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MG MGB Technical - Rubber Bumper ignition fault

Hi All

I have a rubber bumpre B in for an ex system fit and cure running on....turns out running on is actually not switching off on ignition key, no ignition light either, any ideas please?

Thanks

Peter
Peter Burgess Tuning

Peter,
IIRC there was something like this recently - obviously I can't remember the outcome but if I find the thread I'll report back

usually something to do with switch or coil wiring(?)

could the red light be a second separate fault(?)

having model year could help with differences to wiring(?)

more actual help-
wiring diagrams in colour (via Paul's site) - http://www.advanceautowire.com/mgb.pdf

and loads of info on Paul's site - http://www.mgb-stuff.org.uk/electricstext1.htm#
Nigel Atkins

Hi Nigel 1976 GHD5 374675

Thanks

Peter
Peter Burgess Tuning

First thought was the ignition relay sticking, which was a common fault and led to most of the circuits being taken off the relay in 1978, but the relay wasn't provided until 1977.

Either the ignition switch simply isn't disconnecting 12v from the white circuit, or possibly the ignition ballast bypass circuit from the starter solenoid is powered all the time providing an alternative supply when the ignition switch is off.

You should be able to detect which with a voltmeter on the white at the fusebox. The former will show 14v or so with the ignition switch off, the latter probably a lower voltage. The latter would cause the coil to run much hotter than normal. The latter will also result in 14v or so on the coil +ve with the engine running, whereas it should show 9 or 10v - on an analogue meter anyway.
Paul Hunt

Hi again,

getting closer. Keith discovered the ig light bulb holder has been hot and the connections are a little melted, wobbled bulb and the light started working and engine now switches off on key....any reason for this? As you can tell we are not autoelectricians. Bulb extingushes as revs rise and charging commences.

Peter
Peter Burgess Tuning

thankfully the cavalry has arrived, I couldn't find what I was looking for, 'ignition ballast bypass circuit' sounds vaguely familiar but that was from a thread longer back than I was thinking
Nigel Atkins

I was typing while you posted

hopefully Paul will be back

I'm wondering at alternator, its plug and connections and also if it could be anything to with the o/d wiring (which I think might be joined by a connector to the white wires so could perhaps be disconnected from there to eliminate it
Nigel Atkins

Peter

The ignition warning light in normal operation with ignition switched on and light extinguished, has 12v on either side of it. One feed comes via the ignition switch the other comes from the alternator, hence with 12v on either side the bulb has zero volts across it and is off.

In your case with the bulb shorted out, once the ignition switch is turned off the alternator would continue to supply 12v to the coil and hence keep the engine running.

Bob
R.A Davis

what's caused the melting connections?
Nigel Atkins

Thanks Bob, just what we were thinking Nigel why has it melted! Have ordered holder and a few bulbs from Moss. Thanks for fast response everyone.

Peter
Peter Burgess Tuning

Nigel

Too high a wattage bulb (unlikely as the lamp is only lit for a short while at start up) or more likely an initial short across the bulb holder that then allowed 12v to flow from alternator to coil. This level of current would likely exceed the capabilities of the contacts in the holder generating the heat. In which case the melting would be a symptom not the initial cause but once melted would make matters worse.

Bob
R.A Davis

thanks Bob I thought that might be the case but got confused when I thought you meant the 'with 12v on either side the bulb has zero volts across it and is off' was one side cancelling the other out - I take it (in my terms) as 12v going through 3v wires and connectors for too long

ETA: this has perhaps reminded me that what I was initially thinking of might have been in a John Twist video
Nigel Atkins

Nigel

The ignition warning light is a bit strange in its operation. The bulb holder has 2 wires and neither is connected to ground. One wire is connected to 12v via the ignition switch and only has 12v on it with ignition switched on. The other wire goes to the alternator, with the engine stopped it has 0v on it and has 12v on it once the alternator starts charging.

Therefore, with the ignition off, the bulb has 0v from the ignition switch on one wire and 0v from the alternator on the other and is therefore off.

With ignition on and engine stopped, the bulb has 12v from the ignition switch on one wire and 0v from the alternator on the other and is therefore lit.

Once the engine is started and the alternator starts charging the bulb has 12v from the ignition switch on one side and 12v from the alternator on the other. The net result is 0v and the bulb is off.

Bob

R.A Davis

cheers Bob
Nigel Atkins

The ignition warning light bulb should be 2.2w. While that will get warm to the touch it certainly shouldn't get anywhere near hot enough to melt anything. Unless a much higher wattage bulb has been installed the melting sounds more like either the white or the brown/yellow wires shorting to earth nearby.

On the face of it there is no way a non-functioning bulb could keep the engine running, unless it has a really weird PO modification to the wiring somewhere.

However! If the two bulb holder wires were shorting together, not only would that extinguish the bulb, but with the running engine that would allow the 12v from the alternator on the brown/yellow (which normally extinguishes the bulb on a running engine as Bob says) to pass through directly onto the white and so continue to power the ignition. That would allow more current than usual to pass through the white and brown/yellow wires in the bulb holder, and that together with the probable less than perfect connection between them, could well cause heat damage. So you need to examine that bulb holder and its wires very carefully as it will likely happen again. It may also have damaged the voltage regulator in the alternator.

With the 2.2w bulb in circuit there is too much resistance and hence too little current to allow the alternator output to run the ignition, so the engine stops when the key is turned off, even though there is still some voltage at the coil until the alternator stops outputting. This residual voltage caused a problem when the ignition relay was first fitted, as the current through the bulb was enough to keep that operated when the ignition was switched off. That was immediately discovered by the factory of course, so they moved the ignition warning light white wire from the output of the ignition switch (which goes to the winding of the relay) to the output of the relay. But that was only on UK spec cars. North American spec cars by this time had the anti-runon circuit, which is so effective at stopping combustion, the ignition warning light change was never made. On those cars when you turn off the ignition, the ignition relay remains operated, continuing to power the ignition as well as all the other ignition powered circuits. It's the anti-runon circuit that stops the engine, and it's only when the alternator stops outputting that the ignition relay releases. That's fine until there is a problem with the anti-runon circuit, or pretty-well anywhere in the emissions plumbing (or when that is removed as many do). At which point you can no longer stop the engine with the key, and have to move the ignition light wire the same as on UK cars.

Paul Hunt

Hi Paul

Thanks for that, we are fitting a new holder and correct bulb this afternoon.

Peter
Peter Burgess Tuning

Peter,
just thought (well you know how slow I think) and I could be wrong but when has that stopped me before :)

if you still have the ignition problems after sort bulb and feed could it also be our old friend the 4-way fuse box upside down and/or wired wrong or something causing a shorting under it

arh, I can relax now and have tea (or is it dinner or supper)
Nigel Atkins

All seems to be ok Nigel. We have connected via male/female connectors so owner can 'whip it out' at the drop of a hat should he need to do so.

Peter
Peter Burgess Tuning

it's a funny thing that ignition light set up but so rare to hear of problems from it

good to hear it's sorted and in under 600 posts :D

if the owner has more funny wiring you might want to suggest putting an in line fuse for the o/d based o what I've read recently
Nigel Atkins

This thread was discussed between 05/11/2013 and 06/11/2013

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