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MG MGB Technical - Alternator Wiring

Hi,

I am after advice on electrical issues to ensure I don't melt my wiring over the next few days! Yesterday I had to do an emergency dynamo to alternator conversion on my 67 BGT. I used the alternator from my 70 BGT which I assume is standard rating (is that 45 amps?). As i hadn't expected to do this the only shop I had access to was Halfords. The thickest wiring they had was 3mm2 with a 27 amp rating (which the packaging said was suitable for alternator charging ciruits). I wasn't very happy with this so what I have done is run two lengths of this wire from the solinoid to the alternator (it has 2 output terminals, so one wire on each).

Is this safe to leave like this, or should I get one thicker wire? What sort of wiring rating do you need compared to alternator rating? How does having two different wires compare to one wire? Can I just add the ratings, or are two seperate wires greater or lesser than the combined rating?

Cheers
Iain

I D Cameron

Iain,

The rated oiutput from your alternator is 22 amps so you are well within capacity of the 27 amp cable. Now running two cables will give 54 amp capacity provided of course that they are both of identical length. If for example one is an inch shorter than the other then the current will always take the shortest path and a higher percentage will go down this cable. As you are so far within capacity it doesn't matter and you can leave your new wiring just as it is.
Iain MacKintosh

A 16ACR as used on a 70 MGB is 34 amps, it was the dynamo that was 22 amps. Having said that the car isn't going to demand any more current from the alternator than the dynamo unless you added some electrical loads as well. But whereas a dynamo will begin to reduce output voltage and hence current as it gets towards the maximum factory loads, the alternator won't to the same extent so you will eventually get more current from the alternator than the dynamo. With the standard wiring all that will happen is that you will get a bit of volt-drop in the wiring, which probably won't even be enough for you to feel the difference as heat. To get the most out of the alternator use 34 amp or higher wire (two runs of 27 amp is more than enough) between the alternator and the junction of the three brown wires where the old voltage regulator was. If you only run it to the solenoid whilst it will help with battery charging it will actually *worsen* the situation slightly as to get from the solenoid to the rest of the cars electrics it will have to come up from the solenoid through the existing length of dynamo-rated wire to the voltage regulator before it can start going towards the cars electrics via the ignition switch, lighting switch and fusebox.
Paul Hunt 2

Thanks guys,

Sounds like I am safe anyway. Car only has stadard electric gizzmos (not much then!) and always worked just fine on the dynamo. However, it seized yesterday on the way to work and I had an alternator handy at home from my 70 BGT that I broke recently. The car had already been converted to -ve earth so it was straight forward.

Paul, the wiring is now exactly as you describe in the electrical section of your site. I have left the voltage regulator in situ as I see little benefit in removing it. I guess the ideal solution is to run wires from the solinoid to the ignition switch, lighting switch and fusebox. I I do this at some point what rating wire should I use?

Cheers
Iain
I D Cameron

To be honest unless you increase the electrical load on the car significantly there is little to be gained by uprating the wiring to those points, and potentially (ho ho) something to be lost as aftermarket wiring is rarely as robust as the original with its welded spades and such-like. One time you would uprate the wiring is if uprating the headlights which should be accompanied by relays and fuses. The relay contacts (which supply the beams) should be fed by suitable gauge wiring connected back to wherever the new alternator output goes to. After that the original main lighting and dip switches are only carrying the very small currents. I'd forgotten the remote solenoid was so close to the old regulator, it looks like the brown wires on the regulator could be swung round so that the two from the fusebox and switches, together with the new one from the alternator, could be connected to the solenoid using lugs which are more robust than spades.
Paul Hunt 2

This thread was discussed on 24/10/2006

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