Coil, weak/no spark, what do I check now?

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Model T Ford Forum: Forum 2017: Coil, weak/no spark, what do I check now?
Top of pagePrevious messageNext messageBottom of page Link to this message  By kep on Wednesday, September 20, 2017 - 03:03 am:

This coil doesn't look like any I've previously had.
The windings appear to be encased in rubber. Secondary winding gives a reading of "2.7" on a cheap digital ohm meter set to "20K ohms range"
From what I read in prior threads on coils, This is not unusual.
A HCCT showed 1.35 amps draw while making... no spark. Points still sparked though.
In the car, which is 12 volt, The coils all threw a 1/4 - 3/8th spark from the lead to the head when shorted with a screwdriver except for this one.
The spark will not jump anything near enough for a spark plug but will give a very short spark, fairly hot looking.

My thoughts were it might be shorting internally.
How does one proceed with this type of coil? I'm not familiar with rubber and yellow goop, And cannot replace this coil...

Please see attached picture for... not a lot of detail.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Wednesday, September 20, 2017 - 10:57 am:

Why can you not replace the coil? I would suspect either an open secondary or an internal spark which bypasses the spark plug. Use an ohm meter between the two side contacts on the coil. You should get a very low resistance. You could have a problem with the coil box. such as a loose connection or an internal carbon path. Will this coil work in any other position, and will one of the other coils work in this position in the coil box?
Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hal Davis-SE Georgia on Wednesday, September 20, 2017 - 12:13 pm:

If you're getting a reading across the secondary, I don't think it could be open. Internal sparking is my guess. Could be a result of making it jump that 3/8" gap. 1/4" max gap is what the experts say.


Top of pagePrevious messageNext messageBottom of page Link to this message  By kep on Thursday, September 21, 2017 - 01:56 am:

Oh I didn't force it to jump that gap. It was doing that as I held a screwdriver near the wire.
And yes i did test the coil in other positions in the coil box, Same result.
So nobody has seen a coil made with rubber instead of tar huh? Hope I don't damage it. Replacement coils do not exist and cannot be obtained due to insane postage fees and taxes.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hal Davis-SE Georgia on Thursday, September 21, 2017 - 07:40 am:

Whatever the reason for the failure, my money is still on internal arcing. I don't know any way to fix that. I suppose that if the break in the insulation were in the outer layers of the winding, that you could remove the coil, unwind enough wire to get past the bad spot and re-install the coil, but the problem is that you have no idea where the arcing is taking place.

If I remember correctly, the resistance across the secondary should be 3200 ohms. I am not sure whether your reading of 2700 is showing an internal short or if the manufacturer just saved a bit of wire on the windings.

At any rate, I think your coil is toast. Perhaps some other T'ers in your area have some spare coils they might sell?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Thursday, September 21, 2017 - 08:58 am:

Kap, You have ruled out coil box arcing and defective spark plug by testing the subject coil in other coil box positions. Other variables that could cause missing sparks NOT related to coil is HCCT electrode oxidation. That can be ruled out by testing performance of other known good coils or cleaning the HCCT electrode surfaces. Another is the use of the wrong type of capacitor. 0.47uF 400V are common parameters but it must also have a dv/dt rating of 600V or may fail with usage. I none of these, unfortunately, the coil may be arcing internally of which there is no remedy.

Regarding alternate potting; I do recall Ron Patterson reporting encounters with many alternatives such as RTV, Foam insulation and others which render the coil non-repairable, and therefore, the recommendation to re-builders is to only use Type III Steep Roofing Asphalt. Here is a link to one of many coil potting discussions: http://www.mtfca.com/discus/messages/80257/102411.html


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Thursday, September 21, 2017 - 11:25 am:

Find someone from U.S.A. who is traveling to N.Z. and ask them to bring a coil in their luggage. Hopefully they can bring it without anyone thinking it is a bomb.
Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Garnet on Saturday, September 23, 2017 - 08:03 am:

My eyes can't make sense out of the photo but it looks like the coil has shifted from around the core, or has entirely shifted. Looks like it's in the middle of the box. Regardless, it's a low quality product commonly seen. They've even quit using the super-high insulating cardboard now by the looks of it !! When stacked, two of them are handy for shimming up the front of your toolchest.

Stick with the coils Ford used. Test them and rebuild the ones that pass. Then set the contacts with a rickety old ECCT and yer off to the races Kep.

Regards,
Garnet


Top of pagePrevious messageNext messageBottom of page Link to this message  By kep on Sunday, September 24, 2017 - 06:20 pm:

Careful disassembly shows soot on the wood near the site of this secondary wire leaving the rubber cased windings. The wire exit is burnt and appears to have been arcing to somewhere, But where? The core of the windings may have been an original but is quiet clean. Not sure how I'd unwind this to see how far the burning goes without destroying the coil.

So far I have a good box, points and condenser to rebuild with. If i can find some good secondaries i might be able to repair it somehow...


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