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MG MGB Technical - Car won't start - coil?

1970 MGB - I have just reset the points and the car will not start, after a lot of cranking it appears to cough but will not run, I have replaced the points with an old set, same problem. I am sure the points are in place correctly and the distributor is roughly in the correct position.
I did leave a dwell meter connected to the coil and earthed to the car for about 1.5 hours while I had lunch, and before attempting to start the car I noted that thye coil was hot to the touch.
Have I buggered the coil???
D G Levy


AFAIK a dwell meter is only a volt meter, which measures the average voltage across the points. As such it would only draw a very small current, not enough to do anything to the coil.

I would double check the points installation, making sure that all the insulating washers are present. To easily check this use your dwell meter, connected across the points, stick a piece of paper in the points, so that they are electrically open. Your dwell meter should read full scale, ie full 12V across the points. If it doesn't then the moving arm of the points is somehow connected to ground, check those washers again.
Maybe you have caught the wire from the points to the outside world under the points and crushed the insulation, causing a short to ground.

Your coil may have survived, but until you can guarantee that the points do open and close, electrically, there's no point in worrying about it. Eventually, to be on the safe side, so you don't wind up stuck in the back of nowhere, a new coil might be advisable.

Herb Adler

Sounds like you left the ignition on, which *is* bad for the coil.

Sounds like you may have got things in the wrong order around the points - the points spring condenser and coil wires must all be in contact. There must be a stepped washer under the coil spring with the step seated inside the spring, and another on top of the wire spades with the step going down through them and the spring. This keeps all three away from the mounting stud, if anything touches that or you don't have all three in contact it probably won't start.

Either that or they are adjusted wrong and aren't opening when on top of the cam.

A dwell meter is a little more than a voltmeter, but operates on much the same principle. Both when connected between the points wire and earth, with the ignition on and the engine cranking, should flicker back and fore between two readings. If it doesn't then either the points wire is permanently grounded, open-circuit, there is no 12v feed to the coil positive, or the coil primary is open-circuit. A volt-meter will indicate exactly which.
Paul Hunt 2010

DG. As noted, if the ignition switch was left in the on position, and the points were closed (allowing current to flow from the coil input terminal, through the coil, to the points, then to ground) you have probably fried something. The one time I have seen this, the melted wires extended back to the ignition switch.

First, check for power going to the coil. If you have power to the coil, follow the electrics forwards and check for power through the coil and to the points. If you do not have power to the coil, trace the circuit backwards until you find where you do have power, then forwards to find where the problem is.

Les Bengtson

Thanks for the advice. I appear to be getting a spark at the spark plug which is consistent with properly operating points, but the spark at the plug appears to be very weak (battery is fully charged). Could this be an indication of the coil being faulty? Could someone describe the difference between a 12V spark and a high voltage spark.... Have carried out checks as described above and the circuit appears to be ok.
D G Levy

Getting spark at the plugs - good sign. Have you checked that the plug wires are connected in the correct firing order?

J Tait

If the spark is very weak there is a good chance that the condenser is faulty. Have you a spare that you can substitute. It's worth a try before replacing the coil.
Iain MacKintosh

Indeed, I'd check the condenser. With a failed condenser (or not connected correctly!) the spark will barely jump a plug gap, and you will get a lot of spitting and arcing at the points.
Paul Hunt 2010

Thanks everyone for the help.
Although I am embarrassed I am posting the solution to my problem.
I connected one of the wires on the wrong side of the insulating plastic washer on the points, IE: the wire was shorting out on the post of the points. I have changed the points every year for the last 7 years with out a problem, my only excuse is that I probably became to complacent, the first time I changed the points I would have taken notes and drawn a sketch. In trying to solve the problem all my books stressed that this should be checked first but I was convinced that I had not made such a simple error.
D G Levy

Ah well at least now you know and you will probably never make this mistake again.
Iain MacKintosh

"I have changed the points every year for the last 7 years"

Every year? Crikey, how many miles a year do you do?
Paul Hunt 2010

DG - While our cars are dead simple, they are not "six sigma". I made a similar mistake years ago when I replaced an aftermarket set of points with a Lucas set. The insulation scheme was different, and I put the wires in wrong. It took a quick check with this BBS to set things right, but I was a little embarrassed, too.

I now keep a schematic in my tool chest.

John Zajac

This thread was discussed between 26/06/2010 and 03/07/2010

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