1948 Ford 12 volt coil issues
 

I have been reading the various threads on this issue and after checking my system, I am still having the same issue with my coil overheating.

I have a 1948 ford tudor sedan with the original flathead but converted over to 12 volts negative ground. When cold, the car starts up perfect and runs like a charm. After driving around for 30-45 minutes, I pulled up to a friends house and turn the car off. about 10 minutes later, I got back in to leave, car started up fine and then about 2 minutes down the round, it started to miss and died.

I could get the car to turn over but could not get it to start. I had this issue before so i had a spare coil in the car. I took the old coil off, installed the new coil and boom, car started right up.

What could be causing this issue? The coil is a 12v internally resisted coil. when the car is running at idle, the coil has 12.4 volts going to it. At higher RPM, it is showing 14.4 volts going to it. Is this causing the overheating issue? Do I need to run a ballast resistor with it even though it states its internally resisted?

Any help would be appreciated.

Re: 1948 Ford 12 volt coil issues
 

Poor quality internal-resistor coil. Jack E/NJ

Re: 1948 Ford 12 volt coil issues
 

Jack, what type internally resisted coil should I get?

Re: 1948 Ford 12 volt coil issues
 

Need to take the measurement at the distributor. You want a coil with about 3.5 ohms resistance.

Re: 1948 Ford 12 volt coil issues
 

Flapjack, the current coil has around 4 ohms

Re: 1948 Ford 12 volt coil issues
 

Sorry, no first hand experience with 3+ ohm 12v coils. I'm still using a ~50 year old 1.5+ ohm can with external resistor. Mounted away from engine heat. But I've read that German made Beru & Bosch Blue are likely more reliable and heat tolerant than SMP, Pertronix or never-of-them 3+ ohm coils. Jack E/NJ

Re: 1948 Ford 12 volt coil issues
 

ALL 32 to 48 coils whether 6 or 12 volt coils should have 3.8 to 4.2 volts
to the coil with the ignition switch on. Sounds like you talking about a round
type coil. I don't do round coils. SKIP

Re: 1948 Ford 12 volt coil issues
 

>>>Flapjack, the current coil has around 4 ohms>>>


Heat it up to about 180-200*F in your toaster oven to see what it reads then. Jack E/NJ

Re: 1948 Ford 12 volt coil issues
 

I'm just an amateur, but I have to speak up here, because I don't believe that's possible. That is correct value for a turning distributor, but not when working in the garage with engine stopped. You need to make that distinction when giving that advice.

When I first got my '40 with a no-spark problem, got a fresh Skip-rebuilt coil, Bubba distributor, and Bubba-built condenser. I then tried everything to get the 3.5+ volts to the coil that I had read on forums. Tried five different original resistors. With engine stopped, full current flow, all the resistors produced a voltage drop of 3.0 volts (plus or minus a few tenths, more as they get hot). Finally concluded that with a 6.3 volt battery, is impossible to get more than 3.3 volts to the coil. The info I had read on the forums is just wrong! And my engine started and ran fine with 2.8 volts to the coil.

Then later with engine turning, points open 20% of the time, voltage at the coil will jump up. Duration of points open there is no current and therefore no voltage drop, so will get full battery voltage 20% of the time. Meter will show the higher averaged voltage. Only with engine turning will the voltage at the coil be that 3.8 to 4.2 volts.

Re: 1948 Ford 12 volt coil issues
 

I think GM is referring to input voltage on coils with the ignition switch on, the dropping resistor in between, points closed, engine not running. Should be as he stated. About 4volts. Jack E/NJ

Re: 1948 Ford 12 volt coil issues
 

Yes Jack, I think that's what he is referring to. But based on what I found with the resistors, I don't think you can get up to 4 volts until engine is running. I'm trying to dispel what I think is a myth on the forums, something that caused me a lot of time and aggravation. But maybe I was doing something wrong? I invite people with stock resistor & coil to put a meter on their coil input with engine stopped. Like to hear what others see. (don't forget to turn off the switch when done!)

Re: 1948 Ford 12 volt coil issues
 

I will try a different brand coil and also reroute away from the engine heat just in case.

