MTFCA Forum

I have a 1926 coupe that was converted to a 12 volt distributor system. My problem is after starting the car and driving it for a while it starts running rough and then eventually dies. Usually when this is happening it ends up being the coil that has gone out. So it seems to be heat related with the coil.
I am just wondering why this is happening. I have been told that if you have converted to a 12 volt system that you should use a resistor to keep the coil from burning out. Does anyone have any experience with something like this.

Best to always stand the coil up right on the firewall, I always use a ballast resistor too and this normally cures all problems. An old Dodge style works great.

Hope this Helps,

Hank

50’s-60’s cars with 12 volts always had a resistor in the ignition circuit.
What coil are you using?

MSD 8214 ballast resistor. Summit racing equipment stocks them. NAPA also lists one for less money - but I'll bet it's Chinese. AC Delco still has a part number for the classic ceramic resistor that they used from 1953 through 1972. Addressing the OP, the answer is yes.

Autozone AL795

Napa ECH ICR12

Check your coil, some coils have an internal resistor.

On a 12volt system, the coil and the ballast resistor should be a sum of 3 ohms total.. This limits the current on the points and coil to 4 amperes.
Less than 3 ohms, the points burn and the coil overheats.More than 3 ohms, the spark is weak.
MSD coils are Chinese trash BTW.

12 volt coils don't run on 12 volts. I think they actually only run on 8 volts or so, and if you check, the car manufacturer uses a resister wire to power the coil, which drops the voltage from 12 to whatever actual value the coil is designed to run at. Same thing with 6 volt coils. I think they actually run at 4 volts or so.
You have to put a resistor in, or use "resistor wire", to give the coil the proper voltage, or their life will be extremely short.

SOME new coils contain a resistor, the one Lang's sells does. Where is the 12 volt voltage going into the coil coming from - battery, generator, alternator?

OilyBill wrote: ↑

Wed Sep 30, 2020 1:02 am

12 volt coils don't run on 12 volts. I think they actually only run on 8 volts or so, and if you check, the car manufacturer uses a resister wire to power the coil, which drops the voltage from 12 to whatever actual value the coil is designed to run at. Same thing with 6 volt coils. I think they actually run at 4 volts or so.
You have to put a resistor in, or use "resistor wire", to give the coil the proper voltage, or their life will be extremely short.

12 volt coils operate on 12 volts. When the coil is charged, which takes only milliseconds, the current drops as well as the voltage. This happens so fast, your meter cannot read that fast enough, ergo, your meter is reading the stabilized voltage after the fact.
https://www.corvetteforum.com/forums/c3 ... ke-me.html
The author is a retired Delco ignition designer, and a friend.

Jeffery
That 3 ohm concept is interesting. I will keep it in mind.
Thank you

Just sharing.
If you read some of that thread, Mike goes into the inductance in Henrys, and the deep math, and I get it, but I get a headache.
He was knee deep into combustion engineering at GM,
and is a wealth of knowledge.

John Codman wrote: ↑

Mon Sep 28, 2020 11:21 am

MSD 8214 ballast resistor. Summit racing equipment stocks them. NAPA also lists one for less money - but I'll bet it's Chinese. AC Delco still has a part number for the classic ceramic resistor that they used from 1953 through 1972. Addressing the OP, the answer is yes.

Alright we will check the MSD 8214 resistor. We might need one for the incoming '26 Coupe project. We're gonna take a closer look at her after we finished installing the suspension from 4Wheelonline on the Silverado and cleared the garage.

Wait a second... I have owned a '13 Touring for 6 years running a 12 volt system, driven it hundreds of miles, and never had a problem with burning out the coils. I also follow this Forum even closer than I monitor the "People of Wal-Mart" site, and this is the first time that I have ever read about resistors being needed to protect the coils. I have read about Marvel Mystery Oil, different weights of oil, different types of brakes, and the sins of water pumps... but why has no one mentioned this before. Is this an on-line snipe hunt that I am being invited to?
Regards,
Mike

MikeSommers wrote: ↑

Thu Oct 01, 2020 8:42 am

Wait a second... I have owned a '13 Touring for 6 years running a 12 volt system, driven it hundreds of miles, and never had a problem with burning out the coils. I also follow this Forum even closer than I monitor the "People of Wal-Mart" site, and this is the first time that I have ever read about resistors being needed to protect the coils. I have read about Marvel Mystery Oil, different weights of oil, different types of brakes, and the sins of water pumps... but why has no one mentioned this before. Is this an on-line snipe hunt that I am being invited to?
Regards,
Mike

1) There is a good chance your coil has an internal resistor. Mine does as well.

