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Old 07-01-2019, 02:51 PM   #1
richard instness
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Default Cause of hot ignition coil?

Hi,
I have a model A that the coil seems to be getting hot on. The car runs fine, but after driving for about 30 minutes the coil is pretty hot. Hot enough to only keep your hand on it for a short time. The car is 12v negative ground. I have tried 3 different 12v coils running both internally resited and a coil with a external resister. The first 2 coils showed a resistance of 1.4-1.5 on the ohm meter. The coil that is currently on the car is a Bosch 12 volt blue coil. This coil shows a resistance of 3 .0 on the ohm meter. Currently the amp meter shows a charge of about 10amps while driving. Any ideas what might be causing the coil to heat up?
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Old 07-01-2019, 03:27 PM   #2
30 Closed Cab PU
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Default Re: Cause of hot ignition coil?

I assume the 1st 2 that measure 1.5 ohms were specifically for Model As. You could also ohm each of the 1.5 ohm coils terminals to the high voltage output. Should measure 7k-15k ohms. Not sure what teh reading would be for a 12 v coil


I am unclear on your email. The coils that measure 1.5 ohms require the 1.5 ohm external resistor when used on 12 volts. Otherwise they are running twice their desired current and will heat up.


The 12 v coil should measure about 3 ohms and does not need the resistor.


Then, do not ask why, can not supply a reasonable answer - but would try a different condenser.
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Old 07-01-2019, 05:38 PM   #3
rotorwrench
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Default Re: Cause of hot ignition coil?

The 3-Ohm coil should be able to take the heat. Later types like that are usually insulated with an epoxy resin that is designed to work at the higher temperatures. As was mentioned previously, the 1.5 would need the ballast of 1.3 to 1-5-Ohm. This splits the heat between the resistor and the coil. It makes the older type coils a lot more reliable otherwise they will burn up in a short time. The 1.5-Ohm coils will work on a 6-volt system with no ballast at all.
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Old 07-01-2019, 05:52 PM   #4
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Default Re: Cause of hot ignition coil?

Quote:
Originally Posted by rotorwrench View Post
The 3-Ohm coil should be able to take the heat. Later types like that are usually insulated with an epoxy resin that is designed to work at the higher temperatures. As was mentioned previously, the 1.5 would need the ballast of 1.3 to 1-5-Ohm. This splits the heat between the resistor and the coil. It makes the older type coils a lot more reliable otherwise they will burn up in a short time. The 1.5-Ohm coils will work on a 6-volt system with no ballast at all.
I believe the 1.5 ohm coils are for 6 volt installations. Put them on 12 volts and you draw twice the current and produce 4 times the heat. IMO, there's your answer.
I suspect the coils that overheated will be labelled 6 v (if they're labeled at all) and the 3 ohm one labelled 12 v.
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Old 07-01-2019, 10:14 PM   #5
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Default Re: Cause of hot ignition coil?

Heat in a coil is also a function of 'dwell'. The less your point gap the more time the coil dwells in saturation mode, heating up from primary coil resistance. If you have a "B" dizzy cam it has a profile that will give more dwell for the same point gap, resulting in more coil heat. Checked your point gap lately? Have a dwell meter?
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Old 07-02-2019, 03:27 AM   #6
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Default Re: Cause of hot ignition coil?

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Is your coil wire to the distributor facing up or down? A lot of the new coils are oil filled and only like to sit upright with the coil wire in the up position. When on their side ir upside down the oil doesn’t cover all the windings and they heat up. To go upside down you may have to go with an epoxy filled coil. They’ll go in any direction.
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Old 07-02-2019, 03:32 AM   #7
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Default Re: Cause of hot ignition coil?

as well as all above .... poor condenser (or condenser not earthing correctly) can also cause overheating of coil.)

Last edited by johnbuckley; 07-02-2019 at 07:17 AM.
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Old 07-02-2019, 05:48 AM   #8
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Default Re: Cause of hot ignition coil?

I just had to sort out this issue. johnbuckley is correct ,but Dodge is more probably ..most guys when switching to 12V just mount a coil upside down and it overheats from oil not covering wiring internally. how do I know? because original coil wire to 6V is to short when 12V is mount correct side up......
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Old 07-02-2019, 09:25 AM   #9
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Default Re: Cause of hot ignition coil?

I'd also check the little carbon button in the center of the distributor cap that makes contact with the rotor. A friend burned out 3 coils, and finally found that the carbon button was cracked. This caused resistance in the secondary circuit, and caused the coils to overheat.
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Old 07-02-2019, 09:35 AM   #10
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Default Re: Cause of hot ignition coil?

Heat produced by the natural function of the ignition coil should transfer out to case, so the case will get hot to the touch. Coils with an internal resistor will run hotter.


Check this out:
https://youtu.be/UEmSFDn8Rlw
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Old 07-02-2019, 09:58 AM   #11
Dick Steinkamp
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Default Re: Cause of hot ignition coil?

