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MG MGB Technical - Ignition light question

I have a issue with my ignition light in my factory original B GT V8. When I have left the car for a day and then turn the ignition on the light doesn't illuminate. The car will start ok, and there is no flickering of the light, and no other issue with the vehicle.

When I then turn on and off again the ignition light comes on as soon as the key is turned. I can turn off, remove the key, re-insert the key, turn on again and the light illuminates. So I have an intermittant issue.

The only non standard part of the electrical circuit is the fuel pump, which is a solid state Red Top pump and I assume has a higher current draw than the standard SU pump, as it runs continuously (makes a racket!).

Has anyone had a similar issue. Or recommend a starting point to fault find?

Many thanks James
James Eastwood

Well the obvious first place to start is just the simple connections from the back of the alternator (brown/yellow) to the bulb holder, the bulb and holder, plus the earth loops between the gauges. And the white from the bulb to the bullet connector near the ignition switch.
Allan Reeling

The warning light comes on with 12v from the ignition switch on one side - the white wire, and gets an earth from the alternator via the brown/yellow wire.

When you start the engine and the alternator starts charging there is typically 14.5v both side of the light which is why it goes out again.

The first thing to check is that the alternator plug is in securely, the wires are firmly attached to the spade connectors, and they and the alternator spades are clean.

The second is to remove the bulb holder from the dash socket and make sure the bulb is fitted properly - it's a bayonet so has to be pushed in with two little lumps on the bulb base fitting through two notches in the lip of the holder, against spring pressure, then turned about 90 degrees. Check the wires are securely attached to the holder.

The problem with intermittent faults is that even moving a wire can be enough to make the fault go away, until the next time. If your plug is like mine you should be able to unclip the back of it and reveal the backs of the wiring connectors. Release this now, ready for the next time. A bit of insulation tape should hold it closed and guard against shorts in the meantime.

Then the next time it doesn't come on see if you have 12v on the brown/yellow in the plug, being careful not to short the brown to the valve cover.

If you do, and the lights still isn't on, then the problem is either inside the alternator, or the connection between that wire and the alternator spade.

If you don't see 12v then the fault is back towards the warning light. Pull this out of the back of its dash socket, and see if you have 12v on the bulb holder which has the brown/yellow attached to it (the white connection is internal).

If you do, then the problem is in the brown/yellow between the bulb holder and the alternator, there should not be any connectors in this wire.

If you don't then either the bulb could be faulty (if the filament detaches from a support wire rather than blowing it can move around sometimes touching it and making contact, others not) or the bulb holder could be faulty. Remove the bulb and see if you have 12v on the centre connection of the holder.

If you do then it is either the bulb or how it makes connection with the holder.

If not then it's a fault in the white wire back towards the ignition switch. There is a 4-way bullet connector with this and three other white wires by the steering column, so the one for the warning light (a thin one) could be loose in that connector. In fact if you have been groping under the dash recently this is probably the most likely cause.
PaulH Solihull

Paul, many thanks for taking the time to write such a comprehensive set of instructions. I'm going to print them and head for garage. Thanks James.
James Eastwood

Always remember with electrical problems there could be more than one fault! In the past with intermittent faults I've had to drive around with a meter connected to various points in the circuit so I can see what is happening immediately the problem occurs, and without fiddling with wiring and connectors and possibly disturbing the fault.
PaulH Solihull

Ive found making some jumper wires helps a lot. Along with some alligator clip wires. Helped me a lot with a few things.

This thread was discussed between 23/01/2013 and 24/01/2013

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