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Old 08-28-2020, 04:07 PM   #1
alanwoodieman
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Default over heated coil

49 F1 with 12 volts, Bubba Chevy Distributor, internal resister coil, one Bosc, one NAPA-both over heat and the engine starts to miss and finally dies. voltage is in the 14 volt range. any ideas? do I have two bad coils? too much voltage?
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Old 08-28-2020, 05:52 PM   #2
estout81
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Default Re: over heated coil

Check the primary resistance of the coil. Should be at least 3 ohms.
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Old 08-30-2020, 05:39 PM   #3
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Default Re: over heated coil

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Originally Posted by estout81 View Post
Check the primary resistance of the coil. Should be at least 3 ohms.

This is important! It's also a very good test and a way to eliminate other unnecessary lines of testing.
Once you know what the resistance is, you then can figure the amp draw (and heat) the coil is operating under.
If you have a meter to check this,it's as simple as unhooking both primary wires from the coil and checking OHMS across the two coil terminals.

It'll be good info to know.


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Old 08-28-2020, 06:01 PM   #4
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Default Re: over heated coil

The voltage should be in the 14+ volts range when running (12v setup). Are you sure you are using 12v coils? Like already posted, check the resistance of the coil(s).
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Old 08-28-2020, 06:43 PM   #5
alanwoodieman
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Default Re: over heated coil

yes 12 volt coils--voltage on plus side is 14 volts, neg side is 9 volts, my other truck with a known coil runs 8.8 volts did not check resistance just got tired of messing with it, will do tomorrow. I strongly think I have two BAD coils, coil got so hot you could not touch it!
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Old 08-28-2020, 06:45 PM   #6
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Default Re: over heated coil

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Alan, that's what it sounds like, but I would still check them.
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Old 08-29-2020, 04:30 PM   #7
alanwoodieman
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Default Re: over heated coil

tried two more coils with the same results just exactly how hot does a coil get? these get so hot you cannot even touch them with out burning your fingers
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Old 08-29-2020, 07:19 PM   #8
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Default Re: over heated coil

Not that hot. Do you have any idea what these coils are? It won't make any difference if they are all the wrong ohms.
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Old 08-30-2020, 03:18 PM   #9
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Default Re: over heated coil

OK, I just about give up, have tried everything I know of and still cannot get this truck to run properly. everyone thinks it is ign. but I even tried another carb, another condenser and a coil off my other truck that was running a bubba dizzy. put one of the other coils I was trying on the 49 on the 41 truck and it ran with no problems. Got to be something I am missing but WHAT?
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Old 08-30-2020, 05:10 PM   #10
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Default Re: over heated coil

If it is overheating then it about has to be too much current going through it. When you said the voltage was in the 14v range, was that at the coil? Did you check the voltage at the coil at around 2000 rpm? I'm guessing here, but the generator could be putting out too much voltage at rpm. Don't know if you run a generator or alternator, but it could be the same issue.
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Old 08-31-2020, 11:42 AM   #11
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Default Re: over heated coil

One way to deal with it is to use an external ballast resistor. If they run too hot then they are drawing too much current. The ballast will bleed that off and keep the heat out of the coil. Ignition coils usually only fry themselves if the key is accidentally left on too long. A coil will always be pretty warm to the touch but it shouldn't be so hot as to burn you if you touch it.

Many manufacturers started using ballast resistors when the change to 12-volts came about in 1956. The 1.5-ohm primary type coil works well with a 1.3 to 1.5-ohm ballast. In later years, the epoxy resin insulation inside the coils allowed for a full 3-ohm primary resistance inside the coil. This eliminated the need for an external ballast for 12-volt systems.
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Old 08-31-2020, 03:21 PM   #12
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Default Re: over heated coil

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Originally Posted by rotorwrench View Post
One way to deal with it is to use an external ballast resistor. If they run too hot then they are drawing too much current. The ballast will bleed that off and keep the heat out of the coil. Ignition coils usually only fry themselves if the key is accidentally left on too long. A coil will always be pretty warm to the touch but it shouldn't be so hot as to burn you if you touch it.

