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MG MGA - Totally stumped

Ok, I'm totally stumped and need more help. I have a 1500 MGA that keeps burning up points, condensors etc. The car hardly runs, has no power and backfires out the exhaust. Ignition problem right? I originally found the screws for the point plate to be loose and points burned. I tightened the plate and replace the points and condensor. The vacuum advance and all of the wires inside the distributor were checked and fine.Ran fine for about 30 miles and started the same thing again. Tried new points, rotor and condensor. Ran fine for another 30+ MILES AND DID IT AGAIN! This time I replace the points, rotor, condensor (with a different brand) AND coil as the coil was only showing about 1.5 ohms resistance. I replace the coil with one that has an internal resistor and checked to make sure of the correct 3.5 ohms resistance. After about 30+ miles its doing the same thing again and I'm at a loss. HELP!!!
491

Is the coil polarity wrong : ie have you tried swapping the low tension terminals on the coil over
dominic clancy

A poor engine ground will burn up points very quickly. Check the cable from the left front of the engine to the frame.
- Ken
Ken Doris

Burning up points could mean different things. Are the point mating surfaces severly pitted? Or has the plastic part that sits on the cam melted?

The coil should be above 3+ ohms. At 1.5 ohms , it is a 6 volt coil and should be run with an external ballast resistor. (Not std MGA parts). A 1.5 ohm coil without ballast resistor will melt the plastic within 30 or less miles. Don't aske me how I know. Pitted points that quickly may indicate either a faulty condensor or bad wiring of the condensor to the points. Since you have replaced the condensor, I would recheck the continuity of the condensor wire to the point arm. Perhaps the assembly of the connection is in error.

Chuck



C Schaefer

While it's a long shot check the running voltage of the electrical system at different points starting at the generator and working your way to the battery with the engine at about 2000 rpm. Running you should see about 13.5 to 14.5 volts between any component connected and ground. A run away voltage regulator could be raising the system voltage to high causing the burning. This also will normally burn out light bulbs quickly. As I said, it's a long shot, the info listed above by others is usually the cause.
J Heisenfeldt

I had the same problem earlier this summer. This is the response I received from Barney Gaylord.
Barney response
There is only one thing that makes a coil excessively hot - too much current. That can only happen if the coil has too little resistance in the primary winding. This might result from an aging coil breaking down wire insulation and shorting windings together, but that only happens after some years of use
The more likely cause of too little primary resistance is use of the wrong ignition coil. Standard ignition coil for the MGA should have about 3.2 ohms primary resistance. With 13 volts system voltage this will draw about 4 amps (when the contact points are closed). A high output coil like the Lucas Sport coil will be slightly lower resistance, perhaps 2.7 ohms, drawing more like 5 amp (when points are closed).
To have an intuitive idea of how much heat this generates, 4 amps at 13 volts is 52 watts of power (heat). This is rather like the heat output of a 50 watt incadescent light bulb. Limiting the operating temperature depends on being able to dissipate the heat to the surrounding air by conduction and convection, or through the bracket to the base mount by conduction. The coil will get quite warm when operating, but it should not be so scorching hot that you can't put your fingers on it.
When the engine is running the contact points are open part of the time, stopping current flow and reducing heat in the coil. The stock Lucas distributor has a dwell of 60 degrees, meaning the points are closed for 60 degrees out of every 90 degrees rotation of the distributor. Points being open 1/3 of the time when running reduces coil heat by a proportional amount, down to about 35 watts net.
A ballasted ignition system, as used in later model cars, uses a 1.6 ohm coil wired in series with a 1.6 ohm power resistor (ballast resistor). This is essentially a 6-volt coil with system voltage being cut in half (to the coil) by the ballast resistor. When wired in series you get 6-volts drop across the resistor and 6-volts drop across the coil. Having the same total circuit resistance this will draw the same current and will generate the same total amount of heat. A big difference is that half of the heat goes out through the ballast resistor, and only half of the heat is generated in the coil. This can reduce coil heat to about 18 watts (when running with 60 degree dwell). The MGA does not use this system.
The bad condition can occur if you install a low resistance coil intended for use in a ballasted ignition system, but you use it without the series wired ballast resistor. The 1.6 ohm coil will draw about 8 amps and generate 104 watts of heat (when points are closed). Running with 60 degrees dwell this is still cooking the coil with 70 watts of heat. This is when you may have coils fail one after the other from over heating. Moral of the story is, always check the primary resistance of an ignition coil before installation. Something around 3 ohms is good for the non-ballasted ignition system. Primary resistance around 1.6 ohms is bad, bad, bad. The increased current also has potential to overheat the contact points, possibly melting down the rubbing block, which could result in increasing dwell and higher heating.
The first one was used for about 100 miles before dumping all it’s fluid onto the ground. I had it wired backwards.
Reverse primary wiring makes no difference in coil heat, because it still has the same internal resistance. Otherwise the car runs just as well with reverse polarity at the coil with no problem.




M. E. WADDS

This thread was discussed between 26/09/2007 and 28/09/2007

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