Resistance of Ballast Resistor on old Points ignition

Presumably you know ohms law, just calculate the value to give the 8 volts after starting.

A few ohms is about right. It would never work with 1k.

NT

I also assume you know that the ballast resistor is used to apply a higher voltage to the coil by bridging it on start then opening the bridge when running.

That is correct. The resistor is removed from the circuit while cranking so the engine gets a hotter spark. The reason? Because while cranking, the starter takes a huge amount of current from the battery, lowering the available voltage to the coil. Removing the resistor from the circuit gives more voltage (and current) to the coil, helping the engine start. After the engine starts running, the starter is no longer activated, removing the huge current drain on the battery, raising its available voltage. Then the resistor is inserted into the circuit in order to reduce the current through the points, thereby reducing arcing, which can eat up the contacts rather quickly with high current. That's also the purpose of the condenser... to help reduce arcing across the points.

Cheers!! Dave M

Back in the 70s, I wired a momentary switch across the ballast resistor of my car and mounted it on the throttle shaft of my carb. At full throttle the switch bypassed the resistor.

Between that and turning the air cleaner cover over, the car broke the sound barrier...

Seriously, it did seem to help at high rpm though.

This is NOT complicated.

a) The value is about 1.4 ohms for a 12-volt (nominal) system. b) 1.6 ohms is usually suggested for a 6-volt system (some older VWs and tractors). But that same 1.4 ohm device will work fine. c) They carry A LOT of current, and so are usually ceramic devices with substantial connectors. d) They may be purchased for as little as ~$5 + tax for a generic device that may fit to as much as $20 + tax for a name-brand device that will actually fit. e) They are still stock items in most auto-supply stores and dealerships.

Peter Wieck Melrose Park, PA

Hmm, the thought of Mr Tubeguy doing rooster tails on the tractor would be a sight you dont see every day.

Sounds like a question for yesterdays tractor forum

George H.

One thing people didn't seem to notice about points is that you can't just set the gap. The proper gap changes with wear. What you want is for the poi nts to be closed one third of the time.

The number of cylinders matters not. Look at an old time dwell meter, for a n 8 cylinder it says so many degrees, but that is at a certain percent of t he scale on the meter. For a 6 cylinder the dwell in degrees also hits the same spot on the meter. For a 4 cylinder, with double the degrees of dwell specified STILL needs to be at two thirds, also the same place on the meter scale.

There are a few ways to measure it accurately. The best is with a scope. Sy nc it to any frequency that lines up with graticule lines and make the bott om part twice as long as the top part.

Another way is to use a resistor and Zener and first take a DVM reading wit h the points open, just roll the engine to a spot where they are. Read that voltage, should be that of the Zener. Now start the engine and you should read ONE third of that voltage.

You will have to trial and error it to get them really right. Actually GMs had an Allen screw and a window to allow you to set the points with the eng ine running. The rest you have to try it over a few times. If the "on time" is too short make the gap smaller, if longer make it bigger.

In MOST cases, over the years the lobes wear down on the distributor shaft. What this means is that a smaller gap will be necessary to achieve the cor rect dwell (closed) time. (duty cycle actually if you want it that way) Whe n this gets too bad, the points are not opening as far and the arcs will we ar them out faster. In the old days I noticed people who had to change thei r points like every couple of months. The coil wasn't shorted, the ballast was fine, there weren't any fouled plugs. It was that the lobes were work d own and every time those points opened and made the spark the arc was worse because the contacts wee not pulling back fast enough.

If you have that problem, say if you find the gap is only 0.008" or somethi ng and can't get a new distributor shaft then it can be machined. It takes a pretty good machine to do it, but it is not worth programming a CNC for a one off. You can get a guy to do it with NC, OR find an old timer who real ly knows what he is doing. I am not good enough with that to do it even whe n I had the machines that might. I am simply not a machinist. you cold mayb e find an auto parts place that does their own machine work, they might be able to do it. Some of them though, their machine shop consists of a big be lt sander. Remember how they used to mill cylinder heads ?

