Ford 172 stationary power unit

I have a Ford 172 cubic inch power unit on a brush chipper. Last summer as I was using it the engine shut off just like someone turned the key off. I put in new points and condenser, and spark plugs. Still no spark. I have 11.75 volts at the coil, on the distributor side of the coil I have 11.75 volts. I am leaning towards a bad coil. Does anybody know how to test a coil. Or am I not seeing something. Another thing I thought of was the insulators inside the distributor where the wire supplying 12 volts to the points could be worn out. They look good and it is hard to believe they could loose their insulating capabilities. The engine is around 1960's vintage.

I did get it to run again last fall but have not a clue as to what I did. This spring (yesterday) It did not have spark. I shined up the points and it fired right up. Used the machine for 30 minutes and shut it off to change the oil. After the oil change, no spark, weird! I put in another new condenser. Still no spark.

Anybody have any ideas besides putting dynamite under the machine?

Check and see if that engine has a resistor before the coil Ford autos around that time cut voltage from 12 to less than six. also coils would break down with a lot of heat

There is a resistor. With the key on and the engine not cranking over I get the 11.75 volts. I just checked both sides of the resistor and now I get 11.90 volts going in and coming out of the resistor. The same thing at the distributor. Do you know how to test the coil and what it should read. I am thinking you test the ohms of resistance.

That kind of voltage readings do you get with the points closed? With the key OFF, rotate the engine by hand until the points are closed, then turn on the key and recheck voltages. Should be near zero on the distributor side of the coil, 6 to 8 volts on the coil side of the resistor as I recall.

If your ballast resistor is not reducing the voltage to the coil you can fry the coil. I've burnt of few of them up on a Ford 351. You may want to try replacing the ballast resistor and the coil and see if that helps.

Inspect the coil for overheating. It's filled with oil, and if you see any weeping near the top, or ballooning of the canister, then it's most likely shot.

I did not know it would make a difference in voltage if the points were open or closed. The numbers above is with the points open. With points closed I get 5.91 volts on the side of the resistor going to the coil. On the switch side i have 11.22 volts. On the distributor side of the coil I get .079 volts. Isn't that kind of low on the distributor side of the coil?

Shenandoah said:

If your ballast resistor is not reducing the voltage to the coil you can fry the coil. I've burnt of few of them up on a Ford 351. You may want to try replacing the ballast resistor and the coil and see if that helps.

Inspect the coil for overheating. It's filled with oil, and if you see any weeping near the top, or ballooning of the canister, then it's most likely shot.

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I don't see any ballooning or weeping of the coil. With the points closed it looks like the voltage is being reduced. I am not that sharp on the electrical workings of an engine.

orville said:

I did not know it would make a difference in voltage if the points were open or closed. The numbers above is with the points open. With points closed I get 5.91 volts on the side of the resistor going to the coil. On the switch side i have 11.22 volts. On the distributor side of the coil I get .079 volts. Isn't that kind of low on the distributor side of the coil?

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As far as I can remember, this is correct. the points (contacts) make the spark, that is they control/time the spark. Those voltages mean the contacts are closing, not too dirty, and the resistor and low tension side of the coil are working.

If you take the cap off, with the contacts closed and the high voltage cable coming from the coil to the cap held close to the engine, you should be able to make it spark by opening and closing the points ( or do it with the points open and use a screwdriver to close them).

The low voltage side of the coil should be around 1 ohm or less, the high voltage side in the thousands of ohms, but that will only tell you if a coil is obviously bad, the only way to know that it works is if it works, that is it sparks from opening and closing the contacts.

Working when cold and failing when hot is a typical electrical component failure.

Yep, that's about what I was expecting on the voltage readings and it should be fine. At least while it's not turning. Try a new coil wire and make sure you're getting spark from the coil and wire by laying the wire close to a grounded metal part and spinning her over a bit. Then if it still won't run maybe a rotor and cap. After that I suggest punting if it still won't run.

Delmer said:

As far as I can remember, this is correct. the points (contacts) make the spark, that is they control/time the spark. Those voltages mean the contacts are closing, not too dirty, and the resistor and low tension side of the coil are working.

If you take the cap off, with the contacts closed and the high voltage cable coming from the coil to the cap held close to the engine, you should be able to make it spark by opening and closing the points ( or do it with the points open and use a screwdriver to close them).

The low voltage side of the coil should be around 1 ohm or less, the high voltage side in the thousands of ohms, but that will only tell you if a coil is obviously bad, the only way to know that it works is if it works, that is it sparks from opening and closing the contacts.

Working when cold and failing when hot is a typical electrical component failure.

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Delmer, I am not electrical smart. To test the coil do you put one probe on the distributor terminal and the other where the high voltage wire plugs in? The low voltage side is that from the battery side of the coil to the high voltage wire plug?

From what you have said the coil could check good cold. But when it warms up it could check bad.
It is raining today so if it stops or lets up I will check the coil. I am still leaning towards the coil but don't want to buy one if that is not the problem.

lantraxco said:

Yep, that's about what I was expecting on the voltage readings and it should be fine. At least while it's not turning. Try a new coil wire and make sure you're getting spark from the coil and wire by laying the wire close to a grounded metal part and spinning her over a bit. Then if it still won't run maybe a rotor and cap. After that I suggest punting if it still won't run.

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All the wires are pretty new, the cap looks good, all the terminals are clean, no corrosion. The rotor looks excellent also. Yesterday I did not think to check the spark from the coil going to the distributor. If there is no spark from the coil wire is it a safe bet that the coil is bad?