Quote:
Originally Posted by Dodge View Post
Is your coil wire to the distributor facing up or down? A lot of the new coils are oil filled and only like to sit upright with the coil wire in the up position. When on their side ir upside down the oil doesn’t cover all the windings and they heat up. To go upside down you may have to go with an epoxy filled coil. They’ll go in any direction.
I've heard this many times on this forum, but I think ALL coils were oil filled when Model As were produced and I don't think Epoxy had been invented yet. (5,000,000) Model A coils were originally mounted with the connectors pointed down.

I know (20,000,000) VW Bugs and (500,000) MGBs also mounted the coils with the connectors pointed down (and there are probably others. Seems like a preferred, more weather proof mounting system). I hang out on forums with the Bug and MGB groups. No discussion of coils failing when mounted in the factory orientation.





Is there any science or manufacturer instructions that anyone can point to that backs up the "upside down" oil filled coil claim? Not just anecdotal data, but actual science or manufacturer instructions?
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Old 07-02-2019, 10:10 AM   #12
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Default Re: Cause of hot ignition coil?

Richard - any update?
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Old 07-02-2019, 10:16 AM   #13
Bob Bidonde
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Default Re: Cause of hot ignition coil?

Incorrect timing and or a points gap that has decreased causes the coil to energize longer than normal resulting in its overheating.



A high resistance in the high voltage circuit will cause the coil to overheat and fail. High resistance may be due to a worn-out distributor cap carbon contact, excessive rotor to distributor body electrode gaps, a faulty high voltage coil wire, etc.
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Old 07-02-2019, 10:20 AM   #14
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Default Re: Cause of hot ignition coil?

Quote:
Originally Posted by MikeK View Post
Heat in a coil is also a function of 'dwell'. The less your point gap the more time the coil dwells in saturation mode, heating up from primary coil resistance. If you have a "B" dizzy cam it has a profile that will give more dwell for the same point gap, resulting in more coil heat. Checked your point gap lately? Have a dwell meter?
Mike, just out of curiosity, what is the "dwell" spec for the Model A with a "B" dizzy cam we should look for on our dwell meter?
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Old 07-02-2019, 01:32 PM   #15
Dick Steinkamp
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Default Re: Cause of hot ignition coil?

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Originally Posted by Bob Bidonde View Post
Incorrect timing and or a points gap that has decreased causes the coil to energize longer than normal resulting in its overheating.

Decrease in point gap = an increase in coil saturation time. I can see that.

I can't see how changing the timing one way or another changes the coil saturation time, however.
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Old 07-02-2019, 01:37 PM   #16
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Default Re: Cause of hot ignition coil?

Old coils were filled with pcb’s lilke a PG&E transformer not oil, shake one and see,
When they banned the use of pcb’s they went to oil. The oil filled are meant to go straight up not down and not on their side.

I’m just saying to try it and see if it works better.

There are instructions with the MSD coil that say stand it straight up or use one of their epoxy filled coils. Had the problem on my truck and went to they epoxy coil. No more problems.
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Old 07-02-2019, 02:33 PM   #17
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Default Re: Cause of hot ignition coil?

What's pcb?
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Old 07-02-2019, 02:53 PM   #18
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Default Re: Cause of hot ignition coil?

Here is a Wikipedia link.. It was heavily used in cooling concoctions used in oil filled transformers, coils, and other uses. Was found to be nasty stuff and the U.S. EPA (Environmental Protection Agency) clamped down on its use.


https://en.wikipedia.org/wiki/Polychlorinated_biphenyl
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Old 07-02-2019, 05:00 PM   #19
Dick Steinkamp
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Default Re: Cause of hot ignition coil?

Quote:
Originally Posted by Dodge View Post
Old coils were filled with pcbs lilke a PG&E transformer not oil, shake one and see,
When they banned the use of pcbs they went to oil. The oil filled are meant to go straight up not down and not on their side.

Im just saying to try it and see if it works better.

There are instructions with the MSD coil that say stand it straight up or use one of their epoxy filled coils. Had the problem on my truck and went to they epoxy coil. No more problems.
Thanks, Dodge. That helps.

I'm still going to wonder why for other makes the inverted coil isn't a problem. Maybe they are running old PCB filled ones, or installed epoxy replacements?
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Old 07-02-2019, 06:10 PM   #20
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Default Re: Cause of hot ignition coil?

Quote:
Originally Posted by 30 Closed Cab PU View Post
Here is a Wikipedia link.. It was heavily used in cooling concoctions used in oil filled transformers, coils, and other uses. Was found to be nasty stuff and the U.S. EPA (Environmental Protection Agency) clamped down on its use.


https://en.wikipedia.org/wiki/Polychlorinated_biphenyl
Some were even filled with an asphalt type insulation material which also helped seal the windings.
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