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MG MGA - alternator snag

I probably should have expected this after burning out the front loom recently when wiring up the front running lights.
I have just fitted a lucas alternator which proved to be suprisingly straight forward. I followed Barney's MGA Guru instructions to the letter and everything worked perfectly. I moved the wires on the voltage regulator as instructed and fitted the extra output wire to the battery to prevent any overloading of the standard original generator (yellow) wire.
The ignition light now goes out instantly on start up and the alternator is obviously charging ok.

Today, however, after a 30 mile drive I was surprised to see that the ignition light stayed lit even though the ignition key was switched off.
I quickly turned the battery isolator switch off and then I heard a click from the engine bay and the ignition light then went out. When I tried the engine again later it was fine and the ignition key switched everything off as normal but later on the same thing happened again and the ignition light stayed lit even with the key removed.

I have double checked my wiring and it seems ok.

I recall many years ago I had a similar problem when it turned out that the voltage regulator had stuck on and caused a similar ignition light fault and almost a fire.
Does anyone think it is an alternator fault, my wiring, or that the original voltage regulator is causing this.
My feeling is that I should probably fit another voltage regulator with the innards removed so that it is just acting as a connector block.
Any advice would be appreciated, (one fire per 12 month period is plenty!)
colyn firth

I just read something about this on a tech site (not Barney's I don't think) - had to do with a known issue with GM style alternator conversions on older cars. I'll try to refind the reference for you. Meanwhile, Google it and you might come up with it yourself before I do.

AJ Mail

Found this on the Painless Wiring FAQ board - not the one I was thinking of but similar line of reasoning...

"Faulty alternator/regulator. GM 10SI and 12SI alternators sometimes feedback voltage into the exciter wire even when the engine is shut off. This causes voltage to feed into the ignition circuit which keeps the coil hot with voltage and causes the engine to keep running. Remedy this issue by installing a diode #276-1661 from Radio Shack, inline with the alternator exciter wire. Make sure to install the diode with the line closest to the alternator. By installing this diode you have introduced a one way gate in which voltage can only flow to the alternator and not vice-a-versa on the exciter wire."
AJ Mail

Thanks Jim.
I have just looked in the archives and seen the that someone had similar problems with an alternator conversion which were solved by soldering the wire connections at the original voltage regulator. I noticed that the connectors are a little small to fit 3 wires into so I will try this first but I will see if I can get hold of a diode from our local Maplins electronic store and fit it if the soldering doesnt help.
My car is positive earth so I will have to work out which way to connect the diode and also if it will work in the high temperatures next to the engine.

If anyone sees a black mushroom cloud to the north of Doncaster over the next few days, just ignore it, it will just be me burning out another MGA!
colyn firth


If it's any comfort mine occasionally does the same. Like you, I knock the isolator off (as I always do when I shut down).

I notice the other guys making reference to a GM alternator, but it would seem our Brit versions are wired the same. Looks like a trip to Maplin.


Which is the alternator exciter wire? - My basic electrics knowledge is okay but my electronics is abysmal.

Steve Gyles

I just had the same problem I solved it by taking off the control box and connecting the three terminals together underneath the box as I to found it difficult to connect all the wires into the one terminal. There is a piece on this on barney's site near the end. I whis I had read it a long time before I gave up and asked the question as I was fiddling with the conections an flicking the paddles on the control box for months untill I found the answers on this site.I expect you will find it in the archives but can't remember the thread.I must say when my light was glowing it did not seem to keep the enging running I will keep a watch on this as it runs on a bit when hot.

David swaine

Thanks David, I was thinking along the same lines.
Im going to solder the connections and probably not use the original regulator connectors at all.
Did you say you had left the regulator in place and used it just to cover up the wiring mods?
Also how did you safely connect the wires and insulate them?
Like in your case, my ignition light stays lit even with the key removed but the engine stops.
So Im sure we have had the same problem.
colyn firth

I have been looking into some more alternator threads in the archive and found that the problem with the ignition light is not so unusual.
Like Steve and I, one writer heard a definite click from the engine bay when he switched the battery isolator off.
He, like me, suspected that a voltage regulator coil had cut in but then would not trip out when the ign was switched off.
His solution was to put some insulating material between the contact points inside the regulator and apparently this solved it with no recurrence.

So I will try this first and if it doesnt work I will then solder the wires and not use the regulator connectors.

I will let you know how it goes Steve.

I have managed to locate a 6amp diode from Maplins to fit into the thinner ignition light wire from the alternator as suggested by Jim in NH USA but Im a little concerned that current needs to actually flow in both directions to operate the ignition light.


colyn firth

The problem is common. It is a fault in the old regulator causing the cut-out relay to be engaged when it should never be active. A simple solution is to disconnect the wires from the old regulator and tie them together with a few twist nuts, just stop using the control box for a junction bar. This cured the same problem on my car.
Barney Gaylord

Yep Barneys right. But I liked to keep the loom looking nice so I keep the control box and left all the wires conected to the original terminals There is plenty of room to cut all the tin conecting straps underneath compleatly taking out any functioning of the control box. I then rolled the top three around a thick peice of houshold earth wire and soldered the lot together as per Barneys instructions on his webb site. I also run another thick wire direct from the Alternator to the battery come starter switch, belt and braces.
sorry cant show you a picture of box without taking it all apart but ist an easy job if you have a large soldering Iron.

You are on your way now good luck with it

David swaine

Thanks David
I have had another look at this today, I must admit that I am really reluctant to rip the works out of a perfectly good (brand new) regulator. So I have cheated a little and I have isolated ALL the contacts inside the regulator with thin strips of plastic which seems to have solved the problem.
( No pall of smoke so far!)
I am now looking for an old, non functional, regulator which I will modify as you have done, I think that probably this the best way to go.
Thanks everyone
colyn firth

This thread was discussed between 19/11/2009 and 26/11/2009

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