Coil Testing

I know "Finish a project before you get started on another one". If I did this I'd get nothing started.
Anyway since a got a full head'o steam on the GT might start asking about the TD. I tried to start it a few years ago but it did not turn over. No spark for me but ran when the kid parked it in 1989.
How can I test the coil to see if that is good?
These are the 2 that came with the car so I wan't to rule 1 out.

Toss 'em both & go new - they're not that expensive.

I'd actually like to find out how to test them. I have a pre-war oil coil in my TC that I'd love to test and see if it's working properly. The car runs great, but if the coil is a weak link I'd like to refurbish it. No idea how long it's been in service but it's probably 70 years old now.

Steve, If it's still working and hasn't developed any leaks... no need to test it. But, If it's still working... Good show! When they go... it usually means a part or more of the individual wraps of copper coiled wire have developed a break in the line (or several). If yours is able to be opened and examined/rebuilt... I'd keep running it, and meet a few guys that like to build Tesla Coils. Do a search... Any of them could rebuild your coil from a dead one, and the oil core is a very nice thing that repairs itself as opposed to the modern plastic cored one's that aren't rebuildable at all.

Excuse me for butting in. I normally hang out in the Healey forum. You can ohm the primary side of the coil. You need to know if the car requires a coil with it's own ballast or not. For my AH, the coil was supposed to be 3 ohms. If you hook up an ohm meter and check it, a good coil should be close to the stated balast value. I had a bad coil and it showed much more than the 3 ohms. I think it showed around 15 to 17 ohms instead of 3 ohms. This is an indication that the coil is bad and the output voltage is reduced by the same factor as you found the primary out of spec. For instance, If you find 15 ohms and the coil is supposed to be 3 ohms, that is five times out of spec on the primary side. This will reduce the output voltage inversly by the same factor. If the coil is supposed to put out 15 KV, it will only be putting out 3 KV if the primary side is 5 times the normal value out of spec.... Any lucas coil from the 50s or 60s that has never been replaced is highly suspect in my book, especially if the car is constantly backfiring, or runs bad after it gets hot, and the points are correctly adjusted. If it doesn't run at all, the coil could also be open instead of reading high. It is easy enough to check with an ohm meter across the two small wire connectors.

a much better reply than he first thing that popped into my mind.... it was akin to checking a 9v battery. I highly recommend you do NOT do that with an ignition coil

Kenny, it's still working and gets quite a few admiring comments when I lift the hood. I do have a very experienced British car electrical specialist in my area, Jerry Felper. He already mentioned (after complimenting the coil!) that he can rebuild it if or when the time comes.

To test your coil, the first thing is to do as Ed K suggested and ohm it out. Remember to make your resistance measurements with the low-tension wires removed. Also remember that if you are ever working on a 6v car or a car with ballast ignition, it's coil will have primary resistance less than 3 Ohms.

Once you've ohmed the coil out, you should look at whether your coil is positive or negative ground and you can test the coil operationally. Again, paying attention to the ground of your car, connect the ignition switch side of the coil to the "hot" (not ground side) of your car's battery. (As before, with the primary wires removed). Place a spark plug wire in the center, high-tension terminal of the coil and put a spark plug in the wire's other end. Rest the spark plug on the car's chassis or a good ground point. Now "tap" a jumper lead from the low (distributor) side of the coil to the car's ground. This should give you a nice fat spark on the plug. If it doesn't, there's something wrong with the coil.

Once you are comfortable that the coil will spark, you should still be prepared for its failure. Most of these coils are oil filled and can slowly leak over the years. Without the oil the coil will overheat and die.

Sorry for a potentially rookie like question. 6v vs. 12v.
1 -Why for & what is the difference?

My TD did not come with a battery & I assumed that ff I took my Midget's new battery, I could get her started. All the lights & wiper motor work just can't get it started.

2 - Did the TD originally come positive ground?

Probably positive ground, and 6 volts... 12v will fry somethings on that car.

Uh.... Perhaps it was converted but our TD was twelve volt for as long as I can remember and being the tight Scot that my grandpa was I highly doubt he went to any trouble to convert anything from 6 to 12. I seem to recall that it was positive ground but I can't say for sure - someone who knows will chime in or I'll dig out my TD manual when I get home tonight. I can clearly picture in my mind the beautiful braided ground strap but I can't for the life of me remember which post it was hooked up to.

I only mentioned 6v because my comments were directed at old cars in general... not specifically the TD. I have never touched an MG that old but would presume that since most other LBCs didn't go negative ground until after 1960 that the TD was probably positive ground. Obviously others with more direct experience will know and should correct me if I'm wrong. Keep in mind that there isn't much to changing the polarity so a PO may have switched it to negative ground.

TD had 1 big battery up front - & was positive ground.

If you were to inadvertanly hook up a PG car to NG, what might happen?

Would I still have lights and a Wiper motor that functioned?
Might I not have a spark praytell?
Would there be a large explosion?

You could fry various electronics... not that there are that many, but... voltage regulator comes to mind, and possibly electric fuel pump (blow a diode), and electronic tach... maybe a couple other things.

I don't think you'd be able to start the car if you connected the battery backwards. You'd definitely fry something somewhere in the electics... let the smoke out of some of your components if you will /ubbthreads/images/graemlins/smirk.gif

MG TD was always 12 Volts. I believe they were also single battery but not absolutely sure. They were also all Positive Ground.

If you connect the battery backwards, the car should still run, but the charging system may not work until you re-polarize the coil. I don't know exactly what would happen on a TD, but I do remember accidently connecting a battery backwards and starting the car. If you had electronics in the TD, you'd fry them.

but... how would that work? wouldn't the starter turn the engine over backwards (assuming the solenoid isn't effected by the reversed magnetic field)?

No idea! But it cranked.

All will work. TD's didn't have much in the way of "dainty" electricals. Polarity on the coil (swapping the two small terminals) and "flashing" the generator are about the only things I can think of and it would run fine. Cars with electronic tach, positive ground radio, electric windows, etc. would be more problem.