Cause of hot ignition coil?

[richard instness]

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?

[30 Closed Cab PU]

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.

[rotorwrench]

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.

[Synchro909]

Originally Posted by rotorwrench

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.

[MikeK]

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?

[Dodge]

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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.

[johnbuckley]

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

[canadian]

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......

[Will N]

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.

[Bob Bidonde]

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

[Dick Steinkamp]

Originally Posted by Dodge

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?

[30 Closed Cab PU]

Richard - any update?

[Bob Bidonde]

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.

[Ruth]

Originally Posted by MikeK

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?

[Dick Steinkamp]

Originally Posted by Bob Bidonde

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.

[Dodge]

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.

[johnbuckley]

What's pcb?

[30 Closed Cab PU]

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

[Dick Steinkamp]

Originally Posted by Dodge

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.

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?

[duke36]

Originally Posted by 30 Closed Cab PU

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.

[richard instness]

Today's update on the coil issue.
I forgot to mention in the first post that this car also has an electronic ignition in place of the points. It was on the car when I purchased it and it has ran well so I left it there.
I am still using the Bosch 12v blue coil with a resistance of 3.3, but today I removed the external inline resister. After a long drive the coil temperature was much lower than before and everything else seems to be as it should. The coil is still slightly warmer than my other model A with modern style points, but I think the temperature now is very acceptable. I am going to monitor it for a while and if things continue to be as they should I think I will call that good.
Thanks to all who replied. A lot of good information.

[noboD]

Doesn't a Bosch blue coil have an internal resistor?

[katy]

Currently the amp meter shows a charge of about 10amps while driving.

It sounds like your voltage regulator is set too high.

Old coils were filled with pcb’s, lilke a PG&E transformer, not oil

The coils WERE oil filled, but the oil contained PCBs.

[Chuck Sea/Tac]

I took an oil field coil for Model A and cut it open. It has a spacer on the bottom so the windings are immersed in the oil when it’s upside down the spacer is what is in the air. Now if all of them are that way I don’t know I’ve only cut one open.?

[Chuck Sea/Tac]

Correct me if I’m wrong, but I believe coils are made for plus or minus ground. That is, one or the other not both. If you’re running one in the opposite way it was designed, could that cause it to overheat?

[katy]

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Originally Posted by Chuck Sea/Tac

Correct me if I’m wrong, but I believe coils are made for plus or minus ground. That is, one or the other not both. If you’re running one in the opposite way it was designed, could that cause it to overheat?

It wouldn't cause it to overheat, but it doesn't give as good a HV spark.

[Dodge]

Thanks for cutting one open Chuck. Maybe that’s why some survive the upside down position. Was it one sold by one of the Model A vendors? Does anyone know if a Bosch blue coil is built the same way? As Dick pointed out the Bosch coils for a VW are in the down position.

[30 Closed Cab PU]

Originally Posted by Chuck Sea/Tac

Correct me if I’m wrong, but I believe coils are made for plus or minus ground. That is, one or the other not both. If you’re running one in the opposite way it was designed, could that cause it to overheat?

I do not know if there are different coils for + and - 6V systems. Previously ran across this statement in a string and called a couple of vendors to see if they carried or knew of people that carried different 6V coils, they said they have never heard of this, and carry only one 6v coil.


If you have sources additional info, please post, I would be interested.


Thanks

[Chuck Sea/Tac]

If you look at this diagram, the ground is the side with one end of each winding terminating. So a coil for an opposite grounding system would be labeled backwards. I suppose if you switched the primary, then it would be correct for that system regardless of labeling.
By the way,Bubba of Bubbas ignitions says that once a coil is ran wired wrong, then switched, it won’t put out full voltage because of core magnatism. I’ve never heard that before.

[30 Closed Cab PU]

Interesting about the full voltage statement. When I checked my 6 v positive ground system using the pencil flair test it was correct. Bought a spark tester that has the movable wire in it that is put in series with the dist./coil lead so you can see where the spark starts degrading. Noted this, and then switched primary connections on the coil, and did see about a 40 percent degrease in the reading. Switched the leads on the coil back to correct and got the same reading as teh 1st correct hookup reading, so did not see a decrease. Perhaps you have to run it incorrectly for a while to get the core magnetism issue.


Not familiar with ignition coil constructions - they have a conductive core the windings are wrapped around? Might be a silly question, but would like to know.

[katy]

Bubba of Bubbas ignitions says that once a coil is ran wired wrong, then switched, it won’t put out full voltage because of core magnatism.

I doubt that, the core is made of mild steel laminations, and the residual magnetism can easily be reversed, same as a generator.