Coil Resistor Smoking Hot

F

FordFlatheadFan

Junior Member
I've been diagnosing a no spark issue on my stock '48 Mercury. The car was starting and running fine until two days ago. When cranking the engine with remote starter to check for spark, I noticed the ceramic-type resistor mounted on top of the (after market) coil was literally smoking it had become so hot. I'd appreciate any input on what might be causing this problem. Bad coil, resistor, points, or condenser?
 
I've never liked the ceramic type resistors for the reason you describe although the ones I've used in the past never got quite as hot as you describe although they do get hot. I remedied the issue by using an internal resistance coil. I use a 012 Bosch Blue coil on the 8ba in my '39 p/u. Problem solved. You may however have other issues that others may have ideas about.
 
FordFlatheadFan said:
I've been diagnosing a no spark issue on my stock '48 Mercury. The car was starting and running fine until two days ago. When cranking the engine with remote starter to check for spark, I noticed the ceramic-type resistor mounted on top of the (after market) coil was literally smoking it had become so hot. I'd appreciate any input on what might be causing this problem. Bad coil, resistor, points, or condenser?
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This type of resistor shown in the photo?
That's probably not the issue.
Coil may have failed internally.
Or the points may have failed..... pretty easy to check, they're just an
on-off switch really.
To check a condenser you need a condenser tester. Easier to just replace
it and the points.
 

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Why does it have a resistor in the first place? Unless you have converted to 12v, a 6v system with a can coil does not use a resistor. If it is 12v and using a 6v (1.5 ohm) coil, then you need around a 1.5 ohm resistor.
 
Thanks for all the comments. The coil is an original type which requires a resistor to drop the 6 volt input to the primary side of the coil to about 1.5 volts to prevent damage to the coil circuit (See the picture below). So far I replaced the condenser and tried a direct connection from the negative (power) side of the battery to the BAT. input on top of the coil. I've also tested the high tension output of the coil to ground while cranking the engine with a remote starter (key in on position). In each case, I get no spark arching to ground. Since the car was running perfectly last week I have no reason to suspect the points. I have to think the coil is bad, correct? Just curious if this type of coil is known to fail suddenly, rather than gradually over time. Thanks.
 

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FordFlatheadFan said:
Thanks for all the comments. The coil is an original type which requires a resistor to drop the 6 volt input to the primary side of the coil to about 1.5 volts to prevent damage to the coil circuit (See the picture below). So far I replaced the condenser and tried a direct connection from the negative (power) side of the battery to the BAT. input on top of the coil. I've also tested the high tension output of the coil to ground while cranking the engine with a remote starter (key in on position). In each case, I get no spark arching to ground. Since the car was running perfectly last week I have no reason to suspect the points. I have to think the coil is bad, correct? Just curious if this type of coil is known to fail suddenly, rather than gradually over time. Thanks.
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Sorry about that! You are correct!! I was looking at the other photo posted. Believe you are incorrect on the voltage though, I don't work with the original coils much, but believe it is somewhere around 3.5 to 4.0 volts or so.
 
Resistors are a load so they get hot normally. This is why most ballast resistors are generally housed in a heat resistant enclosure. The original resistors were around 0.8 Ohms. I'm not a fan of some of the reproductions. The originals were good to go until they finally die.

I don't know how reliable that brand of coils was. The OEM Ford ones were not quite the same.
 
They do get very hot normally but if the ignition points are closed too long they see additional work and get even hotter. The resistor is doing it's job only when the points close. If the points are closed for an excessive period then the time the resistor is working increases and so does the heat.
The coil primary resistance should be close to 1.5 Ohm. You need a digital ohmmeter about 0.5 Ohm for the resistor. If the coil primary resistance is very low the resistor will also see additional load and heat.
 
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The amp meter is your friend here!put in series with with wire to coil,see what you are drawing,,I think around 2 amps is what you are looking for,,see if it drops off when points are open,,
 
JSeery said:
Why does it have a resistor in the first place? Unless you have converted to 12v, a 6v system with a can coil does not use a resistor. If it is 12v and using a 6v (1.5 ohm) coil, then you need around a 1.5 ohm resistor.
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Answer In my lifetime always wondered about that. I am sure its a Ford thing "coil wise". NO body at that time had a inline resistor only ford products, until 12 volts came in to play which is not new many of the buses trucks even before WWII had 12 volts 24 Volts. My 46 coil took a dump the first day we bought a melt down. so Napa 6vt beer can coil no resistor been 6 yrs starts before ya drop a dime...
 
