I have been following some threads regarding the Lucas coil on the 1975 4.2l engine. Some of the threads recommend that the ballast be removed and sent to Doug for his collection. My workshop manual says that I should have approx 7v out of the ballast resistor to the + terminal of the coil. Now that that is established, I followed some of the threads and bypassed the ballast resistor and wired directly to the coil. Running the car the coil gets extremely hot. I read somewhere that some of these cars have either a 6V or a 12V coil. I have no markings on mine. the sticker on the coil is not there. It says Lucas on the top and that is all that I have. If I apply 12V to the Ballast, I have 12V out when cold. I am going to assume (I have not checked)that when cold, the ballast resistor will read 12V and as the resistor heats up due to current flow the voltage should drop to the specified 7V. Is the application of 12V directly to the coil causing the coil overheat? If so, the elimination of the Ballast resistor is not recommended for my engine, or maybe I am misinterpreting the information on the threads. The original ignition amplifier was replaced with a CraneCams Fireball XR700 unit which also came with a ballast resistor. I also relocated the coil from its original location to the fender valance. See pics. Any and all comments will be appreciated.

 

Generally when cars used a ballasted coil, the reasoning was that when cranking the battery voltage would drop below the 12v in normal use or when static.
The ignition start switch is used to bypass the resistor allowing full available volts to the coil during cranking. The swich when back in the normal running position removes the bypass and the resistor is back in circuit for normal running.

The coil is designed to run at 6-7v not 12v permanently, hence it gets hot, when the ballast resistior is removed. Mind you they all run a bit warm anyway so how hot is hot. If it gets "too" hot then it's probably a coil requiring a ballast.
If it smokes and catches fire then run!!!!

Modern batteries and coils have largely negated the requirement for this, so i guess that's why people recommend getting rid of the ballast resistor, but I would only do that when replacing the coil with a 12v one.

Your electronic ignition came with a ballast resistor, either because it provides a higher voltage to the coi (unlikely)l or more likely to allow you to fit the system to a car with a ballasted coil.

As to why you are getting 12v at the coil, it may be that the resistor is short circuit, the ballast bypass is permanently on, the wires the incorrect way round on the resistor or the resistor is different due to the ignition system.

 

Those remarks are in reference to the CEI system used on the 80s vintage Ser III XJ6s with the 4.2 engine

Jags with points-type ignition or the older OPUS ignition system are a different kettle of fish.




Most aftermarket system describe in the instructions what spec coil to use and where a ballast is/isn't required.

You mentioned "1975" engine. That would either be points-type ignition or OPUS ignition. I don't know what the OEM coil specs are on those.

The later "CEI" system used a 12v coil with about 1.0 ohm primary resistance.

Cheers
DD

 

Traditionally that's always been the case.

However, and just for the record so to speak, the CEI system used on the Ser III XJ6s with the 4.2 engine was an exception. The ballast was not configured in a "bypass" fashion. It was in-line at all times....not wired/configured to be bypassed when the key is turned to "start".

I should add "when fitted"....as not all of the cars had the ballast fitted

Cheers
DD

 

Doug, this is a 1975 4.2 with the OPUS ignition. Thank you for the info. I reconnected the ballast. With KOEO (key on, engine off) I have 12 volts through the ballast. With KOER(key on,engine running, I have alternator voltage(14.7) at the input of the ballast and 9.8v at the output to the coil, so I guess the ballast is good. My question now is; Is the alternator overcharging? Shouldn't the alternator output drop to around 12v when the battery is fully charged as mine is? Based on the math: 14.7 input, 9.8 output = 4.9 voltage drop across the resistor. The specs in the manual calls for approximately 7 volts at + terminal of coil. So, spec 7v + voltage drop 4.9v = 11.9v which is approximate battery voltage. Am I overcharging? or Is that as good as it gets. Does the alternator voltage decrease based on battery charge with no other load except the engine running?

 

I think the generally accepted guideline is 13.8-14.5 volts when running. You're in the ballpark.....personally I wouldn't worry about being a tad high.


'Tis the output amperage that decreases, not the voltage, as the battery charges,

Cheers
DD

 

Thank you. I appreciate you taking the time off to clear the air on this. Car is pretty much done, Planning to do my AC this weekend

 

Guys can I ask a related question? You are talking about my exact problem. My feed wire from my ignition switch has a short someplace in it so there is no power going to the coil. It just shut off while I was driving at speed. With a simple jump wire from the + battery terminal I ran power directly to the + coil terminal. Ta da.. it started. I had to run a new wire from the ignition switch to the ballast on the coil. There was no way to trace that entire wire all the way back to the switch from the coil. I spliced in a line to the existing white wire off of the ignition switch just as it exits the switch. The rest of the original white wire is still there. The car started right up and ran well. OK so I lied. When I slowed down and came to idle the car stalled in drive an/or the idle went so low it had to be put in park to keep it running. At speed it ran well but idled and stalled intermittently. Sitting in my shop idling the ballast started to smoke. I had changed my amp recently so I switched back to my OEM amp module. It certainly made the car run much better but the ballast still gets very hot and starts to smoke a bit. Shouldn't the ignition switch shut off the constant 12V to the coil once it is in the run position or does it supply continuous 12v to the coil when the key is on and the engine running. Could I have a faulty switch? Thanks, Kev

 

Nope :-)

It isn't wired as a bypass resistor on the Ser III 4.2. The resistor is "on line" at all times




The ignition switch provide 12v to the resistor. The resistor provides something less than that to the coil at all times. I can't remember the voltage reduction off-hand






Quite possible. Common failure point.

Some guys with good fingers open up the ignition switches to clean and repair the contacts. Others just buy a new one.

Often a jiggle of the wires at the back of the switch will prove a defect. Lacking a reaction to the jiggle test you'll have to open it up for a look-see to confirm

As for smoking ballast resitor, well, you can take it off and thow it in the bin and just run 12v to the coil. You wouldn't be the first, believe me

Cheers
DD

 

Agreeing with all said.

I found this "Electrical theory as described by Joseph Lucas" somewhere recently.

I think it clears the air on a lot of the mystery HAHA. Especially the "smoke theory".

Might give some a giggle, might upset some, so be it.

Eletrical theory as per Joseph Lucas.doc

Enjoy, it might even rate a "sticky" mmmmm.

 

If I discard the ballast I can use the same coil? I have several just in case. If the 12V from the switch is constant then I suspect the switch is performing properly. Must have been a short in the 12V white feed wire and the bypass worked. This vehicle is a 1987 XJ-6. Neglected to mention that initially. Thanks for all of your input. I really do appreciate it and again, I apologize for hijacking your post. Going out to the shop now to try it!

 

Yes, you can use the same coil

Not to add to your woes but if it's the original coil, and I suspect it is, it's well past the typical expiration date...so be ready.

Cheers
DD

 

I removed the original coil and bought a BWD E504P ignition coil. That should work, I hope.

 

If it's a 12v coil with roughly 1.0 ohm primary resistance you should be OK

Cheers
DD

 

Gentlemen,
I discarded the ballast and wired the coil direct. It runs better than ever. The coil is warm but not hot. I'll keep a spare with me just in case but your suggestions were fantastic! Thanks so much. Next.. ball joints. Anybody have any tips for this procedure? Kev

 

Took it for a 50 mile Italian tune-up. It ran very well. Thanks everyone!