I am in the process of installing a points distributor system in my model T (SBC). In my research I have read on here some of Bubba's threads and posts recommending 3 ohms resistance in the 12 volt+ feed to the coil. That's with a 1.5 ohm ballast resistor and a 1.5 ohm coil. My ballast resistor is 2.0 ohms and I have a .9 ohm coil. 2.9 total. I figure that is pretty close. I had to wire in the R circuit on the starter to give 12V in start position. While checking that after wiring, I found that there is the full 12 volts on both sides of the BR when the key is in the run position. I thought the idea of the BR was to reduce the voltage to about 8-9 volts for the points in the run position. So shouldn't I see a voltage drop across the BR? I have 2 different Ohm value BRs and I don't see a voltage drop across either. I'm not the brightest bulb in the bunch when it comes to electrical. Can somebody explain this or do I have something fouled up?
Please describe how you wired the R terminal on the starter solenoid. What is voltage at R terminal when car is running. Seems it should be zero?
I got this diagram off the good ol' HAMB. I haven't checked the R terminal while running. I will have to do that. Yes, I would think it should be zero with key in run position. 5 minutes pass........Well, I just checked the R terminal, key on, not running, (I didn't want to run it in the shop). I have 4 volts at the R term. Now I'm really lost. ????? I wouldn't think it would make any difference if the engine was running or not, just that the key is in the run position and not the start position.
Diagram is too small for these old eyes to read. I suspect that source of the 4 Volts is from the other end of the wire connected to the R terminal Now try disconnecting the wire from the R terminal - and measure the voltage at the R terminal.
Your schematic looks good. This narrative follows the same. https://www.hotrodders.com/threads/starter-wiring.152788/ My guess is a problem within the starter. Seems that correct operation is when purple wire activates solenoid, big red battery wire sends juice to R terminal. When start request from ignition switch ends the big red cable stops sending juice to R terminal. Make sense? I'll call for a little more expertise: @Crazy Steve @squirrel
Depends on the ignition switch as well. In the run position there should be 12v to the ballast... Less coming out the other side. In the start position there will not be power coming from the run side of the switch (verify with wire to ballast disconnected). The power at the coil will be coming from the starter solenoid while it is engaged only. Some ignition switches will only have start/run/acc. terminals allowing voltage to go to the coil from the ballast and solenoid at the same time which will cause you to show 12v at both sides of the ballast. There are some ignition switches that have separate ign and run terminals. Ign supplies voltage while cranking, run has power when in on position. These were used on cars without a starter solenoid or relay. Not seen often.
There is only a voltage drop across the resister when current is flowing if the key is in the run position and the points are open there is no flow and you will read 12V on both sides of the resister. Phil
Yeah, that will tell me if there is a problem with the solenoid. I'll give that a try in the AM. I just got the car running on the points and it still needs timed and tuned. Then I will check it running. I just don't want to burn the points up running 12 volts to them all the time. So the consensus is that there should be a voltage drop across a BR if there are no other circuits involved? If that is the case then I should see a voltage drop across the BR as well as zero voltage at the R terminal on the starter when the R wire is removed from the R terminal. and the key is in the run position. Correct?
Beat me to it!! And, to really confuse things, with an autoranging meter it will appear to change with engine speed. At least with mine. Any intermittent flow will get 'averaged'. Even when checking a turn signal circuit on my trailer, it would show 6-8 volts when it was flashing. 12-0-12-0 etc was read as the average flow. Same thing happens on an ignition primary.
What Phil P said. No, voltage at the R terminal will be the same as the coil side of ballast resistor, unless cranking of course. If you followed that diagram you'll be fine.
When it's running the capacitor and the induction of the coil would probably effect your readings but understanding that is above my pay grade. Phil
The voltage at the coil will bounce around a lot when it's running...if you want to see what's really going on, you need an oscilloscope. To do a quick check to see if the ballast resistor is working, make sure the points are closed, or just short the (-) coil wire to ground, turn on the ignition, measure the voltage at the coil (+) terminal. It should be below 12v.
Jim, doesn't the ballest need to be energized for a bit before it shows the voltage drop, this would let the resistor wire heat up some for the drop to happen.
You'll show a drop whether the ballast is 'cold' or 'hot' if the points are closed, but it will vary with the resistor operating temp. If checking with the motor running, a typical voltage value at the coil side of the resistor is 8-9 volts to ground as read by the meter. If the points are open, there's effectively no drop across the resistor. If you want to know the actual drop across the resistor with the points closed and the motor not running, check between the two ends of the resistor. The meter will read the actual drop.
This is a vital point that most guys who haven't studied electronics, can't understand easily. Learning Ohm's Law, and Kirhhoff's Law, will help you understand how this works.
A 2 ohm ballast and 0.9 ohm primary winding in the coil means you have about 4 volts to work with at the coil, and should show about an 8 volt drop ACROSS the ballast when the points are closed. 0.9 is pretty low resistance for a points ignition system coil.
If you use an old analog meter like a Simpson 260 It a Triplett you can see the voltage on the coil with the engine running. They read right at 9-9.2 volts. I use the stock Ballast resistor on my 56 Ford with a Pertronix unit and never had a problem with any of the individual components.
Take a good look at your diagram. The wire attached to the R terminal is also attached to the coil. Therefore you will see whatever voltage the coil see's when the ignition switch is on. The real check of the terminal is to remove the wire to the coil and check. You should only see 12 volts (actually battery voltage, it will be less when cranking) when the starter is cranking the engine. You should see 0 volts with the starter at rest.
Just to clarify, and this is an important point to grasp, the purpose of the ballast resistor is to limit current draw, not voltage. That the voltage is reduced is a symptom of the reduced current, not the purpose. The basics of electronics always have to be kept in mind to do proper troubleshooting. You can easily get twisted sideways and down a wild goose chase if you don't keep the basics in mind.
Stated earlier, the r post will always read what the resisted side of the ballast resistor reads. In other words, coil(+) and r term on the starter will always be the same. Condition 1: Key on points open = battery voltage Condition 2: Key on points closed= 6-9 volts approx Condition 3: Key in “crank” position = battery voltage regardless of points being open or closed
Correct. Another basic fact that helps with electrical system troubleshooting is, all points in a parallel circuit will always have the same voltage (though there could be voltage drop due to insufficient wire gauge). The R terminal, the + post of the coil and the downstream side of the ballast resistor are all parallel in that circuit, the voltage will, or should, be the same at all 3.
That's only partially true. While the resistor does limit current and is it's main purpose, the lower voltage also helps reduce arcing which extends point life. Higher voltage will produce a longer, increased duration arc.
I gotta admit. Electricity never interested me in the least. If it didn't have wheels and gears it didn't catch my attention. I only learned what wire to hook where in order to make things work. I have always wired my own race cars and hot rods with the help of diagrams so I have a rudimentary knowledge of automotive wiring, but that's about it. I think I have enough info now to check the ballast resistor. Thanks guys.
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