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MG Midget and Sprite Technical - Ignition light

Hi,
My ignition light no longer lights!

1275, Mk3, RWA, midget, with alternator conversion - regulator box removed.

The bulb is ok.
The wires are connected at the alternator.
The alternator seems to be charging ok, the battery has not died yet!

What is the best way to diagnose and fix it?

There are two wires on the bulb holder, one comes from the alternator, where does the other one go?

Thanks

Dave
Dave Brown

Poor Earth?
Mark O

The warning light contact not connected to the alternator should be connected to an ignition switch controlled +12V feed from the battery. IIRC it provides a small initial current to start generation and when the 12V+ power is being produced current no longer flows so the warning light goes out.
David Billington

Hi,


On the OE harness there are two wires, one white and a brown/yellow.


As David says the white wire should be +12v with the ignition switch on.


The brown/yellow goes to the small terminsl at the alternator providing an earth when the engine is at rest ie illuminating the bulb.


Once the alternator starts to produce current the brown/yellow wire then has running voltage normally 14v).

The bulb then goes out as it has charging voltage either side of it.


First test is to pull off the alt plug, key on and then earth the small terminal, the bulb should then light. If it does after that test the alternator is defective.


If not check it has a live supply to the bulb from the ignition circuit.


Hope that helps a bit.


Regards Steve
SR Smith 1

Volt gauge, 2 wires pos/neg and done...no more idiot light to tell you the battry is dead when you park at wal mart 100miles from home
Prop and the Blackhole Midget

Hi Steve,
Earth on the little wire at the alternator and the bulkb lights up. So faulty alternator.
What is the general opinion, repair the old one or new one? The old one is not that old so bearings, brushes etc. should still be good.

Thanks for all the advice.

Dave
Dave Brown

Repair if all the other stuff is good.

Easy to repair, and cheap.

http://www.ebay.co.uk/itm/ALTERNATOR-RECTIFIER-FOR-LUCAS-15-16-17-18-ACR-see-notes-in-advert-REF-3923-/111573978155?pt=LH_DefaultDomain_3&hash=item19fa53cc2b

I assume it's a lucas.
Lawrence Slater

Dave - I have a spare 18ACR (43 amp) alternator. Remanufactured about 10 years ago and dry-stored since.
It has the shaft key but not the pulley or fan.
I paid £25 + postage. You can have it for the same if you want. That's roughly the same as the cost of a rebuild kit.

Cheers;
Rod
RS Hughes

And here is a full rebuild kit.

http://www.ebay.co.uk/itm/LUCAS-ALTERNATOR-ACR-REPAIR-KIT-15-16-17-18-LRA100-RECTIFIER-REGULATOR-BRUSHES-/181025062176?pt=LH_DefaultDomain_3&hash=item2a25ef3d20
Lawrence Slater

Lawrence typed faster than me!
If it's the regulator/rectifier at fault (which is highly likely) his solution is easier and cheper then mine. Good luck!

Edit: the full rebuild kit got cheaper since I last looked. It gets better and better for you!
RS Hughes

But 25 quid's cheap for a new alt. So if you aren't handy with a soldering iron, that's a winner too.
Lawrence Slater

Thanks for all the info.
I think I will go for the repair kit.

I am now wondering why it broke.
I did charge the battery from very flat without disconnecting the alternator. I think I have been told to disconnect the alternator if connecting a charger but decided to ignore it this time.
I have charged various batteries in my time and never blown a diode.
Is it a myth that charging batteries with alternator connected will blow the diodes?

Thanks

Dave
Dave Brown

Rectifier kit has been ordered.

Thanks for all the help.

Dave
Dave Brown

I think you're thinking of running the engine without a battery, damages the alternator.

That might happen if the alt' doesn't have an anti-surge diode. But I think most alt's now have one. New ACRs do I think.

Do they? Anybody know for certain?
Lawrence Slater

Hi,
Rectifier fitted yesterday. Quite a straight forward job. 4 screws, a nut and four soldered wires.

All refitted, but still no ignition light... so something else must have has gone in the alternator.

Any suggestions?

Also, how can I test it on the bench, save keep on refitting it?

Thanks

Dave

Dave Brown

This thread was discussed between 14/04/2015 and 26/05/2015

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.