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MG MGB Technical - Lumenition Optronic Ignition

Can anybody help?

The car starts and runs ok when cold but after some 10/20 miles starts to run a bit rough and when I stop the engine it won’t start again. It’s an ignition problem as I have no spark at all at this point.

Leave the car to cool down and it starts ok and runs no problem. Get the engine hot and the problem reoccurs.

I have a Lumenition ‘Optronic’ ignition system installed with MS4 High Energy coil. (I guess I’ve done some 5k miles with it since installation).

I suspect the coil is breaking down and is affected by the heat of the engine.

Any thoughts anybody before I pay out some £35 for a new coil.

Thanks

Brian
Brian Davis

Brian. Failures of the electronic components of the system will often give the symptoms you describe. As the system cools down, the parts begin to function again.

So, some questions. First, what is the year of your vehicle? The early vehicles used a full time 12V input to the distributor system while later vehicles used a full 12V input at starting, then dropped to 6-8V input when running. Which system you have will have an effect on which coil you should be running.

Coils are either 12V, indicating they are designed to run full time with a 12V input, or a "12V for use with an external resistor", which indicates a coil designed for the lower input voltage. What coil you have and what year you have can have an impact if they are mismatched.

You do not need one of the "super coils" so commonly sold with these systems. For street use, the required coil output voltage seldom, if ever, exceeds the 20K volt capability of the factory coil. The rated output voltage of a coil is the potential it can provide, not the actuality of what it is providing. My cars measure about 11K volts in steady state operation and may jump as high as 19K volts with rapid acceleration. A "40K volt coil" can, as its name indicates, provide 40K volts when necessary, but is normally operating at significantly less volts and I have never seen one put out more than 20K volts when the rest of the ignition system is in good working order.

Might it be possible to swap in your old coil and see if the system begins to work properly? That would save you the immediate cost of a new coil and allow you to determine if your problem relates to the coil or the electronic points replacement system, or whether it is a mechanical problem with the distributor itself.

Depending on how the conversion was done, it might also be possible to reinstall the points system as a means of trouble shooting the coil.

There is a tech article on ignition system trouble shooting on the MG side of my website, www.custompistols.com/ some of which applies to the electronic points replacement systems.

Les
Les Bengtson

Les is absolutely right - the coil has to be suspect - and you don't need a special coil with lumenition. I have used the optronic system on a series of cars since it was first introduced and been let down only once - that was the occasion the amplifier had to be replaced because of the symptons you mention - I tried the coil first (coils have proved much less reliable).
Roger
R Walker

Brian

I have a few spare coils, if you want to try one.

daveo138 at yahoo dot co dot uk
Dave O'Neill2

As an aside, and forgive me if I have recounted this story before the old magneto systems of old british bikes would become dead as the temperature rose. It turned out that while cold the coils were intact, but as temperature rose, a break would become evedent with a loos of continuity? Mike
J.M. Doust

Hi Brian
Beware the crappy shorting out when hot rotor arm syndrome, same symptoms you describe. We only use the Distributor Dcotor red ones these days.

Peter
Peter Burgess Tuning

First check you have 12v on the coil +ve and to the ignition system. Then check the ignition system *isn't* connecting a permanent earth to the coil -ve, i.e. you have 10v or so on the coil -ve while cranking. Both tests with a meter.

Then check it isn't the rotor or distributor cap breaking down by connecting the coil lead to a plug laying on the block and cranking, looking for a spark.

If still no spark you can prove it to coil or ignition by simulating points and a condenser and see if that gives you a spark. Remove the Lumenition wires from the coil, then connect a condenser between the coil -ve and earth, and tap an earth on and off the coil -ve. If you get a spark out of the coil - with the king-lead connected to a plug laying on the block, then the coil is OK and it is probably the ignition.
PaulH Solihull

Dave

I have 2 spares myself and will try them over the Easter weekend.

The problem is that it only happens when the engine is hot, after 10 miles or so and I don’t want to breakdown on the road. Hopefully it will simulate the fault in the garage if I run the engine for a while, I'll try first before changing the coil.

Maybe if I’m stuck after trying things we can get together and compare notes, I’m in Balsall Common so just down the road from you in Solihull.

Brian
Brian Davis

This thread was discussed between 03/04/2012 and 05/04/2012

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