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MG MGB Technical - taking power from the alternator

I noticed that only one of the terminals on my ACr alternator is used . Is it safe to take power off the unused terminal to a fuse box to power accesories such as light s and a kenlowe. What size of wire should I use?
philip may

Hi.

The other two terminals are for :

1) battery voltage sensing

2) providing current (via the ignition light) to 'start-up' the alternator.

Neither is suitable for the use you describe.

Don
Don

Phil, do you mean the two big lucars within the plug recess? There are two, probably about 12mm wide.

If these are the ones you mean, then they're both connected together (look inside the alt and you'll probably see that they're actually the same piece of metal). Really you should have both connected to the battery to make sure you get a good connection (look at the wiring diagram and you should see two brown wires running in parallel).

However, if you hooked some accessories to this then I'm sure that no harm would befall you; as long as you make sure all the connections are sound etc. Best bet though would be to put in the second connection to the battery and then connect your accessories to the back of the starter motor.

hth,
--
Olly
Oliver Stephenson

Phil - I have set up a power outlet connector in our MGB to power up an inverter for use with out lap top computer when traveling (Street Atlas is a great navigator). I took the power off of the battery lead where it connects to the alternator using 10 AWG wire and fusing it for 30 amps at the take off point so the wire is protected in case of any short circuits. While I was setting this up, I also put a couple of connectors under the bonnet to connect any diagnostic equipment that I might be using. Good luck - Dave
David DuBois

Hi all....

I understand that some Lucas ACR alternators come in two 'flavours'..

1) Battery sensing, where one of the larger lugs is to be connected to a wire that goes to the starter solenoid, so that the alternator's built in voltage regulator can 'see' the battery voltage, rather than the battery voltage plus the small but inevitable voltage drop in the alternator main feed wire.

2) Machine sensing, where the two larger lugs are connected together inside the alternator, and the voltage regulator sees the output voltage of the alternator, rather than the battery voltage.

I don't know which of these types was fitted as standard, but many B's have replacement alternators by now anyway.

So far as I know, the only way to tell which type you have is to remove the cover and see if the large lugs are strapped together.

Does anyone else know the secret ?.....

HTH... Don
Don

Hey Don, that battery voltage sensing type sounds a bit high-tech for our lowly MGBs! Would like to know what they were fitted to though lest I should need to work on such a car in the future.

ttfn,
--
Olly
Oliver Stephenson

Hi Olly

I suspect that some B's had such an alternator fitted, as they have a thick wire on one of the large terminals, and a thinner wire on the other large terminal, but don't quote me on this !!!!.

The Haynes book 'Automobile Electrical and Electronic Systems' gives some info on the topic, also take a look at www.vtr.org/maintain/alternator-repair/html

Don
Don

Early alts were five-pin and had one thick and one thin brown, the thin brown was for external sensing and shouldn't be used as an output as heavy loads could cause the alt to supply too high a voltage to the rest of the electrics. Then there were two-pins alts with only one thick brown output wire and internal sensing, then three-pin alts, which might or might not have two large output terminals. If your plug has two large spades and only one of them is used, then confirm the two spades on the alt are connected together internally, and if they are you can use the spare one for an additional output. I have done this on my V8 precisely for the fans, and a worthwhile difference it made too, along with local grounds for each motor instead of relying on the relatively thin gauge headlamp ground. Use a gauge similar to the existing thick brown wire.
Paul Hunt

This thread was discussed between 25/04/2003 and 27/04/2003

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