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MG MGB Technical - MGB Ignition Warning Light

A few comments suggested a blown ignition warning light might be the cause of my '72Bs lack of charging. The alternator is only about 3 years old, with barely 2000 miles on it, I would say it is always possible it failed, but not the likely source of my problem. My ignition light no longer works when it is started or running, so how do you easily get at it without long arms or being a contortionist? I am able to reach up and touch it/wiggle it, but not certain how it comes out to inspect if it is blown after 41 years of service.
Joe Dufresne

if your car is the same as a UK in this respect then -

the panel and warning light bulbs are generally in bulb holders - the bulb holders are a push fit into what houses them - these can be a very tight fit

you could try scraping any muck/crud off between the rim of the bulb holder and the end of the housing tube with a very small blade screwdriver

then try to wriggle the bulb holder out of the housing tube or you could also add a quick small spay of electrical contact cleaner

if you can't reach then a thin long bladed screwdriver might be used to lever and/or twist between the bulb holder rim and end of housing tube

I doubt you get near with long nose pliers but if you think you can again you could use a lever action with them against the bulb holder rim

have a look at your Driver's Handbook to find out the type, number and wattage of the bulb you want

at an average of around just 700 miles a year you're putting in more problems to the car than you're taking out plus you're not giving it the chance to run at its potential or optimum or you learn (or remind yourself) what these are and how to drive the car to exploit these and enjoy them
Nigel Atkins

You simply can't say that being only X years old or having done only Y it's unlikely to be the cause of the problem. It's assumptions like that that cause people no end of difficulties in diagnosing faults.

As I said on the Voltage Stabiliser thread, the first thing to do is remove the plug from the alternator, and with the ignition on connect an earth to the brown/yellow wire in the plug. Do not earth the brown wire(s)!!

If the warning light glows then the alternator is at fault.

I'm not going to repeat the rest of the diagnosis steps. The bulb-holder should be a push-fit into a socket in the dash. But before going there it's worth checking that all five white wires are clean and tight in the six-way bullet connector by the ignition switch. That, the bulb holder, and the alternator plug are the only connections in the circuit.
Paul Hunt

Finally got a chance to look at the 72 MGB. I grounded the brown yellow wire to the alternator with the ignition in the on position, no ignition light. Therefore I concluded the alternator is OK. I then removed the ignition warning bulb, it looked OK and I tested it on the bench, and it lit. I looked at the bundle of wires from the ignition switch (which I changed 6 months ago) and they all seem connected. The car starts easily on a full battery charge, so I assume the ground and ignition switch function. So any suggestions as to the next set of diagnostics I can attempt? I have only a good test lamp and a volt/ohm meter, with limited knowledge of electrical troubleshooting.
Joe Dufresne

Another test I made was at the fuse box on the 3rd fuse down (2nd up) green and white. With the ignition on I get 0 volts across the fuse, but when the fuse is removed I get about 11 volts by touching the 2 metal fuse holder prongs. I tried another fuse, to no avail.
Joe Dufresne

Just a warning, you can't assume something *does* work because something else *doesn't* work, both have to be confirmed.

Having said that, because it didn't light with the ignition on and the brown/yellow earthed there *is* a problem with the warning light circuit, at least.

You need to test the voltage on the white terminal inside the ignition light bulb holder, with the ignition on. If you can't tell which is which then test both.

If you have 12v on one of the wires but not on the other, then that is the white wire, and it is working. That indicates the brown/yellow is at fault, so test the continuity between the other terminal in the bulb-holder, i.e. the one that doesn't show 12v, and the brown/yellow terminal in the alternator plug. As before all these tests should be done with the alt plug removed from the alternator.

If you don't have 12v on either of the wires in the ignition light bulb holder, then that indicates there is a break in the white back towards the ignition switch.

This is a simple series circuit ignition switch - white - bulb-holder - bulb - bulb-holder - brown/yellow - alternator. There is a break in at least one place in that circuit, and possibly more than one.

But as I said earlier if you rev the engine to 2k or 3k then even without the warning light connected it should start to charge, then charge normally down to 600rpm. If it doesn't, then the alternator may be faulty as well.

Since you first raised this another fault scenario has come to light (ho ho) and that is when the two wires inside the bulb-holder short to each other. This stops the warning light glowing and can also damage the alternator. Under certain circumstances it can also prevent you switching off the engine.
Paul Hunt

Hi Paul

It was still charging ok though.

Peter
Peter Burgess Tuning

I ran it up over 3000 rpm and it started charging as normal, about 13.8V. It continued charging down to idle. So I am confident the alternator works. Will look to see if there is voltage to the ignition light, then look at the wires at the bundle of wires at the ignition. Will be travelling on business for a week, so it will be a while until I get back into this.
Joe Dufresne

"I ran it up over 3000 rpm and it started charging as normal, about 13.8V. It continued charging down to idle."

OK that's fine then, just a break in the light circuit somewhere. The voltage tests should narrow that down.
Paul Hunt

This thread was discussed between 28/10/2013 and 12/11/2013

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