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MG MGB Technical - Pertronix trouble

I inadvertently left my ignition on, switch is so worn can remoe key in the on positon, this fried my pertronix and the coil. Actually melted the wiresbetween the two and the pertronix unit swelled and burst. Installed a new pertronix and coil. car ran about 20 minutes, then the coil got too hot too even touch and engine stopped. THen I installled a new voltage regulator and another coil, pertronix seemed fine, engine ran a couple of minutes and coil overheated, engine stopped and now will not evne fire. what am I missing here, am considering going back to a points ignition, another new coil and voltage regulator. what experience can you share on this problem. many thanks, Kenn
Kenneth von Wolf Lewis

What coil are you using? If the car is a rubber bumper car, are you using the original wiring? Alternator voltage voltage regulator? Answering these questions may help us get a better idea of what the problem may be.

Clifton
Clifton Gordon

Sounds like the coil requires a ballast resistor which is normally part of the wiring in the later MGB's except those with the constant energy ignition (~1980)

What year car do you have? Is the '+' coil connected to the same wire(s) as original?

some coils come with internal ballast resistors. Do you know what coil you have?
werner haussmann

If you have a rubber bumper B, you need to go to the crome bumper style 12 volt coil, preferrably with 3 Ohms of resistance.
Jeff Schlemmer

Sorry guys, forgot to say that my car is a 1967 BGT, with a generator, Lucas voltage regulator and a Lucas sport coil. To the best of my knowledge this car has never been equiped with a ballast resistor. Thanks, Kenn
Kenneth von Wolf Lewis

The ballast resistor reduces the effective voltage to the coil and the current spikes to the electronic switch. The switching load without the resistor has large voltage spikes. Crane advises with it's 700 ignition that a resistor is necessary - with the upgraded 3000 model no resistor is required. It is supposed to be heavier duty and can handle the voltage and current spikes.

A resistor doesn't do much resisting at low loads (amps). As the load (amps) increases the resistor will drop the voltage in proportion to the load increase. -- in effect it smooths the voltage and amperage spikes.
Barry
Barry Parkinson

The Lucas sport coil has an internal ballast resistor, so lack of a ballast resistor is likely not your problem.

werner haussmann

Doesn't it depend on which Lucas Sport coil you have? I thought there were different Sport coils with different levels of resistance.

Jeff Schlemmer

Kenn. I would suspect that when the damage happened, there were other wires shorted out. The only time I have seen this problem, wires were shorted out all the way back to the power input to the ignition switch. You need to start at the ignition switch itself. Check the brown wire going to the switch for signs of melted insulation and wire strands having burned. Then, using a wiring diagram, follow all of the other wires in the "white wire" circuit which is energised when the ignition switch is in the "run" position. Find any burned wires and either tape them up or replace them as necessary.

Then, take the dizzy out of the car and look for internal damage inside it to see what might have melted and be causing some form of direct short to ground, as that seems to be what is happening and causing your problem.

Les
Les Bengtson

Kenneth

Any potential you have a wire on the wrong termial on the regulator?

BTW. I have a three or 4 regulators that came with my 67. I am running an alternator, so if you need to try another regulator, let me know

cunhab@charter.net.
Bruce Cunha

Am quite sure that the regulator is wired up correctly as I have done that before and am very carefull in so doing. have yet to trace all the wires in the system, actually not usre how o check those buried within the wiring loom, perhaps I just need to replace them with new wire runs. still am puzzled as to why it would run strong for 15 minutes and then just stop and fail to restart, tries to fire, just won't continue. thanks for all input. Kenn
Kenneth von Wolf Lewis

Les, you are correct, there are other damaged wires, at least one of the white wires shows signs of overheating all the way back to the switch, btw the offending switch is to be replaced. will follow the entire circuit soon and repair, then replace all major components, am also returning to a points system, they amy be primitive, but at least their working is visible. Thanks to all. Kenn
Kenneth von Wolf Lewis

Ken,
Glad you found your problem, but you shouldn't be down on the pertronix's...your problem would have fried your points also...wasn't the pertronixs fault! I've had the same Pertronixs in my B for 8 years now, about 10,000 miles a year...try that with points!!!
Robert Dougherty

Robert, am not really "down" on pertronix. Mine worked great for five years, have just decided I perfer the lower tech and easily visible points system. Used points, also, for years with no problems.

Now I have the "great joy" of opening up the wiring loom behind the dash and into the engine bay to remove all "fried" wire and replace same.

Almost gave up on my beloved 67 BGT/Special. my wife talked me out of doing so.
Kenneth von Wolf Lewis

The list of things I'd rather do then work behind the dash is VERY long!!!
Good Luck!
Robert Dougherty

Robert, it was well worth my time to remove the driver's seat when we worked behind the dash. And, since we were changing out the steering wheel, we took that off too. And, since we had a failure going in the multifunction switch for brights, signals and horn (it's a 70 MGB) we took off the column surround, rebuilt that. And, since that was so easy, we inspected all the wiriing from the steering column on back, and replaced the faulty voltage stabilizer. And, since the speedometer read wrong we had that recalibrated. And finally replaced the fan switch, and remounted the radio and the front speakers.

One thing led to another... so be careful!

Total work time, two evenings. Total elapsed time waiting for parts and for the back muscles to loosen up again, two weeks. Total enjoyment of everything functioning well, unlimited.

good luck!
dave
Dave Braun

All coils will get hotter if left with the ignition on than when running (unless the points happen to be open, which is usually not the case), although the 12v coil probably gets hotter than the 6v coil as in the latter case only half the power is being dissipated in the coil and the other half in the ballast resistance.

Whether a coil has *internal* ballast or not is irrelevant, a coil is either a 12v coil or a 6v coil. What is important is the resistance of the coil primary. 3 ohms (or about 2.4 ohms for a sport coil) is a 12v coil and should not have an external ballast, i.e. all chrome bumper cars. 1.5 ohms is a 6v coil and must have an external ballast in the harness, i.e. all rubber bumper cars.
Paul Hunt 2

Thanks to all for yuor input, Les, you where spot on. This past Saturday I spent five hours tracing the ignition wires thru the harness. Found that the white from the coil via the tach to the ingnition switch was not only had melted insulation in several places, one such spot was just above the scuttle antishake bar and clearly was shorting out the system upon the slightest road bump. Replaced with wire running parallel to the harness. Oddly, there seemed to be no insulation melting of this wire within the covered part of the harness, thus no other wires damaged. Also reverted to points and coil, at least with them I can see their condition and replacement is a mere fraction of the cost of the pertronix.

Took her out for a series of errands around town and no problems, brought her back home and gave her a wash and a wax. Thanks again to all, Kenn
Kenneth von Wolf Lewis

Kenn. Glad that you got the problem solved. I know what a pain it can be, in several parts of the body, when you have to trace the wires in this situation. I have been there myself.

It happened a couple of years ago when I was troubleshooting a problem on my daughter's car. Forgot to disconnect the wire coming off the coil and going to the dizzy. While testing the wires for another probem, I notices smoke coming from under the dash. Wire from the coil had melted, as had several of the wires under the dash. I had to replace the brown wire going to the ignition switch.

We have made our mistakes, fixed them, learned our lessons and are the better for it. Congratulations on a job well done.

Les
Les Bengtson

This thread was discussed between 20/08/2006 and 29/08/2006

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