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MG MGB Technical - Coil for Electronic Ignition

Just bought a Newtronic ignition system from Moss. It is an optical type system. Can anyone recommend a suitable coil to use with it? I haven't fitted it yet, but if any one has this system I would be interested in your comments.

Iain.
IJ Maynard

Iain-
The type of ignition triggering system that you put in a Lucas distributor is irrelevant to the type of coil that you need. Unless you've increased the compression ratio or added a hotter camshaft, your stock coil should work just fine.
Stephen Strange

Stephen,

Thanks for the reply. My B has been modified with a fast cam, bored out to 1950cc and a webber carb fitted. I have been finding it difficult to start, especially in the winter. That's why I've decided to give the electronic ignitiom a try. It has a lucas 43d dizzy with no vacuum advance. I also thought I may as well change the coil too.
IJ Maynard

Lucas Sports coil will do the job fine..

I run a Aldon Igniter with the Sports coil with no problems..
K Harris

Iain-
Lucas Sports Ignition coils are available for ignition systems both with and without the ballast resistor. The original Lucas Sports Ignition coil (Lucas Part # DLB105) is the twelve Volts sports ignition coil for ignition systems without a ballast resistor. The alternative Lucas Sports Ignition coil (Lucas Part # DLB110) is for ignition systems with an external ballast resistor. Most Sports Ignition coils are usually six Volts and have a resistor from the ignition live wire to the ignition coil live wire (often called a “ballast” resistor). The only way you can be sure of whether you have a direct (12 Volt) or a ballasted (6 Volt) feed to the coil is to reconnect the white or white/light-green wires to the positive (+) terminal of the coil, connect a ground to the negative (-) terminal of the coil, turn on the ignition, and then measure the voltage at the positive (+) terminal of the coil. If you measure battery voltage, i.e., approximately 12 Volts, then there is no ballast in circuit, in which case you must fit a 12 Volt coil. If you measure less than battery voltage, then you probably have a ballast in series, in which case you must fit a 6 Volt coil. If you measure approximately 9 Volts, then it would appear that you have ballast in circuit, but with a 12 Volt coil, which incorrect. This combination will give reduced spark. However, if you have a 12 Volt coil in series with two lots of ballast, then you will also measure 6 Volts, but this combination will reduce the High Tension voltage by approximately 75%(!), which is why you have to measure both the coil resistance and the voltage in order to make sure they are compatible.
Stephen Strange

If it's not starting properly then there is something else wrong, or the coil itself is faulty. The Weber is more likely to be the cause of poor starting than the coil, or points for that matter.

You don't say the year, but chrome bumper cars used 12v coils that measure about 3 ohms for the primary, rubber bumper use 6v that measure about 1.5 ohms, in series with a harness ballast resistance for running that also measures about 1.5 ohms making the total resistance in circuit also about 3 ohms and hence the same current.

It's essential to get the correct coil for your wiring, a 6v coil without a ballast in circuit will overheat and burn points if you have points, as 12v coil *with* ballast will give weak sparking and poor starting. You must measure the primary resistance of any coil you have, or buy. You must also determine whether you have ballasted wiring or not, someone could have bypassed it already on a rubber bumper car, or added a resistance to a chrome bumper car.

Some people fit 12v coils to rubber bumper cars and bypass the ballast resistance, but that makes starting *worse* as the purpose of the 6v coil and ballast is to give full battery voltage to the coil during cranking to get a more powerful spark to aid starting. Sometimes this is done as part of fitting electronic ignition, which is counter-productive. Some electronic ignition modules indicate that as long as they are powered with a full 12v the coil can still be powered via the ballast and hence stay as 6v. Some (funnily enough) say you must fit a 12v coil (their own of course) and bypass the ballast. Some can be powered from the coil terminal i.e. the 6v feed.
Paul Hunt 2010

Thanks for all your replies. For the record the car is a 1974 Chrome bumber GT. I didn't do the mods so I guess I need to find out whether I have a ballast resistor coil or not.

IJ Maynard

Ok, should be a 12v 3 ohm coil then, or 2.4 ohms if it is a Sport coil, no less than that. Unless someone has installed a ballast resistance ...

Coil is easy to measure, to check for ballast turn on the ignition, make sure the points are closed, and measure the voltage with respect to earth on the two coil terminals. You should see 12v on the white on the +ve, and 0v i.e. an earth on the -ve. That is unballasted. If you see less than that i.e. 9v, or 6v, there is a ballast resistance in circuit.
Paul Hunt 2010

This thread was discussed between 03/04/2010 and 09/04/2010

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