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MG MGB Technical - 79 B ignition light

Hi, folks. On my '79 roadster, I have recently had a problem where the battery will hold fine for a few days (just sitting, parked) and then it will go dead overnight (it sat all winter with no problem starting in the spring). The ignition light never is illuminated at all, but the alternator still put out 14+v when the motor is running.

I assume that the ignition light is burned out, but can't figure out how to remove it to replace it. None of my repair manuals mention how to replace the bulb. Does it come out through the front of the dash, or from the back? Once I get this tested/fixed, I can carry on to see what else might be causing the problems.

Thanks for any insight you can give.
Howard Battan

Update - I managed to get the bulb out - it just pulled out. Now to get it back in again.

The bulb is fine - tested OK. Any ideas why this light is always off, even with the ignition on but the motor off?
Howard Battan

Hi Howard.

I suggest you take a look here :


Thanks, Don. I guess I didn't search recently enough. I searched the archives first but didn't readily find any info on it. Thanks again for the link.

Howard Battan

OK, I found that the brown/yellow wire in the wiring plug to the alternator was not connecting properly because the plug was broken. I removed all the wires from the plug connector, and connected them directly to the terminals on the alternator. Now the "Ignition" light works properly.

The question is - was this the cause of the baterry going dead after sitting for a few days? I guess I'll just have to wait to find out. Wish me luck.

Howard Battan

Howard - My understanding is that the ignition light is part of the regulator circuit for an alternator. If this is true, then the loose wire to the ignition light would cause improper charging of the battery, hence it's going flat after a few days of sitting idle. You might take a look at Paul Hunt's web site, the Pages of Bee and Vee at: Click on Spanners, Electrics, then Alternators/Dynamos. Cheers - Dave
David DuBois

Hi all.

The alternator needs a 'start-up' current to set up it's internal magnetic fields before it can start producing output, though it MIGHT start producing output without it due to residual magnetism.

The start-up current comes from the battery via the ignition light bulb, so a bad connection in this circuit could result in the car being driven around with the alternator not producing any output, which will eventually flatten the battery. I suspect that the battery didn't go flat during sitting for a few days, it was already near flat when the vehicle was left.

When the alternator starts producing output there is no need for start-up current and the ignition light should go out as the alternator's output voltage rises.

Batteries suffer sulphation if they are left in a low state of charge, prolonged gentle trickle charging might help it to regain any lost capacity.


Disconnection of the warning light circuit will definitely *not* cause the battery to discharge when parked. However as previously mentioned, with it disconnected the alternator may never start charging, and so could discharge the battery over a journey and leave it with insufficient charge to crank the engine next time. In my experience (two cars) with it disconnected a used alternator will start charging when revved to about 3k rpm (the previously mentioned residual magnetism) and once that has happened will then charge normally down to about 600 rpm as usual. However a brand-new alternator out of the box may not have this residual magnetism and so may not charge at any revs.

If the battery is flattening overnight some times and not others then there is some other drain on the battery. This could be something like the boot or interior light switches not operating correctly, or possibly even an intermittent partial short. This last should be of some concern, as it *could* develop into a full short which could start a fire.

From what I've heard if a battery goes flat even normal charging from the alternator, let alone gentle trickle charging, will not be enough to restore full capacity, you stand a better chance with a higher output charger giving 15v or 16v. There was a time a few years ago when Mercedes for example were having to select alternator voltage regulators with a control value at the higher end of the range as those at the lower end were allowing the batteries to gradually lose charge and capacity. Typical charging voltages with dynamo could be as high as 15.5v, whereas with an alternator can be as low as 14.3v.
Paul Hunt 2

Thanks, everyone, for the additional information.

FWIW, when I first had this problem, be battery was totally flat. None of the panel lights lit - brake indicator, ignition, or the courtesy lamp in the dash. The fuel pump did not click, and there was no 'click' when I tried to start the car.

I charged it with a standard battery charger overnight, and all was normal. I only drove the car for less than 10 miles and parked it. Two days later, the battery was totally flat again. I repeated the charge process, and about 3 days later I tried it, and it was flat again. I failed to mention that the battery is only about one year old. The unusual thing is that the car sat for about 5 months with no battery tender, etc., and it still started fine this spring. It was two days after that, that it was flat. I checked then that I hadn't left anything on that would draw power, however I did not check the trunk light. I'll check it.

Thanks, Agan.

Howard Battan

Have the battery load tested. Even though the battery is not that old, it maybe not good.

This thread was discussed between 29/06/2006 and 01/07/2006

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