Overcurrent burning coils

[Robo33]

1949-50ish 8N with front mount distributor. 2 year old 12v conversion. Has run great with exception of a few sets of points that were my fault for not turning off the ignition a few times when it ran out of gas. Last fall, about an hour or two into mowing, it died. There had been no recent work done on the tractor. Had been running great all summer. Found that it had no spark. Traced it to a melted coil. Replaced the coil and it started right up but the ammeter pegs to +30A about 1-2 seconds after it starts. So I turned it off immediately and started looking for the short. I’ve tried everything. Replaced the ballast resistor, twice just in case. Installed a completely new wiring harness. Replaced the alternator, distributor, solenoid, and ignition switch. Triple checked battery wires. I fear running it too long but I did notice if I let it run for about 10 seconds the ammeter drops off from pegged to around +20. I only did that once and after I shut it off I noticed the wire from the solenoid (battery) to the ammeter had already begun melting the end of the insulation at both ends. What am I missing?? I’ve literally replaced every electrical component on it.

 

[56_800 Ford]

It's normal for the charging system to go high right after start up. If you are melting wires at 20A I would check the size of the wire. 30A requires a Number 10 wire. 20A a number 12 wire. Check the size of the wire from the solenoid to the ammeter.

 

[Btzj02]

I am going to assume that the increased amperage is a change from normal operation. Two things will cause an increase in amperage, increased voltage, (possible alternator internal voltage regulator problem) or/and decreased resistance, (possible short). I prefer using a volt meter for trouble shooting. Start with engine off and ignition key on and take and record voltage readings at every terminal screw and battery post you can find from the alternator to the battery and the coil. While the engine is off do this twice, once with points closed and once with points open. Obviously pay attention to the overheating of the wire to prevent any problems. Then perform a third test like the first two while the engine is running, (high amperage). Have someone help you so you can minimize the time running. Compare readings from each test and look for voltage changes at each terminal screw and battery post.

 

[showcrop]

Over charging is frequently found to be a bad ground, most likely in or from the voltage regulator.

 

[Eman85]

You converted to 12v so I'll assume you used a GM style alternator with an internal regulator. Bad connections or improper sized wire is usually the cause of melted wires. Since it's now wired for 12v how it's wired is the question. When running did you monitor voltage? What gauge wire runs from the large terminal on the back of the alternator to the battery and how is it routed?

 

[Jim L WA]

My first thought is, what caused the coil to melt after two years? Are you using a 12 volt coil?

 

[showcrop]

My first thought is, what caused the coil to melt after two years? Are you using a 12 volt coil?

A failed ground happens gradually.

 

[Hobo,NC]

From your description of the short duration you spoke of the charging system is working as it should.
BTW + is not a short your charging system is working hard to put back what you used to crank the engine. It will do this for a a short time that is why you will see it tapper off over time. You need to post the voltage readings while all this is going on. Your wimpy dash amp meter is noting but eye candy it offers little to diagnosis modern 12V starting charging system issues. Chunk it and put a voltmeter in that hole. There is a time to use amp meter but its not now its when you get into serious diagnosing of the charging system. At this time voltage is the primary concern. Without a verified voltage reading its anyone's best guess are a shadetree diagnoses.

 

[sotxbill]

1949-50ish 8N with front mount distributor. 2 year old 12v conversion. Has run great with exception of a few sets of points that were my fault for not turning off the ignition a few times when it ran out of gas. Last fall, about an hour or two into mowing, it died. There had been no recent work done on the tractor. Had been running great all summer. Found that it had no spark. Traced it to a melted coil. Replaced the coil and it started right up but the ammeter pegs to +30A about 1-2 seconds after it starts. So I turned it off immediately and started looking for the short. I’ve tried everything. Replaced the ballast resistor, twice just in case. Installed a completely new wiring harness. Replaced the alternator, distributor, solenoid, and ignition switch. Triple checked battery wires. I fear running it too long but I did notice if I let it run for about 10 seconds the ammeter drops off from pegged to around +20. I only did that once and after I shut it off I noticed the wire from the solenoid (battery) to the ammeter had already begun melting the end of the insulation at both ends. What am I missing?? I’ve literally replaced every electrical component on it.

Do you have a (dropping) ballast resistor inline with the coil? Your charging system is working correctly.

 

[Hemmjo]

SNIP...... I only did that once and after I shut it off I noticed the wire from the solenoid (battery) to the ammeter had already begun melting the end of the insulation at both ends. What am I missing?? I’ve literally replaced every electrical component on it.

To me the key could be right here. "already begun melting the end of the insulation at both ends". If everything is correct the whole wire should melt as the same time. The fact the wire only melted on the ends indicates bad connections. Bad connections create high resistance and HEAT.