It’s odd because it only happens on hot summer days here in Texas. That being said, all my other classic cars have the coil mounted on top or near the engine snd don’t have this issue.

Hopefully I just had a bad quality coil. I will also change spark plugs and check plug wires.

Re: 1948 Ford 12 volt coil issues
 

I have experienced the same with my 39. I have also changed resistors with no difference.(lovely spot to work in) my engine runs fine and will start faster than anything I have owned.

Re: 1948 Ford 12 volt coil issues
 

While there may be myths when referring to human interaction, there are no myths in basic electricity.

When folks speak of internally resisted coils you just need to know that a resistor is a coil too so all coils have resistance built into them. The wire size and number of turns is what controls that resistance.

The coils used on the old 6-volt cars were manufactured in such a way that they needed a ballast resistor in the line between the switch and the coil's power terminal. The can types could take more heat such as used in the model A and those used later in the 8BA era. The can types did not use a ballast on 6-volts.

When changing over to 12-volts, a person can change over to a modern 3 Ohm ignition coil, or they can use an earlier type 1.5 Ohm coil with a 1.5 Ohm ballast resistor mounted in the line between the switch and the coil. The original ballast resistor used from approximately 1932 till 1948 is too low a resistance to work with a 12-volt system so it should be removed or at least bypassed.

The modern 3-Ohm coils build more heat but they are manufactured with modern materials that can take much more heat than what were used in the older coils that were tar filled and the like. This doesn't mean that all of them are well made though. A lot of modern reproductions of the early style ignition components are made by folks that don't care if they work or not so keep that in mind. Modern cars don't use this stuff any more.

An ignition coil in a motor vehicle that is not running with the breaker points closed & switch on is just an electromagnet. It will get hot if the ignition switch is on too long. This has killed more coils than we will ever know since they can overheat and damage the wire windings. A cracked winding wire in a coil may make enough electrical contact to work until it gets hot and expands. If it expands too much the crack will open up and the electrical contact will be lost. These are heat related symptoms. It can happen to the coil or a ballast resistor. Condensers can also exhibit heat related problems so those are the first and generally the easiest things to replace while troubleshooting.

Re: 1948 Ford 12 volt coil issues
 

I had a similar situation when I converted my 47 coupe to 12 volts. I put a jumper wire across the factory resistor and used a parts store internal resistor 12 volt coil. I solved the problem by replacing the made in China coil with a Bosch 1200 coil. No problem since!

Re: 1948 Ford 12 volt coil issues
 

Jay I think you need a new volt meter. With only 2.8 volts it will not start.
When you engauge the starter the amp load of the starter will drop the 2.8
volts even lower. G.M.

Re: 1948 Ford 12 volt coil issues
 

This is a no brainer. Get Skip to rebuild a 32 to 48 original Ford coil. It will start
your engine and run it hot or cold. Plus Skip has NEVER charged to repair one of
his coils that failed other than a few the ignition switch was left on. G.M.

Re: 1948 Ford 12 volt coil issues
 

,Some good points above ,I don't think coils like running above 200 F ,( or Like Roter wrench says Coil deterioration .) have run a 6 v beige Yanky coil on my 34 for 25 years up by the generator recently I did a run down the freeway /motorway then got into crawling trafic by the time i got to drop of some brake shoes it was at 200 F and after wouldn't start I switched out the coil and bigo started ,Like AD 48 I shifted the coil to a cooler place ,I am montering how it goes but the Radiator is likely a contributor ,For JayChicago s question if you have a 6 volt battery and a 3.5 to 4 volt dropping resistor your left with ? if the points are open or closed that might have a berring on the test ,I don't know but think there are maybe other factors that come into it to ,I like a open to the atmosfear ballast resistor ,Cold air .

Re: 1948 Ford 12 volt coil issues
 

Jack the best way to heat the coil is to apply 12 volts to the coil.
When you can no longer pick it up it has melted the tar inside. G.M.

Re: 1948 Ford 12 volt coil issues
 

GM, I just mailed Skip my original coil to do the conversion. I am going to run the Petronix coil for now until I get my skip unit back