2) Coincidence is amazing. The author of that article on CF, is named Mike, and retired to.....wait for it.....Liberty,Mo.
SO, if you see a Blue 69 Corvette driving around, flag him down and say BigBird sent you, and he will faint, and my phone will ring.

MikeSommers wrote: ↑

Thu Oct 01, 2020 8:42 am

Wait a second... I have owned a '13 Touring for 6 years running a 12 volt system, driven it hundreds of miles, and never had a problem with burning out the coils. I also follow this Forum even closer than I monitor the "People of Wal-Mart" site, and this is the first time that I have ever read about resistors being needed to protect the coils. I have read about Marvel Mystery Oil, different weights of oil, different types of brakes, and the sins of water pumps... but why has no one mentioned this before. Is this an on-line snipe hunt that I am being invited to?
Regards,
Mike

Did you run it with a distributor like the author of this posting.

is a resistor needed in a 12 volt system
by jims » Mon Sep 28, 2020 9:45 am
I have a 1926 coupe that was converted to a 12 volt distributor system. My problem is after starting the car and driving it for a while it starts running rough and then eventually dies. Usually when this is happening it ends up being the coil that has gone out. So it seems to be heat related with the coil.
I am just wondering why this is happening. I have been told that if you have converted to a 12 volt system that you should use a resistor to keep the coil from burning out. Does anyone have any experience with something like this.

Some coils have a resistors others do not. Should be printed on the side if it has or needs one, or look up the parts number. How hot is the coil getting when it dies? Too hot to touch or just warm? If the car dies after a time, it could also be the condenser is bad. Once it gets hot it stops working. What does the contact surfaces of your points look like?

Is he using a 12 volt coil?

big2bird wrote: ↑

Fri Oct 02, 2020 11:44 am

Is he using a 12 volt coil?

It has been assumed that he is using a 12 volt coil. No idea what it is ( with/without a resistor or even a 6volt one). If several were purchased it would help to have a part number. I'm sure someone could confirm what it is.
He has not responded to any questions or recommendations since the discussion was started.

While he was last on line same day as his post, the 28th, he could be watching as one of those guest we see listed at the bottom. If someone ask a question it is fantastic getting replies, but the person asking the question, would be nice (in my OP) to update and let us know what the outcome is or to answer any questions that might need answering when trying to help.

I have a 12 volt coil and 12 volt system. I have a ballast resister wired into one position of my ignition switch and wired out in the other position. The idea was to start the car without the resistor in the circuit and then switch over for running. I bought a high performance coil, the best coil I could find. I have forgotten which position of the ignition switch has the ballast resistor in the circuit. It does not seem to make any difference.

If the coil is cutting out when it gets warm it is a bad coil or not a very good quality coil. Buy a new coil, a good quality coil, maybe a racing coil. And beware of poor quality knockoffs of name brand coils. Also, beware that a coil can be ruined by turning the engine over without the spark plugs hooked up. The spark has to go somewhere and it could burn out the coil if there is nowhere else for it to go.

big2bird wrote: ↑

Wed Sep 30, 2020 6:31 am

OilyBill wrote: ↑

Wed Sep 30, 2020 1:02 am

12 volt coils don't run on 12 volts. I think they actually only run on 8 volts or so, and if you check, the car manufacturer uses a resister wire to power the coil, which drops the voltage from 12 to whatever actual value the coil is designed to run at. Same thing with 6 volt coils. I think they actually run at 4 volts or so.
You have to put a resistor in, or use "resistor wire", to give the coil the proper voltage, or their life will be extremely short.

12 volt coils operate on 12 volts. When the coil is charged, which takes only milliseconds, the current drops as well as the voltage. This happens so fast, your meter cannot read that fast enough, ergo, your meter is reading the stabilized voltage after the fact.
https://www.corvetteforum.com/forums/c3 ... ke-me.html
The author is a retired Delco ignition designer, and a friend.

This is only true on the HEI ignition systems after 1975. Prior to then, the coil operated at 12 volts for starting, but this was switched through a resister wire or a physical resistor after starting, to operate on from 6-8 volts. Ford used a resistor wire. Chrysler used a physical resistor unit. Early 12 volt ignition system coils SHOULD NOT BE OPERATED on 12 volts. It will damage them.

