Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Electrical gremlins affecting headlights


I think I have a couple of gremlins fighting each other for attention.

On my 72 gt, when I switch on the lights to the first position there are no sidelights, front or back.

In the second switch position, all the sidelights come on but no headlights at all. Although the bulb showing blue for full beam is lit up, whether I switch from full beam to dip the blue light does not go off and no headlight of any kind.

Is there a problem with the headlight ground?

Any pointers appreciated.



Iwan Jones

First call point:

I guess you have a UK spec so probably sheet 11 or 14.

by the sound of it you may have a combo of things. Try bypassing the light switch by applying 12V to Fuse block pin 1 (red/green) if the sides then come on the switch or wire to P1 is definitely suspect.

If however the gauge lights are on at position 1 then it may just (Just!) be the wire from switch to fuse block

Dips (you don't mention if they work! have a different route. power to switch is the same but comes out on Blue at position 2. Then via stalk switch to Blue/Red to the lamps. When you switch to MB then Blue is transferred from blue/red to Blue/White. (Don't forget that the flash will work but the MB will only work if light switch is at position 2)

The MB 'indicator' is off the Blue/White line and comes on so there is probably power there but it could(!) be a stray supply from the stalk switch.
So basically the sides and main are only linked by the switch therefore if the switch has power (brown wire) then you have 2 directions to check.

Be methodical and do one section at a time and you will get there in the end :)

B Anderson

Yes it sounds like the headlight earth is open-circuit at least. What happens is you turn the dipped beam on they earth backwards through the main beam filaments and the main-beam warning light to the warning light earth behind the dash. The headlamp filaments are low enough resistance to light the warning lamp, but the warning lamp is too high resistance to light the headlamp filaments. On main beam the warning light is illuminated directly, but there is no path at all for the headlights. The headlamp earth should be with the starter relay, near the fusebox, that's if the bullet connectors in the black wires by the right-hand headlight are sound.

As for the sidelights I'm betting that you have the brown wire (12v supply) on the headlight terminal, so nothing will light until the switch is in the headlight position. You will have to do some testing to work out which terminal is which. All three are isolated in the off position. Two are connected together in the sidelight position and the third isolated. In the headlight position this third terminal is joined to the other two, and that is the terminal for the headlight blue wire. Electrically speaking it doesn't really matter which way round the first two are. It could also be that the switch is faulty, or the red sidelight wire is connected to the blue wire instead of its switch terminal, but that's less likely.
PaulH Solihull

Hey thanks, got the headlights sorted now, also the side lights coming on in position one on the light switch. It was more than one thing but I think the main problem was the earthing at the front of the car. Think someone had put on a snap connector on an earth wire for another light (aftermarket spotlight perhaps)and this was affecting the earth.

However, I now have no indicators either side; also, no dash lights.

Will work on those next.

Thanks for the advice.
Iwan Jones

All progress is good progress.

Dash lights could well be the 'dimmer' mine used to be Very DIM and off! flashers are a completely different circuit being part of the green line where panel(dash) are red/green from light switch to dimmer to red/white to the lamps. (In daylight they can be very hard to see)
Good luck and please keep up the feedback. Its always good to know what cures ailments :)
B Anderson


Yep, I think everyone needs to provide feedback into problems. After, checking wires, cleaning contacts checking fuses, the sorting out of the mess to the earthing left by a previous owner was probably the real cause of getting things working. Only to have the indicators and dash lights go. No, probs, going to be working my way through logically through the problems today after the helpful comments above.

However, this is odd, I think there is a real gremlin at work here!

Switched the ignition on turned the light switch on, dash lights working! Moved the indicator stalk, left and right lights, they was all there.

Feel cheated, no ...... worried as I don't now know what caused it and it may happen again.
Iwan Jones

Simply being near any bad connections can do it, as can temperature changes or phases of the moon. Replace all the sleeve connectors on the car, as a start. There are about 20 doubles and 10 singles in an MGB, cost about $10 US in toto. The high load ones fail first, but they all crack from aging, and 30-35 years appears to be the critical time. Since they are inside the rubber sleeve, you cannot see the cracks, but the connections will be dodgy and V annoying.

Use OxGard, InOx, No-Ox, or similar on all connections. This stuff is sold as a anti oxidation grease for mains service cables (where aluminum conductors meet copper ones), so look wherever you would get stuff like that.

FR Millmore

Try bypassing the rheostat, if that makes them brighter than the rheostat on its brightest the rheostat is faulty, like all the ones I've had when the various cars have come to me. All mine are printed-circuit types and part of the printed circuit has burned out, probably due to the lights having shorted out at some time. That destroys the low-resistance path that lights them at full (a relative term) brightness, leaving a higher resistance path so they still work to some extent.

FWIW I've been experimenting with improving the instrument lighting for some time. Recently came across a multi-element LED which I had in mind for improving the pilot light in the V8 headlights, but it made no difference. Nothing to lose so tried it in the instruments and the difference was pretty amazing, see attached. That is a standard tungsten in the speedo, and the LED in the tach. True comparison, no Photoshopping.

I'd tried a number of single-element types in the fuel and tach gauges before but none of them made much difference. However in the oil/temp the best of the single-element also worked well, whereas in the fuel gauge nothing makes any difference. That's because in the oil/temp the bulb is at the top and shines right through the concealed hole at the top of the rear part of the face, whereas in the fuel the bulb is at the bottom so has to bounce around inside the gauge before any of it manages to find its way through said hole. Experiments with fuel gauge continuing.

PaulH Solihull

Hey Paul, that is a huge improvement. Where did you get the LED's? I know I can get Petzl head-torch LED's but they are quite expensive.


Iwan Jones

2 for a pair and free postage! I can't remember if I or a pal found them first but I started off with wedge types for the pilot lights, which aren't ideal for the instruments. Then found bayonet types which are equally as good illumination-wise, but still need the bulb holders to be replaced as all mine take screw bulbs. Kept looking for screw versions but only just found them here, currently on order so don't know if there any installation issues yet. I've used the wedge and bayonet versions in the main instruments on the roadster and V8, and there were some installation issues as they project forwards more than the tungsten bulbs, and I noticed the bayonet type was touching the mechanicals inside the speedo - fortunately before I drove it and possibly damaged it. Modified the holder to correct that. The wedge types also had an issue in that they didn't push into the holder as far as they should, so again stuck out further than they should, and could also fall out (into the back of the roadster speedo ...). Modified those holders also, so apart from the fuel gauges which I'm still working on I'm very pleased with the rest. There may be more installation issues on 77 and later instruments with the green glow as they have a green plastic bubble over the end of the mounting tube, which will limit how much light from the four radial elements on those bulbs can get into the casing.

Full story here

They take one tenth the current of tungsten, so with all LEDs the dimmer no longer dims as the current/voltage/illumination characteristics are completely different, but will turn them off if it has an 'off' position. If you have a tungsten left elsewhere like in the cigar lighter then they will dim a little, or you could add a dummy load of a suitable value and it would dim normally.

PaulH Solihull

This thread was discussed between 25/02/2013 and 03/03/2013

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.