Alternator wiring UPDATED 25/2/07

ihatesissycars · Post by **ihatesissycars** » Thu Dec 21, 2006 10:53 am

Hello all,

I will be fitting and wiring up my new alternator soon i hope but i'm not entirely sure of the two smaller wires and which one does what/goes where.

The big wire terminal has a B+ next to it, thats obviously the main power/charge lead. That one i know what to do with.

The first of the smaller ones has D+, this one is in the middle.

Lastly, the other small one has a W next to it.

What do the small ones do? Is one for the charging warning light?

All help is appreciated!

Gav

Kev · Post by **Kev** » Thu Dec 21, 2006 1:53 pm

B+ is as you say the main o/p. Make sure this is a good connection, alternators don't like to run oc it buggers them up.
D+ is the signal to the ignition light, and W is not used. W is sometimes used as a tacho signal but can be left oc.

Kev

Post by **katanaman** » Thu Dec 21, 2006 2:17 pm

remember and upgrade your wiring for the rating of the new alternator while your at it. I am guessing the standard alternator for the car was nothing like the 100A that you have now.

ihatesissycars · Post by **ihatesissycars** » Thu Dec 21, 2006 3:34 pm

Thanks for your help guys!

Yes Marki thats one thing i'm thinking lots about, any ideas what thickness wire i'd need? Its a 115 amp alternator.

Post by **katanaman** » Thu Dec 21, 2006 4:11 pm

should be 16mm/sq unless you can get thin wall/tri rated then you can drop it a bit.

ihatesissycars · Post by **ihatesissycars** » Thu Dec 21, 2006 4:35 pm

16mm? As in 16mm dia wire core? That huge! Thats bigger than my battery cable! Am i reading that right?

ihatesissycars · Post by **ihatesissycars** » Thu Dec 21, 2006 5:03 pm

Would i be right in saying that because the cars not going to be drawing anymore power now than it was before the alternator change then the alternator won't be kicking out a higher ampage, enough to challenge the existing wiring and becuase of that the existing wiring should still be ok? Any extra electrical parts could in future be wired directly to the alternator therefore not overloading the existing wiring. Does this sound ok?

Post by **katanaman** » Thu Dec 21, 2006 5:16 pm

No not diameter its square (cross section area) its a standard measure of cable will be about 8mm diameter. Fair enough your not going to use anymore power but what if you get a fault in the wiring? Its up to you but if you dont and something happens you have a fire. For the sake of £4 do it right. http://www.vehicle-wiring-products.co.u ... ables.html

Should also say wiring direct from the alternator isn't a good idea either.

r2d2hp · Post by **r2d2hp** » Thu Dec 21, 2006 5:19 pm

Hi Gav,

It does not sound ok to me, all items should be taken from the battery as that is going to be the place with the least noise. The further from the battery you take a feed the lower the voltage and higher the interferance will be.

The cable refered to has an outside od or 7.5mm rated at 110 Amps - see below

Ref. Cable. mm2. O/D Amps M.
126 126/0.40 16 7.5mm 110 £2.08

http://www.vehicle-wiring-products.co.u ... html#gen10

Kev · Post by **Kev** » Thu Dec 21, 2006 5:31 pm

Maximum current draw from the alternator is when the battery is low, ie just after you've been jazzing the engine over on a cold morning. In this state the alternator is recharging the battery and maybe feeding the lights, engine coils(s) and anything else on the load. It will be putting out 10s of amps at this stage and the wire has to be rated for maximum load not typical load.
IMO it's OK to have a remote battery(in the boot) and a very low resistance cable to it from the starter motor, and use that point as a takeoff to the alternator. Normal low current wiring can be off a different feed from the battery to the main fuse. To be clear, one huge cable battery to starter/alternator, one normal cable battery to main fuse.

Kev

tetlow · Post by **tetlow** » Thu Dec 21, 2006 6:41 pm

The MGB has exactly this arrangement.

All the power is taken from the starter stud. There is a 16sq mm cable to the battery/s at the rear.
A 4 sqmm cable runs from the starter to the fuse box and altenator.
Dave

jwriyadh · Post by **jwriyadh** » Thu Dec 21, 2006 6:44 pm

If you are upgrading your alternator output cable please don't forget that the same output current will also flow through the earth return from engine block to battery negative. At a minimum clean and refit the existing terminals.

I know the starter current flows though the earthing strap/cable but the alternator does it for longer generating more heat from any poor connections.

jw

Post by **Coops** » Thu Dec 21, 2006 7:03 pm

r2d2hp wrote:Hi Gav,

It does not sound ok to me, all items should be taken from the battery as that is going to be the place with the least noise. The further from the battery you take a feed the lower the voltage and higher the interferance will be.

WITH THE EXCEPTION OF THE ECU EARTH GAV,
I know we are on about feeds,
BUT thought would say about the ecu earth and to earth that to the engine block,

r2d2hp · Post by **r2d2hp** » Thu Dec 21, 2006 7:07 pm

I was only refering to the +ve supply.

Post by **Coops** » Thu Dec 21, 2006 7:08 pm

errrr didnt i put that above