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MG MGB Technical - Dash board lights !!

Coming almost to the end of my two year renovation project I decided to upgrade the lights on my GT. The side lights, stop and tail and indicator all upgraded to LED are brilliant as is the interior and console festoons, only the map light and instrument bulbs to source, and this is where I ran into a problem. The original 12v 2.2w bulbs are just not up to the job,I have seen brighter fireflies, I have tried 12v MES fitting 100degree LED's,again not successful. The only 12v 5w filament bulbs Ive found unfortunately have too big a glass bulb head to enter the instruments, due to the larger filament. The only thing I haven't tried is removing, testing, and possibly cleaning the dash rheostat. Can anybody tell me if they have successfully made their instrument lights brighter,if so how, either by bulbs or perhaps changing the rheostat or perhaps an earthing problem?? Help needed please. Peter

Yes to the brighter dash lights. First by-pass the rheostat. Who needs to dim the dash lights???? Second I took the instruments apart and lined the metal backing with glued on aluminium foil.
Allan Reeling


Painting the inside of the cans gloss white helps.

Herb Adler

I may be wrong but I didn't think a rheostat worked with LEDs. Like others I shorted out the rheostat which made a big difference . Also repainted the can white. People have used halogen but they got too hot. There are bits about illuminecscent wire in the archives too. It will never be like a modern car but as you know at what "angle" every needle should be I decided to live with it!
Michael Beswick

The standard rheostat won't dim LEDs as the voltage/ light output characteristics are completely different to incandescent. Even if you only have *one* LED that won't dim even though the remaining incandescents do. The printed-circuit rheostat (which is the only type I've come across in two MGBs and various other cars of the era) has a failure mode where it still dims and brightens the lights but they are no where near full brightness at max on the rheostat. I've tried multi-element MES 12v LEDs in the past and they are slightly brighter, but don't seem to be available anymore. More recently single-element LEDs make no difference at all. BA9 high-brightness might be better but are bayonet, and again the heads might be too big. There *could* be an earthing problem, use a meter to see if there is any voltage between the gauge cases and a known good body earthing point, but again it wouldn't affect LEDs

By coincidence a pal has sent me info on flexible 12v LED strip that he cut into 100mm sections and stuck in the can and the comparative photo (attached, yes that's the unmodified version you can barely see bottom left) he sent is pretty remarkable (OK, not the camera shake). He also attached a potentiometer (500 ohm 3 watt wire wound) plus a 390 ohm half watt resistor to the back of the rheostat bezel, wired between the red/white and earth (resistor on the earth side of the pot, slider feeding the LEDs) to dim them. Given the apparent brightness it's probably the first time any MGB has needed a dimmer. In the UK probably has the cheapest strip at 13 per metre, which can be cut into 50mm sections.

PaulH Solihull

Herb's LED strip here: and scroll down a bit.
PaulH Solihull

Side issue so hope ok - my rheostat is broken and it seems as though the dashboard lights are at max dim. Can one bypass the rheostat to get max brightness? If not how does one repair the rheostat?

My rheostat (1980) has four terminals - two on each side with only one wire to each side - it's a simple matter to move one wire to the other side, thus bypassing the rheostat.
Geoff E

Years ago, I swapped my stock Lucas alternator for a 1978-80
Ford Fiesta Bosch (#13107) 55 amp alternator.

Ever since, every single light bulb in the car shines much
brighter (including the gauges and interior lamp) and the
battery is charged more completely (= stronger engine spin
when starting).

Just a thought.
Daniel Wong

If the dash lights are working at all then you probably have the printed-circuit type of rheostat. This has series and parallel circuits for the current and when the lower resistance of the two burns out (see, as been the case on all the three or four I've had to replace in different cars over the years, the lights go dim although they can still be turned down to off. The wire-wound rheostat (which I've not seen), if that goes, would normally cut the lights off altogether. I did wonder if the printed-circuit type weren't up to the load, but later cars had many more lamps, so I suspect it was more a case of the dash lights having been shorted out at some time. Replacement rheostats are very expensive, for the use they are. My 73 is the same as Geoff describes, but some years and markets had two wires on the one side in different spades making that simple bypass more difficult. If you do have the rheostat type and the resistance wire is broken near an end tag it may be repairable.
PaulH Solihull

This thread was discussed between 26/08/2011 and 30/08/2011

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