my car has no Ballast Resistor at the Coil. I understand it was removed. What exactly are they for?

 

The ballast resistor reduces the voltage to the coil.

Jaguar tech literature was vague as to why one was fitted to the 4.2 FI engine, saying only "to improve ignition performance".....or words of that nature. I recall the TSB saying the ballast was installed beginning with VIN 421xxx but I'm not sure if that's entirely accurate. Anyhow, a different amplifier was used (DAC3848 vs DAC2087) although I've never found anything describing the difference. Must've been a subtle change.

The coil was different as well (DBC1140 vs DAC3001), with the 1140 having the resistor installed on the "+" post.

Cheers
DD

 

As for "why are they removed?", well, because they sometimes fail. There's no reason to remove a functioning ballast.

Cheers
DD

 

Thanks Doug.

 

Doug's post is better than my thoughts. But here they are.

1. Reduce battery voltage. Common in older ppoint and condensor ignition systems.
Protrct botj cil and poinyts. Some were conected in such a way so as to allow full battery on crank to facilitate the start, then revert to reduced btteryvoltage on run.

Way back when we junk raced on an old dirt track, we removed the resistor and ran full battery.Indeed, the points would not last fr the full event. So many of us had a spare distributor all refreshed,. Swap it in if we made the main event. Went home with two ditributors to refresh. Limped or towed.

Schoo chum and I had similar 34 strip down Ford coupe. hHis was full time drive. At times, i drove mine to work and or school. I had one set of plates for my two cars!!!
Carl

 

Generally, a ballast resistor is used with a lower voltage coil, usually 9 volt. If you take away the resistor, you should also swap the coil to a 12 volt. Otherwise, the coil may could fail.

 

As others have mentioned, in the age of poorer battery performance, poorer ignition systems and carburetted fuel systems, cold starting in winter particularly could be a problem. The ballast resistor system was set up with a 9 volt coil fitted, so that in normal running, starter NOT engaged, the ballast resistor somehow reduced the 12v battery input to the coil to 9 volts.
When the starter was engaged though, the full 12v was sent to the 9 volt coil - in effect the coil was run above its designed voltage - and so the spark created was stronger.
Even on the HE v12, Grant Francis recommends that a relay (using 12 v for the power circuit from the firewall battery post) is used to send the 12v from the ignition switch to the coil, rather than just relying on the switch sending 12v direct. This is so the coil is getting a full 12v and the load on the ignition switch is reduced too. I have done this on mine and it does improve starting, no question.

 

In some systems, the ballast resistor was a simple resistor. When the starter was operated, it was shorted so the coil received whatever voltage the battery could deliver while simultaneously driving the starter. Once the starter was released, the supply to the coil went via the resistor and would limited to around 9 volts. This produces a better spark across the operating range without overloading the coil in normal running.

As I understand it, the Jaguar system in the series 3 XJ was a bit more sophisticated. The supply to the coil is always via the resistor. The resistor is current limiting; that is, its resistance increases with current flow through it. In that way, its resistance is lower when the supply voltage is reduced by other loads on the battery. It may also reduce at higher engine speed with less time to charge the coil. No doubt the more sophisticated resistor was also less reliable, certainly less than no resistor at all. It's simpler and more reliable to do as Greg and Grant recommend and ensure the best possible supply to the coil when starting.

 

Correct on every car with a ballast resistor that I can think of......except the 4.2 FI used in a Series III XJ6! Voltage supply to the coil goes thru the resistor at all times.

Cheers
DD

 

Never knew it was "current limiting". Very interesting !

Cheers
DD

 

The Haynes manual implies the resistor is something cleverer than a simple linear resistance. However, in a lot of electric circuit design, a current limiting resistor is a normal linear resistor placed in the circuit to avoid high currents through something like a diode. As the series 3 engine sitting in my workshop doesn't have its ignition system with it, I've nothing to test.

 

Even on the HE v12, Grant Francis recommends that a relay (using 12 v for the power circuit from the firewall battery post) is used to send the 12v from the ignition switch to the coil, rather than just relying on the switch sending 12v direct. This is so the coil is getting a full 12v and the load on the ignition switch is reduced too. I have done this on mine and it does improve starting, no question.[/QUOTE]

Greg,
If a V12 has the later Ducellier coil, is this mod still worthwhile?
Rob

 

It certainly is, the same principle applies: getting a clean 12v to the coil on starting, and taking the load off the ignition switch.