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MG Midget and Sprite Technical - no spark

Hi,

I have been restorimg my car over last 18 months. 1275 engine, dizzy fitted with pertronix elec ignition unit and matching flamethrower coil. Before restoration, it started and ran very well.
Engine not started for over a year, so I came to try and no spark at plugs. Checked battery fully charged (12.7v), cleaned terminals connections etc, new dizzy cap and red rotor from Simon BBC already fitted. With ignition on,I've got 12.4v at the + terminal on the coil, same on the - terminal. Checked coil resistance 3.1 ohms. I read Les Bengston's article and checked the voltage coming out of the coil; 12.3v. Is the coil cooked or is it something else? I also removed the king lead from the dizzy and checked for a spark but nothing.Any ideas please!
Andrew
aj robinson

Maybe this will help:


Dave Rhine ('78 1500)

Thanks, but I haven't got round to checking the pertronix unit yet as i thought I'd look at the coil/ht leads etc first.Archive gave me some clues, particularly Les Bengston's article on ignition troubleshooting. That's why I am suspecting the coil at the moment.
regards,

Andrew
aj robinson

Yes, but the coil won't fire if it's not getting the proper on/off signal to the (-) terminal, just like with a set of points.

The same voltage at both (+) & (-) terminals could indicate a problem with the trigger module.

Just a thought...
Dave Rhine ('78 1500)

Andrew. As Dave notes, there must be a make/break circuit to cause a magnetic charge to build up in the coil (make) then collapse (break) inducing a charge in the secondary windings which causes the coil lead to produce a spark when held near the block.

Disconnect the Petronix unit from the coil, leaving only the input wire(s) on the positive terminal of the coil. Hook up a length of wire to the negative terminal of coil, making sure that you have about a half inch of bare wire on the end not connected to anything.

Pull the coil lead free at the distributor cap and have it near the block. Turn the ignition switch to the on, or "run", position. Hold the coil lead near the block while touching the bare end of the wire from the negative terminal of the coil to a good ground for one second (one thousand one). Remove the coil (-) wire from the ground while watching for a spark at the end of the coil lead. If you have a good spark, you have a good coil and good wiring up until that point. The problem would, then, be with the make/brake circuit (points or points replacement system). This, by the way, is a method of testing I learned from Bob (Robert) which makes for a very easy "quick check".

If you do not get a good spark when using the jumper wire, I would check the connections at the coil terminals to make sure they are good and, if they are, replace the coil with one of the proper type (i.e. one having about 3.1 ohms resistance in the primary windings and designed for use on a 12V system without a ballast resistor).

I think the fact that you are receiving 12.3 volts on the negative terminal is an indication that there is a problem with your Petronix unit. With a points type system, or most other brands of points replacement system, the current flow if from the input at the positive coil (full system voltage) through the coil where the resistance lowers the voltage at the negative (ground) terminal of the coil about 3-4 volts, then to the points and, through them, to ground. Fairly easy to diagnose. Having the Petronix unit connected to both the positive and the negative terminals of the coil MAY give you the voltages you have recorded if the points replacement system is not working correctly. But, not having had the opportunity to trouble shoot an ignition system having a Petronix module, all I have are ideas, no facts.

Please let us know what you find out. Thanks.

Les
Les Bengtson

Yes to fire the coil the same technique is used no matter what the type of distributor/points or electronic.

What happens is that voltage is applied to the positive of the coil. Hence with ignition on measure 12V between positive of coil and earth. Now measure between negative of coil and earth. If that is 12v then the ignition is not fired IE points are open. Now turn engine to see coil fire when the meter reading at the neg of the coil goes to a low value. If this does not happen then the fault is with the points or in your case the petronics.
Robert (Bob) Midget Turbo

Les

You said you haven't had the opportunity to troubleshoot a Pertronix unit...they're really simple, using a Hall Effect cell in the module & magnets at the trigger points on the distributor shaft, you basically troubleshoot them the same way you would a set of points.

I've been using them for 20+ years & the beauty is the simplicity & reliablity.

They've proven to be absolutely reliable for me, but if you reverse the polarity at the coil terminals, you'll fry the module.

Of course, like everything else these days, they may be now manufactured in China :( , I hope not...

Dave Rhine ('78 1500)

Thanks Guys, all the info is great. I will hopefully do a test tomorrow and rfeport back.
Dave, I can't quite read all the text from the image you posted. Is it possible to send me a scanned copy please, or a link to a website where I can see it a little clearer?
Regards,

Andrew
aj robinson

why complicate things

try to get it working on good old points and if you have it running switch to the pertronix kit.

KISS is always the way to go just add one probleme at a time
Onno Könemann

KISS?
Andrew
aj robinson

KISS - I'd guess at - keep it simple, stupid

simular to - when all else fails check the obvious

RTFM or RTFI - read the f manual/instructions
Nigel Atkins

Wow! Success, of sorts.
Did the test on the coil as advised by Les and got good spark at the coil lead, so i guess the pertronix is fried. I 'm not sure how, maybe just misuse. So, back to points or a new pertronix? As the timing is set for a pertronix, might be an easier route, cos I hated points.
Many thanks to all, I will let you know how things progress.
Andrew
aj robinson

if you can afford one the 1-2-3 fully electronic dissy is very good
Nigel Atkins

123 is a bit expensive for me. May try the Simon BBC unit which is cheap and very similar to pertronix I believe.
Andrew
aj robinson

just try points first it is a cheap and easy option.
if you get it running with that then go for a electronic setup.

And yes that is the correct explanation for KISS.
I try to remind my self of that motto each time i run in to a problem.
A great way to make sure you don't end up as Prop
Onno Könemann

I thought kiss meant...

(Knights, Kings, Kids) In Satens Service.

Gene Simmions.. wow, who knew he was such a family Man


Prop

Opps sorry wrong photo


Prop

Doh...sorry agian wrong photo...this is the correct one of Knights in satens service = Kiss


Prop

Prop

How's the Black Hole running?
Dave Rhine ('78 1500)

This thread was discussed between 29/08/2010 and 02/09/2010

MG Midget and Sprite Technical index

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