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MG MGB Technical - Ignition Mystery

Question for Les Bengtson-Les I have used your pages many times, and they are really a great help-but I have experienced an ignition problem that your "trouble shooting" article covers mostly-except I am still having a problem that I need help with.Car is a 70 B -mostly stock throughout except for pentronic ignition kit-engine is detoxed-runs-starts great-good power--but---once in a while the engine dies very briefly--no electrics-red lite bright-volt meter dead-then immediately restarts while in gear, and acts like nothing happened---never quits completely-no loss of power once restarted-so I can not trace any fault.You cover both LT and HT problems, but these can only be tested when the motor is dead--I have used dielectric grease on all suspected connections-checked all ( I think) possibility of broken wires--coil-ground wires-battery connections--etc. to no avail-only does this maybe once every hundred miles or so-then for only a second--where else can I check ? Thanks Bob
R.A. Snider

Hi R.A.
I'm not Les, but I have a thought. You have a voltmeter in the car. I would think
that as long as the ignition switch is on you should have about 12V from the battery and just a guess without looking it up about 13.5 - 14V when running, When the car quits running I would think that if you glance at the voltmeter it should still be at least 12V and if it has dropped to zero this might be your clue. If that is the case I would look for a bad ground possibly the ground strap at the engine mount. Sand and clean the mounting points and the strap terminals itself. If the headlights are on do they go out too? That would definitely point to a bad ground. One time I was following a guy in a B at night and the lights kept blinking and the engine missing. It ended up being that his engine ground strap was loose. If it is purely ignition how about the fine flexible wire in the distributor? You could eliminate the Petronix by installing the points setup again for a while. Just a few thoughts. Good Luck.
Ralph
Ralph

R.A. Ralph is on one of the possible right tracks. A grounding problem, either at the battery or at the engine ground strap might cause what you are seeing. In any event, cleaning and re-making grounds is never a bad idea.

Without knowing how your volt meter is hooked up, I cannot tell you what it will indicate if it drops to zero. I suspect that it is hooked up through the ignition switch in some manner (in other words, it is not still indicating system voltage when the ignition switch is turned off). One thing that would cause the symptoms you describe (besides bad ground) would be lack of power. Once again, connections starting at the main terminal of the starter and working forwards to the ignition circuit. This is mostly the brown wire circuits. Do not remember if the 70 models had an ignition relay, but your what you describe is a classic indicator of a failing relay on cars having them. I have had this happen twice over the years.

The ignition switch itself is also a possible suspect. Blow up a copy of the wiring diagram for your car then have it laminated. This will allow you to see what wires go where and give you an idea of how many connectors are involved (all need to be cleaned and reassembled).

I would also suggest you post a note to Paul Hunt through his website, "The Pages of Bee and Vee". Much of what I know about electrics has been learned from Paul over the years with a few technique items learned from other members of this board. The best lesson to learn from me on electrics is that, "If Les can learn to do this stuff, I can too". But, thank you for your kind words. Les
Les Bengtson

Thanks Ralph and Les-I will follow up on your suggestions--checking wiring is not an easy project-but having some excellent help thru the MGB tech BBS from guys like you does raise the confidence level-will see what I can find. Bob S.
R.A. Snider

Again-thanks Ralph and Les-apparently it was the ground strapat least no further problem so far-after removing it-cleaning-sanding-giving it a little dielectric grease and tightening-both ends were slightly loose and greasy-so----thanks again--interesting though that I could find no reference to this ground in any of my several work shop or owners manual-should have known better without them I guess.
R.A. Snider

This thread was discussed between 24/06/2004 and 28/06/2004

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