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MG Midget and Sprite Technical - LED light puzzle

I recently replaced the rear side/stop light bulbs with LED equivalents. Tonight, checking for tomorrows MoT test, I found that with ignition on my front side lights illuminated and when I switched my lights on the number plate lights failed to light (as did the reversing lights). I put the old bulbs back in and (phew) everything is back to normal.

What's going on here. Does this suggest I have a wiring or an earth problem or what? Those of you who understand this electricity thing please explain.
Chris Hasluck

is the car pos or neg earth
mark 1500 on the road Preston Lancs

For a pure guess they may be in backwards as LEDs are polarised. Did they work when you fitted them? That could be an issue with a festoon bulb or non polarised bayonet bulb. If that is the issue then putting them in the other way round may get them working. Hopefully they can stand 12V reverse polarity without suffering any damage.
David Billington

HI Chris.
Possibly a polarity problem? LEDs are polarity sensitive and only pass current in one direction. Standard incandescent bulbs don't give a damn which way round they're wired.
If it's possible to put the LEDs in either way round try both orientations.
If the locating pins are not staggered it's worth a try.
Usually bubs with staggered pins are for the likes of headlights where it's important to get them the right way round so the High and Low beams are correct, or in tail lights so the side light and stop light are in the correct circuits.
If it's physically possible to fit the LEDs in both orientations it's got to be worth a try.
At least you're good to go for the MOT so you've got some breathing space.
Good luck!
Cheers;
Rod.

Edit: Chris had the same idea but typed faster than me!
RS Hughes

Negative earth. Bulbs can only be fitted one way because of the bayonet pins (to ensure the 20W bit is the brake light), The LEDs worked fine both tail lights and brake lights. I did not notice the side lights coming on when ignition turned on until tonight cos I did the tests in the dark.
Chris Hasluck

Okaaaay........that's a headscratcher Chris.
I'll have a keek at the wiring diagram and a bit of a think.
At first thought it still seems to suggest a polarity issue - it may be that a PO has jury-rigged the wiring to get round a previous problem for example. The reason I say that is that LEDs are usually hugely reliable devices and it's unusual to find new ones to be faulty.
It may not have shown up before because there weren't any polarity sensitive loads present until you used LEDs.
A wise man recently reminded me that our old cars are very likely to be non-standard here and there. So the wiring may be a bit ad lib. I certainly found a few curiosities in mine.
Do you have a wiring diagram handy? If not I can maybe help.
BTW do the front sidelights still light up from the ignition switch with the original incandescent bulbs fitted?
RS Hughes

Chris,

Yes the stop/tail should be OK unless the wiring has been changed or polarity reversed as they can only go in one way but those weren't the lights you said didn't work. The ones you said didn't work can normally, in my experience, be fitted either way and so you may have a 50% chance of them not working, have you tried fitting them again and try each way. Maybe too late this evening but worth a try after the MOT.
David Billington

Yep - like David says, try turning around those that can be turned around.
As before - good luck.
RS Hughes

This thread was discussed on 02/03/2015

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