Brass top coils and insulating washers

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Model T Ford Forum: Forum 2013: Brass top coils and insulating washers
Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Pellow on Thursday, October 31, 2013 - 02:48 pm:

My partner and I have rebuilt several brass top coils and have never worried about the insulating washers on the upper brass points bridge.

We have never checked for continuity between the adjuster bolt and the upper points bridge.

But all of our coils were checked and adjusted to spec on our hand cranked coil tester.

We gave two to go however and we are replacing the secondary winding in both of these coils. Should we be rechecking our work on the other coils and looking out for the remaining two coils?


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Thursday, October 31, 2013 - 08:30 pm:

The upper bridge MUST be insulated from the adjuster screw which is fastened to the brass top. The regular wood box coil has no brass top so it doesn't need to be insulated from the adjuster screw because the adjuster screw is not fastened to anything metal.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Pellow on Tuesday, November 05, 2013 - 01:23 pm:

John,

This is my second attempt to reply. Are you saying that if the insulating washers are missing that there will be a dead short to another component and that the coil will not work?

I will take a look at the coils we have rebuilt and see what shape the insulating washers are in. Dennis always thought that the adjusting thumb screw did not connect to anything and that the insulating washers had no real function.

I am only talking about brass top coils here.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Wednesday, November 06, 2013 - 02:32 am:

Bob:

What you are missing is that you have not probably built enough metal top coils to see the problem that happens. If the upper bridge is not fully insulated then the coil can in some instances "latch up" and burn up. Here is what happens: Look UNDER the vibrator spring where it proceeds over the front edge of the brass top and then passes over the end of the coil core. Push down on that vibrator to simulate the action of the magnetic core pulling it down. Now look carefully and you will see that there is almost zero clearance between the bottom of the vibrator spring and the top front edge of the metal top. Some metal top coils are worse than others but also the location of the spacers and vibrator play a part in this drama too. Now if you follow the electricity path you will find out that if that vibrator bottom surface touches any part of the metal top then that will complete the circuit in the exact same way that the points complete the circuit. In other words there are 2 paths to power the coil. One is from the upper bridge through the points to the lower vibrator but that path is pulled OPEN by the coil magnetism so the normal operation of the points does not cause any issue of latch up. The second path (when there is no insulating washers) is from the upper bridge to the adjuster screw to the metal top to the bottom of the vibrator spring but this path is NOT pulled open by the magnetism but rather is pulled CLOSED by the magnetism. What happens is that if the vibrator touches that metal top it will NOT let go and will latch there by its own power and stay there until the power is broke elsewhere by the timer or ignition switch. Where this is most dangerous is when the coil is BUZZING with ignition switch turned on and the engine not running. You can turn on the ignition switch, hear perhaps a momentary buzz which stops abruptly and then tar and smoke begin to pour our of the top of your metal top coil that has latched up during buzzing. Since this potential latch up happens AFTER the coil has already fired normally - you only need to insulate the upper bridge from the metal top so that if contact is made between the bottom of the vibrator and the metal top - it is just a noise and the coil will NOT latch up and burn up. Some metal top coils have large amount of clearance down there while others have almost none but sooner or later it happens. Build enough metal top coils the wrong way and you will soon see smoke. Been there - done that.


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