The purpose of the resistor is to prevent points from burning too quickly due to excess current draw. The coil should not burn up due to voltage, if it is well made. Coils have been mounted directly on the engine for years, and withstand heat and vibration. It was very rare to have a coil burn up once, let alone repeatedly. Coils without an internal resistor always have an external one inline somewhere. GM used a resistance wire built in the wiring harness, others use ceramic units. Most manufacturers had a bypass wire from the starter solenoid that gave the coil full voltage while cranking, due to inherent voltage drop on the electrical system due to starter amperage draw. When the starter was off, coil voltage dropped to 6-9 volts on a 12 volt system. 12 volt system is a misnomer, since with charging, most systems run 14-14.6 volts with an alternator. You should be burning points, not coils! A quality epoxy filled coil should hold up fine. Use a volt meter, and check the positive wire from the ignition switch, and see what the voltage to the coil is while running. It should be 6-9 volts, IF your coil doesn't have a resistor built in. If it does have it built in, full system voltage is ok.
Another thing to check is the condenser, as it can cause issues as well, as can a bad ground at the distributer.

Billdizer,Spencer In wrote: ↑

Sat Oct 03, 2020 4:08 am

The purpose of the resistor is to prevent points from burning too quickly due to excess current draw. The coil should not burn up due to voltage, if it is well made. Coils have been mounted directly on the engine for years, and withstand heat and vibration. It was very rare to have a coil burn up once, let alone repeatedly. Coils without an internal resistor always have an external one inline somewhere. GM used a resistance wire built in the wiring harness, others use ceramic units. Most manufacturers had a bypass wire from the starter solenoid that gave the coil full voltage while cranking, due to inherent voltage drop on the electrical system due to starter amperage draw.

This is absolutely correct.

A ballast resistor is Not a voltage regulator, it is a Current limiter. A volt meter cannot pick up what is happening.
You need a scope to understand what is happening.[image][/image]
The coil sees 12v, and as the magnetic field charges, the voltage drops.
When the points open, the field tries to collapse back to the points at approx 250 volts. The capacitor prevents this, sending the energy out the secondary winding.
The induced secondary voltage is regulated by the gap and combustion chamber pressure.
I only wish to explain the dynamics. Jeff

I will try this analogy, then shut up.
When you pump up your tires, you apply 100 lbs, and when you stop, the gauge shows 60 lbs.
Did you apply 60 lbs? No, you used 100lbs, but the gauge shows 60 lbs when full.
Your volt meter is showing the same thing.

I have noticed that when people are trying to debug something electrical on their T they haul out their DIGITAL volt meter and try to take readings. Now they may or may not get anything meaningful. I try very hard to convince them to send me what they are reading using an analog meter but they then make a reading and give me 3 numerical digits which is just about impossible to get using an analog meter so they are just fibbing thinking it will get me to go somewhere else to find the problem. The problem isn't going to move or be found until you run a test that reveals what is going on. I had one fellow reading the 12V supply at the coil with engine running. I asked him to rev the motor and see if that voltage went up since he was also using an alternator and he said his analog meter read a steady 12V (at ALL RPMs) while his digital meter read -.1 0 volts. at all RPMs (YES a minus voltage of .01 VDC). No way to make any sense out of either reading since he clearly was not charging anything with anything and had no ability to make any reading that I could trust to be accurate. There are really good small analog meters for sale at several of the BIG BOX type stores around me and they are less than $15 but most people are convinced that their meter is fine for what they are doing. There is nothing wrong with digital meters other than they CANNOT be trusted once the engine on their Model T fires up since the coil itself and the unshielded plug wires and other components and poor grounding make it a noisy nest to make any sense out of.

I still have my Simpson 260. Still my go to meter, like seeing deflection on a cap, or sweeping motions.

I would like to thank all who responded to my problem. I had been using a coil without a resistor and I only had access to a digital voltmeter which was reading a little over 12 volts, checking both sides of the coil. After driving for about 20 minutes the car began to run very rough, just barely getting back home . I then took another reading which read 12 volts on the positive and very low on the negative. After reading all the responses to my post I decided to call one of the members of our club who happened to have a spare coil with a resistor. After installing it, I took a test drive for about an hour with no problems and have since gone on a 2 day tour with no more problems. I am looking into analog meters at this time and plan to get one soon.

Fantastic and thank you for letting us know the outcome.