Following on from my previous thread, can anyone explain to me coil ballast resistors? The primary resistance of my coil is 0.5 ohms. I have been advised i might need a ballast resistor in the coil wire?

Can anyone explain to me what they do and how i work out whether i need them and what resistance resisitor i would need? It's a ford mondeo 6 cylinder coilpack and keeps getting cooked.

Thanks.

I've just done a search and now i'm now i know what they do. What i don't know is whether i need them? I still can't work out why a coil pack would keep failing other than it's either getting too high a voltage or at high revs the dwell time isn;t high enough ie the sparks are basically on all the time. Anyone confirm i'm making any sense?!

A ballast resistor is use only for old fashioned points systems. The idea is to use a 6v or 9v coil to ensure a good spark when the battery is loaded down by the starter motor. When the engine is running, this would quickly overheat the coil, so a resistor is switched into the circuit to limit the coil current.

Modern coil packs have very low primary resistances in order to ensure the coil can be fully "charged" between sparks at high RPM, where the output of a points system starts to reduce considerably. In order not to burn out the coil, the ECU carefully controls the dwell period, i.e. for a 4ms dwell the ECU powers up the coil 4ms before the next spark is due. They don't use a ballast resistor as this would defeat the purpose of the system.

From what I can gather, some of the the Autronic systems don't control dwell, but leave this up to external coil driver modules that have built in dwell control. These work a little differently to an ECU: they sense the coil current, and compare it against a (pre-set) target value. When the spark trigger occurs, if the coil current hasn't reached the target value, then it switches the coil on sooner to increase dwell, if it has exceeded it then it switches on the coil later to reduce dwell. This gives a constant energy independent of battery voltage.

However, if the coil drivers you are using are not compatible with the Ford coil pack, then it's entirely possible for overheating to occur. There is almost certainly a minimum dwell period the driver can set, if this is too long for the coil then it will cause damage. Given a fixed dwell period, you are pumping more energy into the coil at high RPM, so this ties in with the problem you are observing.

Do you know the Bosch part number of the modules you are using?

[Edited on 12/5/09 by MikeRJ]

Mike,
That makes perfect sense. The bosch numbers are 0 227 100 137