Cooking a Coil

So, my question is, why would leaving the key on until the battery goes dead or the points fry damage the coil? Logically, it doesn't seem to be any worse than a coil's normal day at work, possibly better due to the shorter duration of the event.

Another question is, why can't we do an effective test on a coil with a ohm meter? The primary winding can be read as a low resistance across the + and - terminals, and the large winding can be read as a higher resistance across the - and the coil-to-distributor wire connection terminals. If the coil is fried, one of these windings will be open.

Ramrod

When we setup production to do a new coil design we have to validate how it is made. Typically we put a couple of hundred coils in a machine that cycles from -40C to +150C. Thermal shock stress test.

The mechanical moving of the coil guts when going hot and cold can cause insulation to break down or wires to break. We usually test for 150 cycles. Poor designs usually show up with failures in the first 8 to 10 cycles.

If ya don't ever leave the key on why would a coil ever fail? Poor design?

No wonder things heat up so much when stopped, key on, points closed!

You can test a coil if it is open or shorted with a good meter. (Accurate meter) What you can not test for is an intermittent coil.

When they make coils they start with copper wire that has a very thin coating of insulation on it. They wind a bunch of turns on some type of mandrel, make the primary, then make the secondary. When they put these two coils inside some kind of case they fill the case with another type insulated filler. Dell calls it tar! Ha.
In any event, if a coil has overheated, the thin insulation around the wire can break down or the tar can break down. When the coild gets up to operating temperature, everything swells with heat. When everything gets warm, moves around, wires can touch each other where the insulation has broken down. When things cool down, wires can move, not touch anymore and everything is fine. That creates an intermittent short. Tractor works fine until the coil gets to operating temperature, shorts some of the coils of wire, sparkies drop off, tractor no worky. Coil cools down, short goes away, tractor works fine, for awhile.

Got long winded. Sorry.

Ramrod

Unfortunately, unless you have specialized lab-grade ohmmeter (called a wheatstone bridge), you can NOT measure the reduced resistance effect of one or two turns being shorted out ...but... you can eazily observe the effect in the loss of snappy blue-snott sparkies. The primary winding in the squarecan N-Coil is only about 30 turns and total primary resistance is only about 1-ohm..... .....Dell, former Boeing Cal Lab engr

And yes, even new squarecan frontmount coils will fry from accidently leaving the ignition switch ON long enuff.