Ignition Coil Problem

[stevematta]

I have a 1930 Model A pick-up and I'm finding I need to replace the coil about once every two months! What's going wrong?

It all began months ago...I had trouble starting the truck. Someone suggested it might be a weak coil, so I put in a new one. POW! Started up with little effort. It ran great for several weeks, then back to the same old problem. I tried another coil and once again, it was back up and running.

This past weekend, I attempted to start the truck and it had some difficulty but almost turned over a few times...then nothing. It's acting like the coil is bad again.

So...what am I doing wrong or what should I be looking for? I should add that, yes, it has plenty of gas and the battery is fully charged.

[Tom Wesenberg]

Have you changed the condensor? Have you checked the wire between the top and bottom distributor plates? Have you check the popout or other switch and wires?

Also put a volt meter on the coil primary and see how many volts you have going to it. Maybe the generator is putting out too many volts.

[stevematta]

I have a new distributor with modern plates - that didn't solve the problem. The ignition switch, distributor wire and ignition wire (to the distributor base) are all less than a year old. I'm wondering if the effort to do a "cold start" is creating a problem for the coil. Sometimes it takes a few tries to get the engine to turn over.

[Tom Wesenberg]

Originally Posted by stevematta

I have a new distributor with modern plates - that didn't solve the problem. The ignition switch, distributor wire and ignition wire (to the distributor base) are all less than a year old. I'm wondering if the effort to do a "cold start" is creating a problem for the coil. Sometimes it takes a few tries to get the engine to turn over.

Are you saying that it takes a few tries on the starter button before the bendix engages and turns the engine over? If so, the helix must have heavy oil, grease, dirt or rust on it. Also check the tiny pin and spring on the drive. It needs to be free and have a drop of oil so it doesn't stick. I also put some light oil on the helix.

[stevematta]

No - starter is fine, bendix is engaging. But when the engine is cold and/or hasn't run in several weeks, it takes repeated attempts to start. Somewhere along the way, the coil is getting toasted and nothing will happen when I try to start the engine. If I replace the coil it will likely crank right up (or at least has in the past).

So my quest is to find out what is burning out the coil.

[JoeWay]

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Has the truck been converted to 12 volts, and if so are you using an ignition resistor in series with the coil, or a coil with built-in resistor?

If you have a 6 volt coil running full-time on 12 volts, you can expect problems.

Joe

[fordfixer]

Do you have a wiring diagram? I would check the wiring, even if it is new and was replaced in the last year or so. Is the coil warm to a touch or is it hot? Check terminal box for any stray wires.

[stevematta]

The truck is 6v, not 12. Warm to the touch? Do you mean after attempting to start or just whenever the ignition key is in the "on" position? It seems you suspect a short somewhere. I do have a couple of fuses in the wiring - but they appear okay. But I guess the coil could be "absorbing" the problem if there is one such that the fuse doesn't blow. Am I following correctly fordfixer?

[Kevin in NJ]

It is odd to have that many coils "burn out". You had a problem and change the coil and it worked. You need to determine if you are actually burning out the coil, you really need to test both coils and see if they are really bad as that directs the troubleshooting. If you have bad coils my next questions would ask what voltage are the coils and have you had light bulbs burn out too. If you have a poor connection to the battery and a generator without a regulator then you may have a bad battery connection. This will cause high voltage spikes on your system. If you have an alt then you should be ok, but there could be a failure there too.

What kind of coils are you using? New, used? What are the specs of the coils you are using? Are you running 6 or 12v? Gen with or without a regulator, or alt?

You also need to look at the harness that is new. Are the the connectors properly done, have they been soldered too?

If you do not have a voltmeter or understand electricity then I suggest you find some one to help you. You can spend a lot of money chasing a trivial problem.

The more new parts you add the better the chances you are adding in additional problems. Just because it is a new part does not mean it actually works.

[Aerocraft]

Could it be that you have an eight volt battery?

Gar Williams

[Bob A.]