Not no more. The aluminum heads these days they just blue it up and lay it on a horizontal belt sander and keep on sanding it until all the bluing is gone. A place like that won't be able to do it. You need a GOOD auto machin e shop for this. Racing people. Even though they don't use points anymore t hey should have the skill to do it. you walk in and say "I want to talk to the oldest guy in the place" and then when you get to him "Id you ever redo a distributor shaft, I just can't get one and this thing is eating points, everything else has been checked, the gap is only 0.0XX at 66% dwell.

Something like that. And don't forget, the manufacturer might surprise you and have the shaft in stock for ten bucks.

s after starting. "

That will vary with RPM. It does not only because of more sparks per second , but with cam overlap the effective compression ratio changes. More atmosp here in there means a higher dielectric strength so it needs more firing vo ltage. That will change the current load on the primary and thus if not a s olid voltage of course it will vary. the values they chose for the ballast kinda split the difference. That's why when you put a hot cam in a car you also beef up the ignition system to supply a higher voltage. You usually do the cam for more high end and it will still need enough firing voltage up there.

Yes, I am car people and from the old days. At my Uncle's funeral the pictu re of his car was bigger than the picture of him.

And that is the way he would have wanted it.

We never looked at any book to setup an engine. There was a 1957 Chevy that would pull the front wheels up in second gear, up to 110 MPH, and there wa s my cousin's 442 that was made into a 642 with three deuces and Edelbrock which ripped the front bolts out of the seat. Can you imagine that ? Your c ar is rolling down the road and you are laying down and kinda stuck... I wo uld have liked to have a video of that but not actually be there... He live d though, it took a motorcycle to kill him. Well it was his fault I have to admit. And then there was my 1970 Toronado that ripped the asphalt up off the street, and this other car I forgot what it was exactly but I remember I bought it off of a guy named Dwayne but the title was made out to Dawne. I never got the title in my name, I had spare plates all over the place. Wh y bother ? When I bought it I told him "OK, just sign the title Dawne". LO L My other Uncle was a notary public and he flipped cars, alot of them. If you have more that like 12 in a year you are supposed to have a dealer's li cense in this state, but they didn't go in his name. People sold him a car and just signed the title, when someone bought it he just put their name on it and notarized it. Everything he touched turned to gold. And that is onl y a small tip of the iceberg.

So when it comes to old cars, well...

OOPS ! You want the points closed TWO thirds of the time.

Sorry, I forgot to mention why I mentioned that. It was the car in which I got a ticket in Lakewood, Ohio for flying. Apparently they frown on that.

This one is NOT wired that way. The resistor is in line with the coil all the time. The tractor is a Farmall Super M. I just replaced all the low voltage wires because the old ones were cracking (dry insulation). Anyhow, it goes from battery to a simple two terminal ignition switch (push pull type). On to the resistor and to the coil/points. Thats it...

I just replaced everything in the ignition system except the switch. New low voltage wiring, the resistor, coil, points/condensor, plugs and plug wires. The stock coil that I bought says "For 6 or 12 volt systems". "Resistor must be used on 12 volt systems". These tractors were originally 6 volt. Most are converted to 12 (including mine) these days.

I assume these stock coils are 6v but work on 12v with the ballast resistor. Without it, I would think the coil would be damaged. But I like the idea of full voltage at starting. So I may try it. I'd just need to run another wire from the push type starter switch to the coil. I may give that a try. (the starter switch is a separate switch from the ign switch. It's a push buttom spring loaded contacter).

I also have to add a light to the "dash" to remind me the ignition switch is on. I know that I am supposed to turn off the ignition when the tractor is off, but if I run out of gas, by the time I get more gas, the battery is drained because I never remember to shut off that darn switch. And I imagine that is hard on the coil and points. Possibly why the old coil was weak and the old resistor was not allowing enough voltage to the coil.

After changing all these ign parts, it really runs better, but now I have to rebuild the carburetor. That thing tends to get flakey every so often. But the tractor is over 60 years old, so it was due for all new ignition parts, and a carb rebuild. Plus new fan belts are in order next. They look bad....