Distributor Coil

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Model T Ford Forum: Forum 2018: Distributor Coil
Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry Davis Houston TX on Thursday, June 21, 2018 - 10:31 pm:

I need a sure was to determine if my coil is working. I have 13.7V on the coil plus side and .05v on the negative side. Any help will be greatly appreciated. Jerry.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Zibell, Huntsville, AL on Friday, June 22, 2018 - 10:14 am:

With points open you should have about the same voltage on each side. Take a jumper from ground to the points side momentarily,. When you break the ground connection the coil should fire a good blue spark. However that said, most coil failure occur when the cold gets hot. So a cold test just gives you an idea that it can work. So if the car starts and runs for awhile and then dies, and you test for spark and have none or a yellow spark it is an indication of a failing coil.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Henry K. Lee on Friday, June 22, 2018 - 12:50 pm:

Jerry,

It is always best to use a ballast resister in series with a point/condenser system. Old school USA made coils are available though eBay and look for the ones that require a ballast resistor. Your coil should stand up right away from the exhaust manifold to prevent heating up as well. I use Bosch coils (either made in Germany or Brazil) with a VW 009 condenser made either in Germany or Brazil, not China. I never have a breakdown. Spark is dead on the money.

Hank


Top of pagePrevious messageNext messageBottom of page Link to this message  By Neil Kaminar on Friday, June 22, 2018 - 01:20 pm:

Jerry,

I concur with Henry. I use the same Blue Coil from Bosch (beware of imitations).

If your points are closed you will get the low voltage you measured on the negative side of the coil. Crank the engine over slowly until the points open and the voltage should be the same on both sides.

You can do as John suggested by just opening the closed points by hand with distributor cap off. Make sure you have the wire from the coil that normally goes to the center of the distributor cap about 1/4 inch away from a grounded point, like the top of the head. You should get a blue spark as John described. Sometimes the coil can get an internal short in the high voltage windings if there is no place for the current to go such as when the high voltage wire from the coil is not near a grounded point. This is the cold test as John describes. To test if you have a failure when the coil is hot, replace it with a known good coil. If your problems go away then the old coil is bad.

Check the capacitor (condenser) too. You can check it with an ohm meter (on the ohm setting) by reversing the leads. With the points open and the coil not connected, you should have a small resistance that rapidly changes to an open circuit. Then reverse the leads and test again. You can reverse the leads several times. The best way to test the capacitor is to replace it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Neil Kaminar on Friday, June 22, 2018 - 01:21 pm:

By the way, the Blue Coil from Bosch has an internal ballast resistor so you don't need an external one.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry Davis Houston TX on Friday, June 22, 2018 - 10:20 pm:

Thanks guys. I do use a ballast resistor which drops the coil V from 13.7 to 5.7 on the plus side of the coil but only .05v on the negative side as measured by a true RMS meter. So, what I need to do is to open the points and I should have the same v on both sides. Right?


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