Don't trust anything

Peterv

Active Member
I've been chasing an annoying miss when accelerating and idling for a little while now.
New plugs, Leads, cap, rotor arm, swapped the coil, cleaned all connectors. Just the Lumenition ignition system to go.
Traced the wiring, earth good, negative good, positive goes to the coil positive. Coil a 12 volt one.
Just a minute.... Cant see a separate 12 volt feed, just the factory wires (2 x yellow+white).
Better measure the voltage with the ignition on... 7.2 volts. :mad:. Most of you will know where I'm going with this.....

Seems like whoever installed the Luminition system missed a vital detail with their handy work. An ignition system and fancy coil isn't going to solve any problems if you don't bypass the coil resistor wire. They don't like working at 7.2 volts. (although obviously they can, just, and has been for many many years).
I replaced the coil with a new NGK bypass resistor type and extended the 12 volt for the ignition to a very near by suitable source.
I wanted to keep the factory bypass system as it's there for a very good reason and keeps the engine bay wiring looking smart.
No more misses, and hopefully no more sooting up of the plugs. (another symptom if idling too long.)

20201104_120804.jpg

Cheers, Peter
 
Mine is a bit opposite - starts very well, runs well, no misses....lead at coil measures 12V (or full battery volts). Somebody has removed the ballast methinks? No obvious hacks visible.
 
How does a "bypass resistor" type work then? Just has more turns to boost the voltage? The problem is that doesn't fix the current going in meaning the spark energy is still limited even if the voltage is boosted. I'd fit a clean 12v ignition feed and fit the lowest resistance coil you can.
 
By "can" of course I mean that the module an get away with. With electronic ignition your goal is to remove as much of the fixed resistance as you can. The latest coils are 0.3ohms meaning in theory you could run up to 40 amps but that never happens as it is actively limited by the ignition module. I swapped my crappy Lucas system for a GM HEI module (£16!) and that limits at 7 amps. With a normal system that would be impossible with a ballasted system as you'd never get more than 4amps through it. Why is this important? Because a coil is an active, inductive component. When it is first switched, it presents a very high impedance to the current as the magnetic fields build. Adding any resistance just slows this process down meaning less energy in the spark before it fires - obviously the faster the engine the less time the coil has to charge - and of course a V8 means you do this twice as often - so it is important. What you want is for the module itself to work to protect the ignition by actively limiting the current instead of a crude resistance.
 
By "can" of course I mean that the module an get away with. With electronic ignition your goal is to remove as much of the fixed resistance as you can. The latest coils are 0.3ohms meaning in theory you could run up to 40 amps but that never happens as it is actively limited by the ignition module. I swapped my crappy Lucas system for a GM HEI module (£16!) and that limits at 7 amps. With a normal system that would be impossible with a ballasted system as you'd never get more than 4amps through it. Why is this important? Because a coil is an active, inductive component. When it is first switched, it presents a very high impedance to the current as the magnetic fields build. Adding any resistance just slows this process down meaning less energy in the spark before it fires - obviously the faster the engine the less time the coil has to charge - and of course a V8 means you do this twice as often - so it is important. What you want is for the module itself to work to protect the ignition by actively limiting the current instead of a crude resistance.
you lost me at "electronic" witchcraft me thinks
 
The reasoning for the ballasted ignition system was simply a way to get a better spark during start up. It was well before any electronic ignition systems were available.
During cranking, the voltage to the coil dropped due to the starter motor "taking" all the "power" from the battery. (similarly to the lights dimming in the house when the welder starts up). With a lower voltage, the coil would give a weaker spark (not good when starting). To over come this some vehicle manufacturers used a coil that would give a normal spark at the lower voltage during cranking (say 7 volts). How this worked was by giving the coil 2 power feeds. One directly from the starter motor that was alive only while cranking (at the lower voltage) and the second from the ignition system via a resistor that took over when the starter motor finished cranking. The resistor dropped the normal 12 volts to 7 volts for ordinary running. Without the voltage drop to the coil, it would cook itself and fail in due course. The resistors basically came in 2 forms, a chunky ceramic resistor usually next to the coil or in the Rovers case, the wire from the ignition system would have a section of high resistance wire grafted in the loom. This would dissipate the heat generated by the voltage reduction over it's length. This also makes bypassing the resistance wire a pain as a section of the loom will need to be overlayed with a new wire and finding both ends of the resistance wire is difficult especially the end under the dash.