Do you have the wires correctly on the coil ? (polarity)
A bad starter will draw high amperage when you're cranking it over and will reduce the voltage at the coil which in turn will give you a weak spark. A weak battery while cranking the starter motor also will reduce the voltage at the coil because of the high amperage draw of the starter.
The reason I mentioned polarity, When I bought my '31 the trunk had a half dozen coils in it and they were marked with things like weak, N.G. and etc. I checked them out at my cousins repair shop and they were all good. The previous owner must have had some kind of trouble with coils/ignition so I also checked for polarity at the coil and found that the wires were reversed. I also found that the repo ignition switch was N.G. This had to drive him nuts. Now the the car runs fine.

[Tom Endy]

If you are truely burning out coils it is because you are pulling too much current, or continious current, through the primary winding. A six volt system should have a coil with 1.5 ohm primary. This will give you about a nominal 3 amps of current flow when the points close. If your ignition switch is faulty it could be switched on when you think it is off. If the points are closed when the engine stopped there will be a continious current flow which will overheat the coil. I would also be suspicious of the modern lower plate you have installed in the distributor. If the insulators are shorting where the base and the slider plate attach there could be a current leak. Lift the battery lead to the coil and insert an ammeter and see how much current is flowing with the engine running and when stopped.

Tom Endy

[josh1331]

Could someone post the procedure to use ohm meter to test a coil and also, to test polarity of coil please/ josh

[stevematta]

A very good question, Josh. I'd like to know if the coils I have are good, bad, weak, or other. I assume a continuity test would tell if the coil is broken, but then I understand that the coil body is filled with oil - so that might act as a conductor and give a false reading. I bow to those who know more about this stuff than me! Any suggestions on bench testing an ignition coil are welcomed and appreciated!

[wa4ko]

You may wish to reduce the resistance in your electrical circuit. Remove the short
length of sliced cable that runs from your positive battery terminal and throw it away. Put on a cable (#2),that runs from the battery to a securing lug on the starter. This will make the starter run better, there will be less resistance in the ignition circuit. The old chassis just will not carry 6 volts any more with all the rust and time
Larry

[ford1]

if you keep burning up coils if sounds like you have to much voltage to them, i think the coils only need 4 volts are you using a resistor in the wire going from the ign to the coil?

[Ron/IA]

For those who want to know how to check coil polarity, send me an email to:

[email protected]

I have a Word document how to do this test, but I do not know how to attach it to a thread on Fordbarn.

[Tom Endy]

Bratton's (800-255-1929) sells two devices that will test an installed coil to some degree. The first is part number 17140. It plugs into the top of the distributor cap. When you start the engine one of two LED's blink telling you if the coil is installed with the correct or incorrect polarity. If it indicates incorrect polarity you simply reverse the two wires connected to the coil's primary winding.

The second device is part number 17150. It also plugs into the top of the distributor cap. You start the engine up to a fast idle and with a wooden stick you reach into a small window and move a slider that will create a spark gap. You are able to see the high voltage spark leap across the gap. Markings on the side of the window indicate the voltage value as the spark gap is widened. Both devices sell for about $20 each.

The only other test is to measure the resistance of the primary winding with an ohm meter. A 6-volt coil should have a 1.5 ohm resistance, a 12-volt coil should have a 3 ohm resistance. If you use a 1.5 ohm coil on a 12-volt system you need to add a dropping resister in series with the primary winding.

None of these tests are going to tell you if the coil is going to perform properly under heat and load. The high voltage winding (nominal 20,000 volts) is suseptable to breaking down. The high voltage can internally arc across adjacent windings through the insulation material.

A road test in hot weather is the only real test I know of. Best to carry a known good spare.

Tom Endy

[CWPASADENA]

Lots of very good suggestions.

There is one thing more I would consider looking at. Check the Dwell (Point Gap) if the points are almost completely closed up, the dwell time gets longer and longer which can cause the coil to run hot and also make the engine hard to start.

If the points have not been checked and adjusted for some time, this may be the problem. The rubbing block on the points will wear over time which will cause the points to close up. A little point lube or clear silicone grease on the distributor cam helps to minize rubbing block wear.

Let us know what you find.

Again, just a suggestion,

Chris