factory 4 pin HEI module wiring

I suspect it is to convert the input from pull down to pull up to tricgger the coil.
Points are the weakest link in a points ignition system there is little gain in converting to a transistor system without getting rid of the breaker points.
With a home brewed mixture of systems there is no way of matching the dwell time to coil's specification.

I'm a little confused. You still want to use the mechanical points?

Okay, so in your drawings what you are doing is using the existing points to create the signal that a pick-up coil would create and you are using that to trigger the module. This is a really bad design if you ask me. Why not just get a distributor with a pick-up coil in it? I'm assuming you are having a hard time finding one?

I did this conversion on an old harley davidson golf cart. The igntion module died and it was hard to find. This was a popular conversion online. You retrofit the points system from an earlier model golf cart and use it to trigger the hei module.

The points take the place of the pickup coil.

Lately its been giving me problems. The cart stopped reving up quickly and didn't have enough power to pull a small hill. The problem was fixed by replacing the transistor. Until the next day. Now the cart doesn't run again.

I have a scope that I'm learning how to use.

I have a couple questions.

1.)
When I hook the scope up as shown below, I get a constant 12v engine cranking.


When I disconnect the transistor ground, I can now see the voltage drop when the points open, as long as the engine is cranking.


If I turn the motor by hand and stop with the points open, the voltage does not drop. I have 12v with the points open. Unless the motor is cranking. Why?

2.)
I'm trying to understand why the resistor and transistor are necessary. The G terminal will be seeing the emitter's ground while the points are closed, correct? What should the G terminal see when the points are open? Is this the purpose of the resistor?



I love learning this stuff. I've been a golf cart mechanic for 10 years or so. Working with my dad. I always have a car or truck I'm working on. Things get slow in the winter time. I wish I would have known about your class when I went to college. I am grateful for your videos. Great job teaching.

12v measuring the way you are means you have a constant ground on that circuit. Points open, it should be 0 (equal potential) and when the points close it should read 12 (because your now grounding the circuit)

Your transistor is shorted and is grounding the circuit all the time.
And since it is the second bad transistor, you need to focus on that resistor being bad (shorted) as the cause of repeat failure.

The resistor is used to prevent the transistor from allowing to much current to pass through the base to the collector. You only need a small amount of current to turn on the transistor.

The transistor also is what turn on the coil. Its acting like a switch.