Ignition sparks are a lot better with the addition of modern ignition systems but the coil voltages still need to be what-ever your coil is designed to use whether or not you use the ballasted coil.

In my case, they used an ordinary coil (12 v) on a ballasted feed (7 v) and to top it off, the electronic ignition module was fed from there also. (running a 12 volt ignition system from 7 volts is never going to have outstanding results)
 
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Hi, Ahh, What makes you think that the coil is a 12v one? Is it the fact that it has 12v stamped on the bottom? That doesn't mean it's an unballasted coil, it just means that it's for use on a 12v system vehicle. The only ways to be sure are by the part number or the resistance reading of the coil. Heed your own thread title! If you want to stick with the electronic ignition then run an unfused supply from the ignition switch for the coil and module.

Colin
 
Hmm, not so sure... Strictly speaking they aren't really aligned per voltage, although in practice it might make it simpler to sell this way. An inductive coil fed by voltages that aren't orders of magnitude apart is neither here nor there. In effect if the module clips the current it does this by effectively increasing its resistance across the power transistor (usually some form of Darlington pair) and hence the voltage across the coil drops anyway as if it had a ballast. what the module does is act exactly the same as the compensation you talk about during starting, except it does this for each firing when the initial high impedance of the coil must be overcome. What will cook a coil is ultimately too much current flowing though the primary and that can actually peak fairly high during the initial charge of the coil, it's the long-term RMS current that will heat it. At the extreme the turns ratio could cause the secondary insultation to break down at really high voltages, this will never happen on the primary side. Although that is pretty unlikely as the ratios don't really differ so much. So basically a "low voltage" coil might just be a polite way of saying crappier quality primary windings.
 
Anyway the long short of it is use a GM HEI module, they cost less than £20 and are far better than any other system that can easily be adapted. It annoys me greatly that lots of people looked at my car and none managed to spot the ignition was poor. That and a somewhat low output from the alternator. Both now fixed and the car runs much better. It start easily and I have no misfires.

Here's a very, very good description of why the GM solution is better than Ford and Chrysler (and I assume Lucas) HEI Electronic Ignition Retrofit How-To - Slant Six Forum
 
Looking at the specs, to me it looks like the turn ratio for these coils is 1:70 as opposed to 1:100 for lower voltages. That makes sense as these practically need to top-out somewhere around 45kV. So the limit is probably insulation breakdown somewhere in the system. But again, the voltage isn't the whole story, it's the spark energy delivered to the plugs.
 
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Hi, Ahh, What makes you think that the coil is a 12v one? Is it the fact that it has 12v stamped on the bottom? That doesn't mean it's an unballasted coil, it just means that it's for use on a 12v system vehicle. The only ways to be sure are by the part number or the resistance reading of the coil. Heed your own thread title! If you want to stick with the electronic ignition then run an unfused supply from the ignition switch for the coil and module.

Colin

The coil I had was a Bosh GT40. The ballasted version comes with 'to be used with ballasted etc' written all over them. (and it has the same resistance as the GT40 in one of my other cars without a ballasted system) .Yep, found a proper 12 supply for the ignition . Much nicer spark when tested.
 
The coil I had was a Bosh GT40. The ballasted version comes with 'to be used with ballasted etc' written all over them. (and it has the same resistance as the GT40 in one of my other cars without a ballasted system) .Yep, found a proper 12 supply for the ignition . Much nicer spark when tested.

The ballasted Bosch coil is the GT40R. R being for resistor. I have GT40 coils on both my Rovers. The do not sell them in USA, due to a Ford trademark issue, so people ask me about it when I show the car. The Rover in Australia has had the GT40 coil on it for at least 30 years, and has worked very well.
 
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