2 ignition coils failed within minutes

1951 john deere a - did a complete tune up, points,
condenser, rotor, cap, wires, plugs and coil. tractor started right up and ran fine for a few minutes then started misfiring and back firing. had a condenser cause these symptoms before, changed it but still misfiring/backfiring. swapped out the coil, tractor started right up and ran fine again for a few minutes then started misfiring and back firing again. have one used but good coil left,but don't want to use it until I know whats causing this.

found a few possibilities on line. 1- altenator overcharging because of bad regulator but wouldnt fuse blow. 2 - to much resistence in plug wires, they are brand new copper/tinned wires. 3 - bad ballast resistor, which are in the coils, i dont see one anywhere.

what would cause this ?? anyone ever have this happen ??

(quoted from post at 17:53:32 03/21/18) 1951 john deere a - did a complete tune up, points,
condenser, rotor, cap, wires, plugs and coil. tractor started right up and ran fine for a few minutes then started misfiring and back firing. had a condenser cause these symptoms before, changed it but still misfiring/backfiring. swapped out the coil, tractor started right up and ran fine again for a few minutes then started misfiring and back firing again. have one used but good coil left,but don't want to use it until I know whats causing this.

found a few possibilities on line. 1- altenator overcharging because of bad regulator but wouldnt fuse blow. 2 - to much resistence in plug wires, they are brand new copper/tinned wires. 3 - bad ballast resistor, which are in the coils, i dont see one anywhere.

what would cause this ?? anyone ever have this happen ??

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What is the voltage on the electrical system when operating ? What is the writing on the coil ? What resistance does the coil measure across the primaries. There has not been an internally ballasted coil installed in new equipment since the 1940's or so.

Are you using a 6v coil?

Only way an alternator could cause that is if you have no battery in place since the battery is a buffer so as to not let to high a voltage into the system.

A 6 volt coil with out the correct ballast resister can cause that but not common to have a coil burn up that fast unless it has a problem already

its a 12 volt system with an alternator

didnt check operating voltage while running on either one I didn't expect them to fail like they did,now it wont run well enough to check it unless i sacrifice the last coil i have -which guess i will have to do

no writing on first coil it was new from all state ag- 12 volt,with
3.0ohms internal resistor. second coil was the old delco remy that was on it - no writing on it.

as far as resistence across the primaries - how would i measure that i have a multi tester, which contacts ? while running or not running?

one more thing just to be sure, on a 12 volt negative ground system the negative terminal on the coil goes to the distributor, correct?

(quoted from post at 20:40:59 03/21/18) one more thing just to be sure, on a 12 volt negative ground system the negative terminal on the coil goes to the distributor, correct?

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That is correct.

Are you sure your coils are defective? It is possible something else is causing the problem, and cools down while you are changing the coil and then works again when you start back up causing you to suspect the coil.

To check coil, take your multi-tester, set it to lowest ohms scale. Short your probes together and note the resistance reading. Then connect probes to the small terminals on the coil and note resistance reading, then subtract shorted probe reading. If the result is about 1 1/2 ohms you have a 6 volt coil, if about 3 ohms you have a direct 12 volt no external resistor coil. If you get a very high or open reading across the small terminals then the coil is defective.

If above check shows normal resistance try the coil again. Something else may be causing the failure.

When engine fails, immediately turn off ignition switch and check resistance across coil again.

1 1/2 ohm and 3 ohm coils both have one continuous primary winding in the coil can. All the resistance is in the primary winding, there is no separate resistor in series with the winding in a 3 ohm coil

Unless someone has modified the original there is no fuse in the charging system to blow. A panel fuse would be in the light circuit and have no effect on charging

I have seen some defective alternator regulators allow the charge voltage to slowly climb above normal charge level. That is why you need to check battery voltage with engine running

What points are you using and what gap? El cheapo points could be causing your problem.

You mention negative ground. It came from the factory with positive ground.

(quoted from post at 21:06:50 03/22/18) You mention negative ground. It came from the factory with positive ground.

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But he also mentioned that it had an alternator. So chances are that it was switched over.

have not been back out to shop to try these different things. should have time this weekend. alot of good ideas to eliminate the problem. will post back with my findings.

rvirgil you were right. checked things out and discovered coil that was supposed to have 3 ohms internal resistor only had 1.2. condenser was shot and points were burned. bought and installed another coil that showed 3.1 ohms and new points and condenser and all is well. generator putting out 13.82 volts at idle and 13.89 wide open.

whats the maximum voltage that should be going to distributor from negative side of coil ?? system is twelve volt neg. ground

"whats the maximum voltage that should be going to distributor from negative side of coil ?? system is twelve volt neg. ground"

Near battery voltage with points open, near zero volts with points closed