Web page updated

October 2017  by

Stephen KImber stevek100@yahoo.co.uk


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Steve's technical page

How does a ballast  ignition system work?


For the older modern cars (contradiction in terms) there are two systems used-with and without a ballast resistor.

The theory is that when running, a car does not need 12 volts in the primary circuit to produce the sparks and by doing so means the coil is not so efficient. In a non ballast setup the full 12 volts is directly across the coil primary windings and switched by the contact breaker points in the distributor. If you put a dc meter on the + coil teminal (-ve earthed cars) and the points are closed you should read 12 Volts.

Basically a ballast coil works at 6 volts instead of 12. This means it is not so electrically stressed as a 12v unit. However running at 6 v is ok for the engine running, but when cranking the engine at startup the full 12v is applied to the coil to ensure a good spark. To run at 6 volt a ballast resistor is used in series with the coil. This resistor can be a ceramic package or on some cars like MGB it is a resistive wire as part of the wiring loom. That's a basic explanation. A non ballast coil has a primary resistance of 3 ohms, a ballast coil around 1.5 ohms. The ballast resistor can be a ceramic block with two contacts or as in the case of the MGB a resistive wire which runs in the wiring loom around the front of the car for cooling purposes.

All coils all look the same and its easy for them to be both miswired and misused.


Should ignition coils be mounted face up or down?


My preference is face up with the contacts to the top. The main reason for this is the wiring is easy to see and it is relatively easy to get a meter to the contacts. Coils get hot in operation and are cooled in two ways. Normally they are bolted to the inner wing or a large heatsink area. They are also filled with oil and this conducts the heat away from the windings. To keep the oil inside the top of the coil is normally crimped or bonded to the top of the metal can. If the coil is face downwards the oil can begin to leak away through this join (personal experience) and then this results in coil breakdown and intermittant running.


What function does the condenser have in the distributor?


When the points open the resulting high voltage spike can cause the points surface to deteriorate which leads to the points gap closing with time. It also closes with the wear of the heel but thats another matter.  The condensor (or modern name capacitor) reduces this spike and absorbs the charge as the coil fires. It then discharges as soon as the points then close .

If the condensor is faulty then there would be rough running (if at all) and points sparking and damage. A condensor is normally made up like a swiss roll with layers with an electrolyte between and with the continual use it can fail eventually.


How to check a spark plus is working?


Firstly there is lots of high voltage around so be careful. Dont do what I did in my youth and that is disconnect a HT lead whilst the car was running as the HT then grounded through me. My mate thought that was very funny. I didnt agree with him.

The easiest way is to buy a spark tester and with the engine off put it in line between the plug and the plug lead. When the car is started then you should see a spark.

However if you dont have a tester then just remove the plug and leave it against the block so it is earthed and then turn the engine over on the key. You should see a spark at the electrode briefly. Dont crank the engine for long whilst doing this.





These are just my personal views and tips. If there are any other topics you would like to see covered then drop me an e mail to stevek100@yahoo.co.uk