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Farmall B coil

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Dave H (MI)

07-27-2007 15:09:17

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Thought I posted on this but can't find it! After I cleaned the contacts and charged the battery on the B, I installed a house light switch to replace the bad switch temporarily. When I pushed the starter rod the starter cranked nicely. Since the tractor still does not start I checked spark on all four cylinders and have none.
So I went back to the beginning and that led me to the coil. The coil has the big center connector to the distributor and two small posts.
The one small post has power when grounded to the frame. The other small post is a ground and I get no reading on my meter when I ground the poser post to it...should I??? I guess I am a little confused on how coils work and what all the posts are for and how they relate to each other. Does the coil need to ground through the second small post or can it ground through it's frame? Could the coil be bad? Thanks guys!

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Dave H (MI)

07-27-2007 18:26:55

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Re: Farmall B coil in reply to Dave H (MI), 07-27-2007 15:09:17

I'm sorry guys but you are just fractions of an inch over my mechanical head. If I am getting the wind of this right, the coil is inseperable from the distributor and I cannot test the coil without working the points.....but it is not clear. I'll try to search around and see if I can find something basic on coil operation. I guess one point of confusion is how you can have voltage on both small posts of the coil when one looks like a ground to me(?). Maybe I am overthinking it?

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El Toro

07-28-2007 04:22:34

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Re: Farmall B coil in reply to Dave H (MI), 07-27-2007 18:26:55

You need a wire from your ignition switch to the
negative terminal on the coil and another wire from the + terminal on the coil to your distributor. Its not going to any ground. Take a look at this picture. This is a mag that's been converted to a distributor and the wiring is the same as your distributor. Hal

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Sam#3

07-27-2007 20:51:07

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Re: Farmall B coil in reply to Dave H (MI), 07-27-2007 18:26:55

First, the term coil is a misnomer. It's a transformer which it made up of two coils the primary and the secondary. Inside the can the primary is connected to the two small terminals and the secondary is connected to the - or neg(.)itive terminal and the large socket at the end of the can. The primary consists of a few turns of wire and the secondary is made of many turns of wire and in this application is considered a step-up transformer. The primary circuit consists of the breaker points (with the plate connected to ground), the primary winding, probably a switch, and a source or electromotive force (measured in volts.). The secondary circuit consists of the 'points, secondary winding, the distributor cap, the rotor, cap, and spark plug(s).
The operation goes like this: As the distributor rotates the points are opened and closed, When the points close the primary circuit is completed and current flows through the primary winding forming what is known as an electromagnetic charge. When the point open the charge decays and by an action called 'mutual inductance' an electrical current begins to flow in the secondary winding. By the step-up action of the 'coil' this creates a very large voltage in the secondary circuit (The blue spark.). That�s pretty much it in a nutshell.
The testing techniques given by others are just simulating the rotation of the distributor.
Electrical fundamental: In an open DC circuit the voltage measured at any point in the circuit will equal the open circuit voltage of the source.
If the points are open battery will be measured anywhere in the circuit. Rotate the engine a bit to close the points and you can see the voltage drops across the circuit.
Good luck and visit your local library for a "Basic Electronics" textbook.

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Janicholson

07-27-2007 20:48:28

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Re: Farmall B coil in reply to Dave H (MI), 07-27-2007 18:26:55

Simple test. On the side of the distributor, there is a connection that goes to the coil. Loosen the nut and remove the wire.
Turn on the house wire switch and check the end of that wire for volts. It should be battery volts, or minus a couple from battery volts. If so, it is probably an OK coil,(but not for sure)If there is no voltage at the wire in this experiment, and there is on the switch side of it, the coil is toast
If yes, take the distributor cap off and look at the points. If they are closed (together) turn on the switch again and touch the disconnected wire to that terminal you took it off of. A small,but clear spark should happen where it is touched when it is touched and removed. The wire from the center of the distributor if held with insulated handled pliers, or a dry stick, next to the block, about 1/4" away, will also spark a hot blue spark. If this does not happen, the problem could be in the points, or condenser.
Put the wire back on the distributor. With a small plastic handled screwdriver, and the switch on, reach in and open the points carefully. getting the screwdriver touching ground will not hurt anything, so do not be nervous. If this causes spark, the problem is probably that the points need to be cleaned. A fine emery board, followed by a piece of a folded dollar bill will make them clean enough to work. They should be set at .020" when the little breaker cam is opening them on a high point. Hand turning the engine is best for this, with the switch off. Try these items then get back to us. JimN

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Dave H (MI)

07-28-2007 08:26:08

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Re: Farmall B coil in reply to Janicholson, 07-27-2007 20:48:28

Thanks Jim and Hal and the rest! I think one of my mental stumbles here was an assumption that those little connections on the coil were plus and minus because one appears to be a ground. I will copy all this and take it out there this afternoon and post back tonight. Sure appreciate the help! I am getting quite an education!

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El Toro

07-27-2007 17:08:44

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Re: Farmall B coil in reply to Dave H (MI), 07-27-2007 15:09:17

You need to use a volt meter or test lite to see if have battery voltage on both small terminals with the points open and your ignition switch on.
If you do have voltage on both terminals pull the primary coil wire from the distributor cap and hold it near a good ground on the tractor. Then have someone to snap those points open and close few times and look for the coil to spark. I would clean those points with a piece of 320 wet or dry until they are shiny. Set the gap at .020".
Hal

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old

07-27-2007 15:45:49

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Re: Farmall B coil in reply to Dave H (MI), 07-27-2007 15:09:17

One side of the coil goes to the ignition switch and the other side to the points. When the points close it then puts that side of the coil to ground and when they open that side is an open circuit. I would check you points and make sure they are set right and clean. Pull the cap off turn on the ignition and by hand open and close the points holding the center wire from the coil close to the block. When you do that you should get spark, if you don't watch the points if you don't see spark at the points the points could be bad or need cleaned. You could also have a bad cap and or rotor or maybe you forgot to put the rotor back in. Yep happens to the best of us and I did that just the other day and low and behold I had no spark at the plugs. LOL

Hobby farm

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