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MG MGB Technical - Ignition light ???

On my 75 mgb the ignition light came on very bright and the tack died at the same time.The light would go out and come on again and of course the tack would come back to life [once it even back fired] I did manage to drive it home of course not making any stops

Any ideas on where to start looking for the problem.

It has had a new alt. and a electronic ignition [pertonic]installed a couple of years back

Thanks
Wally
wally armstrong

Wally - Does this happen when you step on the brake? I had a similar situation several years ago on a trip in our MGB and it turned out to be the green/purple wire going to the left brake light was being crushed against the metal socket by some stuff I had in the fender well that was pressing up against the tail/brake light assembly causing a short to ground for the brake light voltage. Cheers - Dave
David DuBois

No Dave It was under light load each time that it happened

Thanks

Wally
wally armstrong

Hi,

Check

Your ignition switch
Tach dying when engine dies is a fault in the ignition wiring, which energizes the tach and coil.

Points / capacitor in the dizzy, points open ( too big a gap)or not opening (too small a gap), cap shorting.

Herb
Herb Adler

Disconnections around the coil and points, or a short-circuit condenser will cause the tach to drop, but will not cause the ignition light to glow. That happens when the 12v feed to the white has been lost from the ignition switch, and the engine is still spinning. All of these will cause the engine to cut out, which I assume is what is happening. Problems in the distributor cap (or rotor) will only cause the engine to cut out, they won't affect the tach or warning light (until the engine stops altogether ...)

Could be inside the switch, multi-way plug that connects the switch to the harness, or possibly loss of 12v from the brown feeding the switch.
PaulH Solihull

Short to ground anywhere on the switch side of the warning light will do it. As Dave said in his example. Any W, G, even N wire. Power is running from Alt through lamp to earth when this happens, and there is no opposing B+ from the IGN switch side.

FRM
FR Millmore

That would burn the wiring as it would put an earth/ground on unfused 12v. Doesn't need a short to light the lamp, if the 12v is missing for any reason it will earth through the coil and other ignition powered components enough to light it near normally.
PaulH Solihull

Paul-
"That would burn the wiring as it would put an earth/ground on unfused 12v."
Not necessarily, depends on the duration and quality of the intermittent earthing. In Dave's example, it was on a fused G, but apparently didn't even blow the fuse. However,should the intermittent earth decide to be more reliable, things could well get toasty.

Agree that it doesn't "need" a short to light the lamp; I was suggesting that a short might cause the problem.
Since the lamp lights, a simple B+ failure would have to be in the N to, or W from, the IGN switch, but before the split of the W to lamp and coil. If the W twixt lamp and coil/lamp split is open, the lamp will not have either B+ or earth, but if it is short to earth, all symptoms fit.

I note that the W in this area are a favorite place for incompetent radio/accessory installers to chop into wires and use stupid connectors and bad tape joints. The other end of whatever wire they install is also highly suspect; in-console wiring faults are common. I couldn't tell you how many times I've cut a rat's nest of wires out and then hooked the radio up to the nice & fused factory radio supply lead. Gets extra interesting now that radios require constant B+, Ign switched ACC, and a light feed.

There are also all kinds of aberrant things done to the bizarro seat belt interlock control apparatus on these cars, don't know if you have those on your side. Lots of wires that get disconnected or cut and stuffed up in the console area, where they mate with the other strange things and produce pesky offspring!

FRM
FR Millmore

Nope, never had the interlock thing, just an even simpler version of the later warning buzzer.

Green fuse not blowing is quite often caused by bad connections at the fuse - not bad enough to noticeably affect things working (except slowing of the flashers which is very common anyway), but bad enough to stop fusing current (35A or thereabouts) flowing. PO chops aside breaks are more likely than shorts, I would have said, but as you say it is an alternative possibility.
PaulH Solihull

This thread was discussed between 28/06/2011 and 30/06/2011

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