The Stovebolt Site Forums The Shop Area The Electrical Bay 1946 Chevy 1/2Ton - Ballast Resistor

How does the ballast resistor work? With the key off - no voltage to the ballast resistor. With key on - 12.17v on both sides. I thought the resistor would cut down the voltage to 8v or 9v before going to the coil?

During operation (ignition on) the voltage is reduced but normally bypassed during starting.

Thank you! I'll pick up another. Is there a way to test with a multimeter? Like OHMs side to side?

John, I amended my answer to be more clear but I think you understand correctly what's going on. When testing the voltage, the points should be closed.

Thanks Otto. I'm learning that the resistor should work when current is flowing. If I hold my multimeter on the coil+ and then ground the coil-, it shows around 5+ volts on the multimeter. This is a new resistor (20+ years old) never used, so maybe it is OK. Ohms show about 1.8 - not sure if that is good or not.

Maybe a dumb question: Does the ballast resister have a "input" and "output" side? Or does it matter? I don't remember seeing any markings when I installed it.

Nope, doesn't matter. Resistors work in either direction. It's diodes or LEDs that you need to worry about polarity.

5.24v after the ballast resistor (should be the same as the coil + terminal) is too low.

With the distributor cap off, turn the engine over until the points are closed. With the key still on .....
... measure coil + to ground. Should be around 8.5 to 9.5 volts.
... measure coil - to ground. Should be zero volts.

Then turn the engine over until the points are open. With the key still on ....
... measure coil + to ground. Should be battery voltage.
... measure coil - to ground. Should be battery voltage.

Results:

With the distributor cap off, turn the engine over until the points are closed. With the key still on .....
... measure coil + to ground. Should be around 8.5 to 9.5 volts.
Mine was 5.19 volts
... measure coil - to ground. Should be zero volts.
Mine was 0.11 volts

Then turn the engine over until the points are open. With the key still on ....
... measure coil + to ground. Should be battery voltage.
Mine was 12.38 volts
... measure coil - to ground. Should be battery voltage.
Mine was 12.38 volts

Maybe I need to pick up a new ballast resistor at NAPA tomorrow.

I'm going to pick up a new ballast resistor tomorrow. My current resistor is reading 5.19 volts and I believe that is too low. Do all ballast resistors lower 12 volts to a certain voltage? I think I'm looking for 8 to 9 volts?

Could it be ignition switch, wire to coil, or coil? Just thoughts. Or like you mentioned, the resistor?

can you take a pic of how you have the starter wired?

... and the 0.11 volts with the points closed indicates one or more ....
a. poor wiring or a bad connection from the coil - to the moveable arm of points
b. dirty/corroded points (most likely)
c. the fixed part of the points not getting a good ground

Probably won't make much difference until high RPM, but if you fix it now you won't have to worry about it later.

Before buying a new ballast resistor do one more test.

With the distributor cap off, turn the engine over until the points are closed. With the key still on measure the "other" side of the ballast resistor to ground. Should be very near the reading with the points open. This is just to test for very little resistance between the battery and the ballast resistor.

John in Utah:

Your same problem is being addressed in your DITY thread with more detail.

Some Bolters are trying to help on this thread and some on the other thread with mixed results.

Maybe J Lucas (moderator for DITY) and/or Jon G (moderator for Electrical Bay) can help get this straightened out.

[edit by klhansen] moved those more detailed responses to this thread.

The resistance of the coil primary winding and the condition of the points will also factor into the amount of voltage the resistor drops. Dirty contacts in the ignition switch could also be a factor. It's only one link in a pretty complex chain of events between the battery and the ground the points provide. With the proper coil installed and a good set of points, the voltage at the downstream side of the ballast resistor should be around 9 to 9.5 when the engine is running.
Jerry

Bill, my thoughts are that the detail responses should be moved here. John tries to separate out specific problems that get mentioned in his DITY thread with posts in the specific shop forums. I think that's a good approach to keep the DITY thread cleaner. I may have been the culprit asking him if the truck dies when he lets off the starter.

[on edit] I moved the responses on this particular issue her to the Electrical Bay, so John's DITY thread is a bit cleaner.

Sorry about that Bill. I was feeling guilty that I was asking questions in the wrong place. I thought maybe I should move to the "Electrical" section until I got things figured out. Again, my apologies.

Thanks Jerry. I did a lot of searching on this forum last night and read many threads listing things to check/verify. I'm trying to learn and apply that to my situation.

Thank you Kevin!

John, no worries. I like your approach. Your ballast resistor issue definitely belongs in the Electrical Bay. Helps others to find it.

I've moved those replies here to consolidate them with a little cleanup. I'll post in your DITY thread directing folks here on the ballast resistor issue.

Got my "To Do" list and headed out to the garage to do my measuring and testing. Will update when completed.