On a lincoln welder and a B john deere I have the points and coils. I also converted both from 6 volts. I had new coils on them and they would start well. Kept burning up points. Eventually the tractor quit, it was the coil. Than the welder quit and it was the coil. A local parts guy kept telling me I had to have a 12 to 6 volt resistor ahead of the coil. He ended up selling me a 12 volt coil with the resistor built in. So far so good, although the B dosen't start as quickly as it did.

orville said:

All the wires are pretty new, the cap looks good, all the terminals are clean, no corrosion. The rotor looks excellent also. Yesterday I did not think to check the spark from the coil going to the distributor. If there is no spark from the coil wire is it a safe bet that the coil is bad?

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Yes, could be a shorted coil, or intermittent. I have seen coil wires fail internally, the old style wires had actual wire in them so you could test them with an ohm meter. Also on the cap and rotor, look closely for fine tracks that will look like pencil lines. We used to call it carbon tracking, sometimes they develop pathways for the high voltage to leak off, in some cases through the rotor to the steel shaft believe it or not. That's pretty rare though.

One thing, sometimes on these old gas engines if you set the point gap to factory spec, they aren't closing right. Try setting the gap a couple thousandths tight and see if that helps. Then if you get it running check the point dwell with a dwell meter.

zhkent said:

On a lincoln welder and a B john deere I have the points and coils. I also converted both from 6 volts. I had new coils on them and they would start well. Kept burning up points. Eventually the tractor quit, it was the coil. Than the welder quit and it was the coil. A local parts guy kept telling me I had to have a 12 to 6 volt resistor ahead of the coil. He ended up selling me a 12 volt coil with the resistor built in. So far so good, although the B dosen't start as quickly as it did.

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If you had the 6 volt coil and the ballast resistor, you could jump 12 volts to the coil for starting. Most 12 volt systems with the external resistor worked that way, there was a contact on the starter that sent 12 volts to the coil when the starter engaged. Can be done with a relay, or in one case I know of, I won't say who, a guy made the circuit by running a wire from the starter motor terminal to the coil with a 5 amp diode in line. Worked.

Cheers

orville said:

All the wires are pretty new, the cap looks good, all the terminals are clean, no corrosion. The rotor looks excellent also. Yesterday I did not think to check the spark from the coil going to the distributor. If there is no spark from the coil wire is it a safe bet that the coil is bad?

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Yes, it sounded like your points were closing and opening, so if you don't get a spark from the coil wire, you coil is bad, or the condenser, I don't know which is more likely to fail, but the coil is a lot easier to swap out, any known good coil will work to swap out for a test, just do what Lantraxco said, bypass the resistor for the test.

To test the ohms on the high voltage side, touch one lead to the case of the coil (ground) and the other to the big terminal going to the distributor cap. The low voltage side is the regular terminals, just make sure one of the wires is disconnected so you're measuring the coil, and not the wiring harness.

Lantraxco, Delmer, You guys put me on the right track. Over the years I had forgotten the check the spark from the coil thing. I also never fully understood how the ignition system worked in the first place. I know how it works now thanks to you two guys. Except the part where the points opening and closing makes the coil produce a spark. I don't want to admit how I really thought the ignition system worked.

Today I used a screwdriver to open and close the points to get the coil to produce a spark and no luck. I should have spun the engine with the starter motor also but did not. I borrowed a coil from another similar engine and put that on. I used the screwdriver again on the points and no luck producing a spark. I spun the engine with the starter motor and had lots of spark. I am now up and running again, just one coil short. I tried to test this coil with an ohms meter last summer but was doing it wrong from Delmer instructions. Looks like this coil would fail when it got warm. The engine was hard to start also, starts right up now.
Thanks Delmer, Lantraxco, you guys educated me and gave me good advice. Thanks again

Stinkycat you were on the money, probably a combination of age and the current days heat.

orville said:

I also never fully understood how the ignition system worked in the first place.

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With the old breaker points/coil system it's actually quite simple orville. Electricity works a lot like water, it flows, and it's under pressure. Voltage from battery (via ignition switch) comes to + terminal of coil.

The coil has two sets of windings in it, the primary and secondary winding. The primary winding is low resistance wiring, electricity flows easily through the primary winding. Inside the coil the primary winding is connected to the + terminal and the - terminal of coil. The secondary winding is high resistance wiring, resists flow of electricity. Inside the coil the secondary winding is connected to the + terminal and the coil wire output terminal. The - terminal of the coil is connected to the breaker points, the breaker points are essentially an earth ground.

The theory of operation is voltage comes to + terminal of coil, electricity flows to primary and secondary winding looking for ground. And being like water, it flows through the path of least resistance, so it will flow through the low resistance primary winding, flow out the - terminal, and continue flow to breaker points when they're closed, thereby finding earth ground.

When breaker points open, this cuts off that "easy flow path" of electricity. Electrical pressure builds inside coil until it reaches the point that it shoves the flow of electricity through the high resistance secondary winding and flows out coil wire to a spark plug to find ground. By now the voltage has increased from the pressure to shove electricity through secondary winding which gives you a hefty "arc" at spark plug.

Thanks for the explanation Willie. Electricity still confuses me. I did wire the house I live in, I had to ask a few questions though. That ignition system I never fully understood, I know now how to diagnosis the ignition problem on a primitive machine and fix it thanks to Delmer and Lantraxco. Mostly what I have done most of my life is maintenance welding and fabricating things. If I don't do things repeatedly often enough I do forget. I do appreciate the advice and help you and the other members on this site are willing to give.