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MG Midget and Sprite Technical - Ignition light not lighting, alternator issue?

Doh! Just as I was getting some confidence in the reliability of the car, something else happens. Could do with it tomorrow too.

I have just had the alternator off to replace the bearings inside as they were making all kinds of noise. All seemed to be fine in the replacement and reassembly.

Having reconnected the alt. when I turn the ignition on, the ignition light does not come on. Disconnect the alt. same situation.

Ideas on a postcard please, whilst I go and check obvious things like the bulb. :-)

Cheers,
Malcolm
Malcolm Le Chevalier

OK, bit of diagnostic probing reveals that with igntion in the on position I am getting +12v on the signal wire from the alternator (without engine running), thus "cancelling" the +12v ignition feed, stopping the bulb illuminating.

So it seems to me some sort of alternator wiring issue.

I shall dig further.
Malcolm Le Chevalier

Presumably the "signal wire" from the alternator should be coming from the alternator output connection. Is it perhaps connected to the alternator input energising wire?
Guyw

Thanks for the input Guy, I think the issue is a bit more complex. I have pulled things apart and one of the rotor to slip ring wires is broken.

Can't quite figure out why this would cause said issue, but it's an obvious fault that I need to fix somehow.

Cheers,
Malcolm
Malcolm Le Chevalier

Can't see how a broken slip ring wire would result in an output feed to the signal wire. I clearly haven't a clue how these things work - although at one time I thought I understood them !!
Guyw

I can't figure it out either, but repaired and working again now.

Used one of the conductors from a bit of ethernet cable to repair it. It was all I could think of that was small enough! :-)

Glad I found a fix as I need the car for a couple of days work. If I had to take the train it would be barely worth it as tickets cost so much.

Cheers,
Malcolm

Pic 1, broken wire on left.


Malcolm Le Chevalier

Repaired.

Note to self, next sign of any alternator trouble, just buy a new one. This one is pretty past it!


Malcolm Le Chevalier

Well done Malcolm. Another satisfactory day!
Guyw

Are you sure you were measuring 12v from the alternator and not the battery?

You will see 12v on both sides of any bulb that isn't grounded.
Dave O'Neill 2

Nice fix Malc.....I've done a lot of those kind of repairs in the past...I must admit you were really quick on that one. Necessity usually expedites the process...wouldn't you say! lol!



S

Well done malc

but i have to laugh... i was like you not to long ago... it has to be fixed properly ...no matter what

now... im like ...wire cutters (check) find the offending issue(check) ... clip and done ....lets roll

tomarrows a new day ill fix it correctly then....LoL

and yet ive still got a new alt sitting next to my front door waiting to be installed in the midget... which reminds me... i got to recharge the battry when i get home tonight

prop
Prop and the Blackhole Midget

Malcolm
If you disconnect the bulb wire from the alt with ignition on then you will measure 12v on the wire because there is no volt drop through the ignition bulb when it is not connected to earth via the alt. With "signal" wire connected to alt, the bulb wouldn't light when the field wire was broken as the earth link is broken (but you'd still measure 12v at alt connection).
Bill
W Bretherton

This thread was discussed between 23/08/2016 and 25/08/2016

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