Ballasted Ignition System

My MGB came with Luminition Magnetronic Ignition which has worked well. I knew from the previous owner that the car originally had a ballasted coil – he had a standard 12v sports coil fitted when he had the car rewired. Fortunately when the car had a new loom it was wired up correctly to take a ballast resistor if it had been fitted – as was standard for rubber bumper MGBs.
Even though the car has always started ok when a Lumenition Magnetronic coil came on eBay at a good price I decided to buy it – and it came with a new ballast resistor.

The theory of the ballasted coil is that it provides an extra voltage boost whilst the engine is cranking to give a stronger spark whilst starting up.

The new ballasted coil is designed to provide a normal spark at 8v rather than 12v. The ballast resistor reduces the ignition voltage down to the 8v whilst the engine is running – but during starting the ballast resistor is bypassed giving full voltage for a short period of time. In fact the full voltage whist the engine is cranking is about 10v not the normal 12v due to the battery having to work at maximum whilst starting the engine.

In fact with the old coil when the engine is starting it has to provide a spark on a reduced voltage of 10v weakening the spark …….with the new coil that normally runs at 8v it gets a short boost of 10v whilst starting providing a stronger spark.

After fitting i checked the various voltages with a voltmeter and it does…… “what out says on the tin”.

There was definitely a noticeable difference when starting the car from cold. It literally started instantly rather than after 2 or 3 compression strokes.

See my follow up post at Rubber Bumper #MGB – check you have the correct coil?