Weak spark

I have an Allis Chalmers WD45 with distributor and has been converter to 12 Volts. It would not start this summer gas is good, traced down to weak spark, cleaned points checked gap still weak spark. Put bought new coil from NAPA with internal resister and it started right up, great ran for a while so decided to go for a ride started out drive and it died checked it out weak spark again. Is there anything that kills coils? Any suggestions. Thanks

Check to be sure there's full battery voltage to the coil.

A poor connection(s) or a failing ignition switch can case voltage drop to the coil and result in a weak spark.

Check with the breaker points CLOSED so current is flowing through the circuit, making any voltage drop show up.

Make the test in a quick fashion and then shut the ignition switch "OFF" to avoid overheating the breaker points and coil.

This post was edited by wore out on 07/22/2021 at 11:33 am.

Easy way to test things is with a simple piece of wire if not out in the bright sun. Hook the wire to the ignition side of the battery then touch the ignition side of the coil as in not the distributor side. With the points closed you should get a small spark. With them open no spark. If you don't get a spark with them close the points are bad or dirty. If you get a spark when open you have a short in the system

With intent to educate, there is no internal resistor in modern coils. The statement printed on them is misleading in that if there is no external resistor needed that there is one inside. The coil is just manufactured with appropriate wire size and number of turns of that wire to operate correctly at charging voltage. I also agree that a failing supply voltage. A wire with a break internally, or a bad connection, or failing switch. Jim

The mag on Farmall would make a good spark until it warmed up.
When condensation inside mag warmed up, steam shorted out the cap on mag.
Only happened after it ran of 10-15 minutes.
You might have water inside distributor. Remove distributor cap when you have a problem and look for water.

Ted you ask a good question ' Is there anything that kills coils?'

Sure, excess current or excess heat are a couple that can. If you have a 12 volt tractor and a CORRECT full true 12 volt rated coil (around 3 ohms) Id expect no more then 4 or so amps of coil current. If its much greater like 6 amps the coil can overheat and be damaged. That could be caused by an incorrect too low of resistance coil. A full true 12 volt rated coil would have around 3 ohms of LV Primary resistance thereby allowing about 4 amps of coil current HOWEVER if it was a 6 volt coil (around 1.5 ohms) and drew 8 amps it would overheat and be damaged. CHECK YOUR COIL RESISTANCE AND CURRENT WITH IGNITION ON AND POINTS CLOSED Id expect 3 to 4 amps if allllllllll is well ???????

If the coil is NOT the problem but the spark is weak, you could have burned pitted corroded mis gapped or carboned up points (Always check and suspect points first),,,,,,,,, A faulty condensor,,,,,,,,,,,,,A bad/resistive connection causing excess voltage drop (check coil voltage with ignition on and points closed, if its way low that can cause a weak spark),,,,,,,,,,,A faulty resistive voltage dropping ignition switch (again check coil voltage)

While sure coils can go bad, I suspect the points or condensor or a wiring or connection or switch problem FIRST. After that bad plug wires or a dist cap or rotor problem.

Run my Ignition Troubleshooting Procedure may help

Best wishes good luck

John T
John Ts Ignition Troubleshooting

Thanks I will try those suggestions.

In addition to all the good advice I would point out that the spark is going to ground so the ground connection is very important also.

Double check what coil you actually have. Go by the part number actually on the coil or measure the ohms across the primary terminals out of circuit. Should be around 3 ohms.

Those coils are dinosaurs, often reboxed and not in the computer correctly.

Also check the distributor shaft for side play. If worn, the points will not stay set, they will arc and burn, cause the coil to overheat.

A correct coil running as it should will not be hot to the touch, should be about the same temperature as the surrounding underhood components.

Try an old condenser if you have one. The new ones are known for being bad out of the box.

To test the ignition switch, turn the engine until the points are closed, put a volt meter from the + coil terminal to ground. Turn on the ignition. The voltage should be very near battery voltage. Wiggle the ignition switch lever or key, the voltage should remain steady. Try moving the wiring harness, especially behind the switch. But work quickly, the coil will overheat if left on and grounded very long

Try a different condenser.

The plug bodies are the ground, the battery ground, the points fixed bracket, and the condenser are the other grounds. Jim