Bread Crumb Link


 Maintained by:
 Electronic ignition kits for classic Cars

Shop search

Shop categories

Shop pages

  • Ballast ignition guide
Using Colortune
MG Electronic ignition Accu Spark






AccuSpark ™ Help


One of the most misunderstood and misinformed questions with ignition is

Do I have a ballast ignition ?

And how to test for it ?



First a quick explanation of how a ballast system works

When a car is first started, the oil is thick and the engine is cold and hard to turn, so the starter will draw a large amount of current. Whilst cranking, you may notice the lights dim as the engine turns, this also means that the coil is robbed of current which in turn causes a reduced spark that will make the car hard to start. Often you will find the car starts at the very moment you let go of the key as this is the point it full voltage returns.

To overcome this the ballast ignition was developed. The car is fitted with a coil that runs on a reduced voltage. The coil is then fed through a ballast resistor or wire that reduces the current to the correct voltage for the coil.

Now the clever bit. When the engine is cranking the starter solenoid feeds the coil directly with 12 volts, bypassing the resistor. Although the car is cranking and all the voltage is reduced throughout the electrical system, the coil is receiving more volts than it requires which compensates for the voltage drop, thus giving a full spark.



How to test

Most people will tell you to test the feed for the coil. This tells you nothing - it will always show around 12 Volts because there is no load on the system, the resistor has nothing to resist ! ( unless the points are closed )

If there is no ballast resistor visible you may have a ballast wire inside the loom. To test for it proceed as follows:

Check voltage of battery with volt meter

Remove the wires from the negative side of the coil ( negative earth cars )

Connect a temporary wire from the negative terminal of the coil to earth

Turn ignition on ( nothing else switched on )

Now check the voltage on the coil, put red probe on + side of coil and the - probe to earth

If the reading is less than 80% of battery voltage there is portably a resistor in the system. If it is more than 80% you probably have a standard system

Remove the temporary wire and reconnect wires .

If your reading is less than 80% you should use a ballast coil , AccuSpark Blue
If you reading is more than 80% you should use a non ballast coil , AccuSpark Red


As a final note :
A standard 12 volt will only have a single 12 Volt feed to coil
A ballast system should have 2 feeds. One resisted feed and a second full 12 volt feed when the starter is engaged

Do I have the correct coil ? Testing your coil

To test coil set voltmeter to Ohms, test between -and + terminals. A reading of around 3 is a standard coil, a reading of around 1.5 is a ballast coil.

Removing Ballast resistor

What do do if you wish to remove the ballast system and go with a standard 12 Volt system ?

If you have a visible resistor you just need to remove the resistor and join all the wires.

If you have a ballast wire in the system proceed as follows:

Remove the 2 feeds into the coil and tape up
You can run a new wire from the coil to the back of the ignition key
You can also take a feed from the fuse box. This must be switched by the key and be on the live side not the fuse side. To test, it should still work with the fuse removed


Any feedback if this was helpful would be appreciated











Additional site navigation