The condenser is the first and easiest thing to check but modern "checking" is usually replacement with one known to be in working order in absence of a good capacitor tester. I'm not sure if the old OEM Mallory style coils were 1.5 Ohms on the primary or not. It should be close to that though. That's about the only check a person can do on a coil when they don't have a coil tester. Both coils and condensers can fail with heat build up. Coils that may have a cracked strand in the primary coil wire can function when cold and as they heat up the coil expands and the crack opens up. More than a few coils have been damaged when the operator forgets to turn off the ignition switch and the points just happen to be closed. The coil is just an electro-magnet when powered up with no breaker opening & closing and they get hot relatively quick.

I don't know if Skip Haney repairs non OEM coils but his repaired coils do function better than the original ever could due to the modern materials used in the rebuild.
 
Thanks to everyone for your help. Since my last post I have replaced the coil, resistor, condenser & rotor. I have also, cleaned the distributor cap contacts and inspected and gaped the points which had no evidence of burning. I'm still getting no spark to the plugs so I plan to test the new components to see if I have any issues. I know how to test everything but am not clear on what the ohms readings should be on the coil, both on the primary and secondary circuits. Any information on what these reading should be will be appreciated. Again, ignition is factory original 6v system on '48 Mercury.
 
Take a test light. see if you are getting voltage into the distributor. (key on of course). If not, work you way backwards with your test light. Maybe a helper will be handy. Probably something simple when you find it.
 
A little bit of history about stock Ford ceramic resistors reveals that they do indeed get hot and that was not always well known, even within Ford engineering. The location for the V8 resistor at Job #1, March 10, 1932, was on the back of the ignition switch on the steering column bracket, less than two inches from the lock cylinder. Imagine how many burnt fingers resulted from that location from the simple act of inserting or removing the key or turning the key back back and forth. Needless to say it was quickly relocated well away from the ignition switch.
 
The resistor in post #3 is a yblock resistor. Ford used them to run 12v with a 6v ignition system and a internal coil resistance. A very good proven resistor. Putting a 1-2v resistor like you posted for a 6v system would certainly smoke some things under 12 volts.


When you put in a new wire resistor it'll smoke as the wire gets hot burning off the coating. They do get hot, having to take off the extra voltage.



.
 
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Make sure you have a good engine ground. Most electrical issues are ground issues.


Best of luck!
 
FordFlatheadFan said:
Thanks for all the comments. The coil is an original type which requires a resistor to drop the 6 volt input to the primary side of the coil to about 1.5 volts to prevent damage to the coil circuit (See the picture below). So far I replaced the condenser and tried a direct connection from the negative (power) side of the battery to the BAT. input on top of the coil. I've also tested the high tension output of the coil to ground while cranking the engine with a remote starter (key in on position). In each case, I get no spark arching to ground. Since the car was running perfectly last week I have no reason to suspect the points. I have to think the coil is bad, correct? Just curious if this type of coil is known to fail suddenly, rather than gradually over time. Thanks.
Click to expand...


Any coil can fail over time. Is that a copper line going to the sparkplug? or am I seeing things again? :confused::eek:
 
The OP mentions replacing the coil. Was it one of the new restoration reproductions or was it an original that has been rebuilt by Skip? I'm not trusting of the new reproduction stuff.

A person can test any coil with a relatively simple home brew test set up. A person needs a breaker set. Any old points type distributor can be used as long as it is functional. Jumper leads are required to connect the coil and distributor to a power source or battery of the normal system voltage. The system ballast resistor can be used and don't worry if it smokes. Just make sure it has the proper resistance and is properly connected to the power side of the coil. The breaker set needs to be functional and insulated so a person doesn't get shocked. The condenser has to be connected in the breaker line between the coil and the breaker points with it's outer case connected to the system ground. The ground has to be shared with the battery or other power source and the distributor body. A known to be good spark plug and high tension coil wire can be connected to the coil secondary tower with the spark plug body connected to the system ground. I'm including this simple diagram to help.
https://www.quora.com/How-do-you-wire-the-points-in-a-distributor

When power is connected through the ballast to the coil primary at the distributor connect terminal and the system grounds are connected to the power source, the test can begin. Rotate the distributor and see that the points open and close with little or no arcing. A spark should occur at the plug every time the points open and close. If all is properly connected with a proper voltage power source and there is still no spark, there is a problem with the coil. This is assuming that all the components used in the test are also functional.

Check your old coil and check your new one. You may have not had a coil issue. You can bypass the ballast but this may give problematic results if done too long. A regular can type coil as used on model A and 8BA family of vehicles after 1947, would work fine without a ballast but the Mallory type coils used by Ford were designed to use a ballast of around 0.8 Ohms. This was largely due to the design of the coil to keep reliability and longevity.
 
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