Many manufacturers started using ballast resistors when the change to 12-volts came about in 1956. The 1.5-ohm primary type coil works well with a 1.3 to 1.5-ohm ballast. In later years, the epoxy resin insulation inside the coils allowed for a full 3-ohm primary resistance inside the coil. This eliminated the need for an external ballast for 12-volt systems.
So are you suggesting using a resistor on a 12v coil with an internal resistor? Does the blue bosch have an internal resistor?
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Old 08-31-2020, 05:00 PM   #13
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Default Re: over heated coil

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Originally Posted by GB SISSON View Post
So are you suggesting using a resistor on a 12v coil with an internal resistor? Does the blue bosch have an internal resistor?
The issue here is we have no idea what coils are being used and at what rpm the voltage is being checked. What you want is somewhere in the range of 3 to 4 ohms in the circuit. If it requires a ballast resistor or not depends on the resistance in the coil (and the rest of the circuit). If the coils are overheating then there is most likely an excess of current flowing through them. This could be the wrong resistance in the coil(s) itself or excessive voltage being supplied to it. Just switching out coils is not going to tell you much unless you know what they are. A coil might work fine in one vehicle and not in another. Only way to know is how it is set up in the original vehicle and how the voltages compare between the two at higher rpms.

Heat is energy and the energy to heat it up has to come from somewhere.
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Old 08-31-2020, 05:49 PM   #14
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Default Re: over heated coil

There is only one resistor in an ignition coil. It's called the primary coil. They are manufactured in various different ohm specification for various applications and most applications are voltage related. A person can measure the amps it is drawing with a multi-meter. Ballasts are used to control current flow but should only be necessary when it is drawing too much current. The old Ford V8 coils had to have a 0.8 ohm ballast to control the current flow on them since they were sensitive to current in the Mallory design.
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Old 08-31-2020, 08:00 PM   #15
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Default Re: over heated coil

OK some of you guys are going off the deep end. turns out the alternator internal regulator was spiking voltage as high as 16 and this was causing most if not all the problem
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Old 08-31-2020, 08:33 PM   #16
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Default Re: over heated coil

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Originally Posted by alanwoodieman View Post
OK some of you guys are going off the deep end. turns out the alternator internal regulator was spiking voltage as high as 16 and this was causing most if not all the problem
So, in order to check mine I put a voltmeter to the battery while it's running and then rev the engine to 2500 or so and see what the voltage is? I know it's supposed to be a bit over 14. I'll check mine in the morning.
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Old 08-31-2020, 08:38 PM   #17
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Default Re: over heated coil

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Originally Posted by alanwoodieman View Post
OK some of you guys are going off the deep end. turns out the alternator internal regulator was spiking voltage as high as 16 and this was causing most if not all the problem
Think I mentioned that several times!
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Old 09-01-2020, 07:19 AM   #18
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Default Re: over heated coil

I had to sit and wait for the voltage spikes, just did not go there and stay at 16 volts--up/down--repeat-repeat
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Old 09-04-2020, 11:36 PM   #19
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Default Re: over heated coil

I have been through a lot of problems on the woodie this month with sputtering and missing on the hills. Not so much on the start of the trip to town (24 mile round trip), but the way home after she heats up. Tonight after sputtering on the long grades I popped the hood after I pulled into the shed. Here's what I found. Coil, So hot I could not keep my hand on, alternator 2" away was warm but could keep my hand on fine. Steel tube up to radiator, no way could I hold onto. Too hot. I don't have a lot of test equipment but do have a point and shoot temperature meter. I don't have a multi-meter but I do have a clear test light with a spike and a clip that reads volts on a green screen. Would that show these 'spikes' if that is also what I have and can anybody say what my temp gun should read on the coil? I can do more checking on the weekend. Thanks
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Old 09-05-2020, 06:40 AM   #20
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Default Re: over heated coil

Instead of messing around till the cows come home you need to measure the current draw (AMPS) the coil primary is drawing with the engine running. To do this set your multi meter to DC AMPS scale which on most meters with this feature is 10 amps and insert your test leads into the common and amps sockets. Remove either of the coil primary wires and connect your meter in line between the coil terminal and the removed wire. With engine running you should have something like 3 or 4 amps draw reading. It will read a bit lower with higher revs. When you have done that report back. Regards, Kevin.
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