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MG MGA - Alternator Wiring

I have converted to neg earth recently with the existing generator and have now replaced the generator with a Hella Alternator . The mechanical installation is finished and I now need to wire up .There is a group of 3 spade connectors on the back ( top one smaller than bottom two) and a separate single spade. The instructions are useless ..but a European Connector was provided for fitting three wires. I have read up Barney's site and others and not 100% sure and do not wish to stuff the unit up. Can the wise advise?
Neil Ferguson

on my Lucas alternator the larger spade connectors are in fact the same main output terminal connected together.
This connects to the thicker yellow wire on the loom.

The smaller terminal is connected to the yellow/green wire which connects directly to the ignition light on the dashboard.

If your Hella alternator is wired the same as this you can then follow Barneys wiring instructions without any major problems.

I connected to the OE Control Box as he shows on MGAGuru and it worked but there was a problem with the voltage cut-out in the regulator still trying to operate. (the cut out sometimes still clicked in and could have caused a potentially damaging short circuit through the alternator)

So I would recommend that you dont use the regulator as a connector block but use his 2nd option of wiring it up without the original Control Box.
You can still leave it in place for appearance sake if you prefer.

Also, as recommended by Barney, I did fit the extra yellow wire directly to the battery,s positive terminal from the large output terminal to cope with the alternators much higher amperage output.

It was a really easy mod to do to the car and it works brilliantly.

Colyn Firth

Do not guess about alternator connections. There are several different connection methods for various alternators, and if you get it wrong you're stuffed. You might get the connection information from the alternator manufacturer. Otherwise you need to know what car it came from, and get the wiring diagram for that car. I do not have connection information for every alternator in the world.
Barney Gaylord

Neil, you need to check that your alternator is the Hella equivalent of the Lucas 16ACR, 17ACR or 18ACR.
(These were all fitted to the MGB and the 16ACR will give you more than enough amperage output for your MGAs power requirements )

If it is not the equivalent of the Lucas design then you really should contact Hella (or an auto electrician)to get the wiring diagram to see how it can be wired to the MGAs charging system.

Best of luck

Colyn Firth

Try this link - a match?

I can see why you think it's useless...
but, according to another article I found (

B+ is the high current output from the alternator (B is for battery, I am guessing)

D is for Diode. D+ is the signal to the warning light on the dash. D- is ground.

This might be clear-er:

Use Google - there are many articles out there.

Good luck!


AJ Mail

If worried about underbonnet appearance you could gut the regulater and just use it as a junction box. A couple of wires will need moving.I did it that way on a Sprite with a 106 regulater.
A Anstead

Just viewed the Hella Alternator catalogue online and it is a Hella model CA18 ,43 amp unit ( with built in regulator ) and the equivalent and replacement for a Lucas 18ACR on an MGB so believe I should be safe wiring it up on the same basis.
Neil Ferguson

Alan, I should have done what you suggested and gutted the control box to turn it into a junction box.
I was reluctant though to ruin a perfectly good control box.

After fitting the alternator I had the problem of the regulator cut-out operating even after the ignition was switched off. This caused the ignition light to stay on even though the key was out of the switch and meant that the battery was shorting through the alternator.
Luckily I have a battery cut out switch and turning this off would make the cut-out switch off.

I cured this by putting thin strips of plastic insulation between all the contact points inside the control box and I have no problems since then.

I suppose the next owner of the car can reverse this if they wish to convert back to a generator.

Neil, let us know how the Hella alternator conversion goes.

Colyn Firth

This thread was discussed between 15/04/2012 and 17/04/2012

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