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MG Midget and Sprite Technical - Indicator Mystery

My N/S front indicator is not working, the sidelight is flashing instead but not at full brightness and at double rate, which I assume is down to the lower wattage.

The indicator bulb works when tested off the car.

The sidelight bulb goes out when I remove the indicator bulb.

The rear indicator is working fine apart from flashing at double speed with the sidelight.

The O/S indicators work perfectly.

Testing the indicator socket (bulb removed) with a multi-meter shows a fluctuating 5v-ish, the sidelight doesn't light up.

I've removed all the wrapping from the wiring behind the grille and can't see any breaks or bare wire on either circuit and all bullet connectors are tight. This appears to have made the horn, which was working when I started, stop working, but that's not unusual and is down to a poor connection behind the grille.

I have never removed the sidelight unit and am reluctant to unless it's thought the problem's there as the retaining nuts look as though they are seized. The indicator bulb socket is clean with no sign of rust or corrosion.

What should I try next?

Any suggestions gratefully received.

Colin

C Mee

It sounds like a bad earth connection. Can you try a temporary earth wire with a crocodile clip to a known good earth?
Dave O'Neill 2

Thanks Dave.

Jump lead applied and the indicator works!

I should have known that considering the number of times I've seen it on other people's cars and said it must be due to a poor earth!

Now to remove the unit without shearing the studs!

Cheers

Colin

C Mee

Colin,
The unit should have an earth lead as well as the two power wires.
You may find that pulling the earth bullet connector and checking the unit, may mean you don’t have to remove the unit as the poor earth could be further up the earth wire / ground connection.

R.
richard b

Earth wires weren’t fitted as standard. The lamp body normally earths through the bodywork, unless it’s fibreglass.

It’s not a bad idea to fit them, though.

I have a feeling I may have added one to my B when I replaced one of the lamps, a couple of years ago.
Dave O'Neill 2

Light unit removed - one stud sheared (I was sure it was the nut turning after wire brushing and dousing with penetrating oil). Suitable BA replacement found in a 'might come in useful' box.

Richard, there was no sign of there ever having been an earth lead and no redundant black wires in the bundle beneath the slam panel.

There is a bit of surface rust on the side-light 'bowl', the indicator side is clean. The back has a fair amount of surface rust so has been wire brushed and treated with Kurust prior to zinc priming and painting, though I will make sure there's metal to metal contact around the screw holes for the earth when it all goes back together.

With the grille off I've been able to clean up the horn connections so it's working better than it has for a long time.

What are the official route and mounting/anchorage points for the wiring loom at the front? Mine was just draped across in front of the radiator until I zip-tied it to the top of the central strut beneath the bonnet catch. There is nothing to show the routing in the workshop manual and all the suppliers' parts diagrams just have the loom ending vaguely in the middle of the grille area and do not show the route the wires take out to the light units.
C Mee

The loom should be clipped to the leading edge of the slam panel using item 55 here…

https://www.moss-europe.co.uk/shop-by-model/mg/midget/electrical/wiring-harnesses-fittings/wiring-harness-fittings-sprite-iv-midget-iii-1500-1967-79.html
Dave O'Neill 2

Thanks Dave. I'll add them to my next order.

Many thanks.

Colin
C Mee

Must be age again !
Confusing the headlight wiring !

R.
richard b

What I don't understand is why duff earthing made the indicator stop and the side light start blinking.

They are mounted on the same backing plate so must share the same earthing route. Is it something to do with the resistance created by the poor earth connection?

What route is the current taking to the sidelight if it lights up despite being switched off?

Thoroughly confused.
C Mee

The voltage supply is coming in through the indicator bulb, then through the lamp body, through the sidelight bulb, but in reverse, then it goes to earth through the other three side and tail light bulbs, as well as the panel light bulbs. As you have possibly as many as seven bulbs in parallel, the resistance is low, so the one sidelight bulb will glow, but the others won’t.
Dave O'Neill 2

Thanks Dave.

I've had to read that several times to get it clear in my head. Thanks for explaining it.

So, presumably, when the indicator earth contact is good, the current takes the line of least resistance and only passes through the indicator bulb to earth and doesn't stray into the sidelight circuit.

Colin
C Mee

This thread was discussed between 17/06/2023 and 18/06/2023

MG Midget and Sprite Technical index

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