I can get 5.35 volts through the ballast resistor to the Coil+, to the Coil-, down to the distributor (outside), to the distributor (inside), then to the points. Battery has 12.24 volts, at the starter 12.24 volts, ignition connection to the ballast resistor has 12.22 volts and through the ballast resistor 5.35 volts. If I get a new ballast resistor for a 1960 Chevy 235, will it be 8v-9v? I thought about by-passing the ballast to see if the engine starts and then shutting it down. Not sure how long you can run without a resistor.

Putting the full 12V to the coil won't make it (the coil) fry immediately, but will shorten it's life a bit.
You said your ballast resistor measured at 1.8 ohms, but have you measured the resistance across the coil terminals? The coil has a weird thing called impedance (which changes with time that it's energized) and that makes it a little hard to narrow down. But I found this "External Resisted Ignition Coils have an internal resistance of 1.5 Ohms and when combined with the Ballast Resistor, you get a total resistance of 3.0 Ohms which is necessary for 12-Volt ignitions systems" on this site. That implies that the ballast resistor should be 1.5 Ohms also. But I haven't seen specs for the offered ballast resistors except this one from Speedway. Lower resistance of the ballast resistor would result in higher voltage into the coil.

Coil+ across Coil- = 1.4 ohms
Coil+ to Coil center = 10.09k ohms
Ballast Resistor = 2.0 ohms

Do coils and resistors go bad from use and/or time? Although new, my coil and resistor are 23+ years old.

I think your problem is the ballast resistor has too much resistance. With the coil at 1.4 Ohms, a 1.4 Ohm Ballast Resistor would give you 6V (or half the battery voltage) out to the coil. So you need a ballast resistor with less resistance, which would boost the voltage to the coil. I'm having a math headache, so can't figure it out right now , but with your 2 Ohm resistor you'll get less than 6V to the coil.

I just picked up another resistor to test and it measures 2.4 Ohms and the voltage to the coil is less than 5 volts. ☹ I'm going in the wrong direction. Maybe I need to pick that one up from Speedway.

I think something else might be wrong. Cut the ballast resister out of the circuit and see if the car starts and runs properly.

The coil isn't marked "12 v" is it?

I did not see any markings on the coil, but I still have the box it came in. The coil and resistor came together with my 6v to 12v conversion kit 23 years old.

I will make a jumper wire and try this tomorrow.

No need to use a jumper- - - -just remove the wire from the upstream side of the resistor and stack it on top of the bottom wire temporarily. That will put full battery voltage to the coil. Take the distributor cap off and tap the starter until the points open. You should see battery voltage at both coil terminals, the side terminal of the distributor, and the movable point. If that happens, change or clean the points. It's also possible that you've got a shorted condenser. With the points closed, you should get almost zero volts at the side terminal of the distributor. If you get more than a fraction of a volt at the side terminal of the distributor with the points closed, change the points.
Jerry

OK - Here's my update: I by-passed the ballast resister and could not start the engine. Feels like it wanted to start a couple times, but no Bueno. I have 12.41 volts on the battery, on starter and through the fuse before the ignition switch. After the ignition switch I have 11.77 volts to the coil+. So, 11.77 volts to the coil+ and it did not start. I bought a couple condensers from "Jon G", so I'll replace the one that's currently in the distributor just to eliminate that possibility. If no start after that, I'm going back to the beginning. Set engine at top dead center, line up the flywheel BB with the pointer and verify distributor rotor is pointing at #1 spark plug.

Note: Not sure why voltage drops from the ignition switch, but I do have another switch I can test and I'm going to do that to see if I get the same voltage drop.

Not defeated yet! Still fighting!!

are you using starter fluid? I would not try to wake an unwoken engine without it.

Charge up the battery. 12.4 volts on a 12V battery is mostly discharged. Your starter is probably dropping the voltage available to the coil while cranking to the point where it can't fire the plugs.

Engine was rebuilt about 4 years ago and I got it started last year without starter fluid. Fuel is making it to the carburetor float bowl and it is wanting to start. But maybe I need to buy a can.


Just put the trickle charger on the battery. 🙂

Glory days! The engine lives!! I swapped out the condenser and after it turned over a couple times it started - this was with me bypassing the ballast resistor. Then, I rewired using the resistor and it continued to start. I need to adjust the idle, but I'm a happy camper for now.

I'm thinking back when I had the alternator wired one way and it was blowing my main fuse. Maybe it blew up my condenser? I rewired the alternator and the fuse was no longer blowing, but maybe the damage was already done. Here's a video of the engine starting. LINK

Thank you to everyone that chimed in with help, suggestions and encouragement. The Stovebolt Team does it AGAIN?! 😎

One thing I forgot to mention was after I swapped the condenser I tried to start the engine. Then I remembered the key needs to be ON. After 2-3 tries I heard something fall on the ground, so walked around the front of the truck and the rotor that was on the fender was now on the ground. Dang thing ain't starting unless that's under the dizzy cap! Finally, I checked everything I could think of and with just a few more cranks, the engine came to life. What a journey these past couple weeks.

I would never forget to install the rotor!!