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How long does it take to fry a coil. I had my better half helping and the key got left on. The coil was warm to the touch. The key was probably on a least 3 minutes maybe more.I've done it too so I couldn't say much. Thanks

Should have said that this is a 54 235 converted to 12 volt

not a problem for a few minutes, would probably take hours to do damage and that'd be if the points were closed to complete the circuit

Bill

It's much more likely to damage the points than the coil. If the distributor stops with the points closed and the switch is left on, there will be a constant flow of 5 amps or less across the points. The coil primary windings can take it, but that's enough to overheat the points and burn them sometimes. To check for excessive point resistance, turn the ignition switch on, bump the starter until the points close, and measure the voltage from the (-) coil terminal to ground. It should be less than 0.25 volts. Any more, and you'll need to clean or replace the points. What the voltmeter is reading is the voltage needed to force the primary current through the point contacts, which should be virtually zero.
Jerry

Boy Jerry, even 250mv drop across the points sound like a lot of resistance, should be a dead short and I go with your last statement, "should be virtually zero" or else clean or replace the points.
dg

Yep, that's the absolute max I'd allow before cleaning or replacing points. The oldtime Sun dwell meters had a "point resistance" scale which measured voltage drop across the points. We used it prior to setting dwell to check for dirty or burned points.

BTW, a "dwell" meter is really nothing more than a voltmeter. It reads an average of the voltage present during "points open" which should be applied voltage, and "points closed" which should be nearly zero. The longer the points stay closed, the lower the indicated voltage will be, so the scale is calibrated to read maximum dwell with the points closed all the time. With a little simple math, I can set point dwell using a DC voltmeter.
Jerry

And you got a formula for that Jerry, I taint bright enough to figure it out on my own????? Got dozens of volt meters around because of my radio work but nota dwell meter one. Well, cept for the Sears "Sequential Dual Meter Automotive Analyzer" that I bought back about 1972 and used once or twice. That would be a neat meter if I had a muscle car but the problem with it is that its for 12v and 24v ignition systems. 24v must have been used in the big semi trucks because I've never seen anything with a 24v system.
Wouldn't mind having a dwell meter in the shop, a mans never got enough tools, right! Been looking for a nice old dwell meter at grarage sales for a few years and haven't run across any.
Seems like it would be a pain in the ars to use on a Stovebolt anyway. Set the points, start it up check dwell, shut 'er down, re-set the points, start it up, check the dwell, shut 'er down again and on and on. Not like the old V8 distributors with the windows in the cap. Or can one get away by just leaving the cap off and adjusting the points while cranking the engine? Seems like the rpm would enter into the formula somewhere, I‘ll have to think about that one when I get the time.
I've always just used a feeler gauge and many times I got by just setting the points by eye till I could get to a feeler gauge. Granted a dwell meter is gonna fine tune the old gal but they've always run at about 98% using a feeler gauge.
DG

Dwell can be checked at cranking speed, no problem. So far as a "formula", it's just a straight percentage calculation. Let's try a V-8, which fires every 90 degrees of crankshaft rotation. Since the distributor runs at half crankshaft speed, each distributor cam lobe has a maximum "points closed" rotation of 45 degrees. "Dwell" indicates the time, in distributor degrees, that the points "dwell together". If the points are closed all the time, that would be 45 degrees of dwell. Most engines run best with the point dwell set at about 66% of the available rotation, or 30 degrees for a V-8. If the voltmeter reads 1/3 of the voltage applied to the coil while running, that means the points are closed 2/3 of the time, or 66%. Check the voltage applied to the coil from the ignition switch, then adjust the point gap so the voltage at the distributor side is 1/3 of applied voltage while cranking or running. Bingo- - - -the dwell is set!

Since a 6-cylinder engine has 60 degrees of available dwell, a 66% reading would be 40 degrees. The pre-1955 distributor specs list a point gap of .016", which is 38-45 degrees. In 55, they increased the gap to .019", which translates to 26-33 degrees of dwell, so set the "dwell" voltage a little higher than the theoretical 66% dwell when working on a 55-62 235/261 or a point-type 230/250/292 engine. Cornfuzed yet?
Jerry

Oh key do key, I'll print that out and leave a copy in by the throne so I can study it in solitude Jerry, thanks for the explanation. Since I'm a visual type guy rather than an abstract thinker I'll need to think about this for a while. Still somewhere back in my mind the response time of the meter movement seems to be gnawing at me. The needle is going to be on/off and depending on the size of the needle or weight of the movement and the frequency I would think that you would need some circuitry that would respond to the average more rapidly than just a mechanical meter movement
But thanks for the food for thought.
DG

My experience with a dwell meter is there is more movement of the needle at cranking speeds and you look for the middle ground. At least that is how my meter has worked. It is pretty steady at idle. It bounces a little when reving the engines but settles to the same value at about any steady rpm.

Chuck