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MG MGB GT V8 Factory Originals Technical - What to do with a Sense wire?

Hi All,

Now I have become immune to the bitter weather I'm pressing on with my wiring.

At the alternator end, there is a second thinner brown wire emerging from the loom, still pretty thick though.

I think this is a sense wire (what is a sense wire BTW?), but what do I do with it? I was just going to wire it direct to any spare +ve terminal on the alternator.

Would it be OK to do this? I don't really feel happy about just cutting it off and blanking it. I like all wires to end up somewhere, and I can't cut it out of the loom at this stage.



Hi Liam,

the smaller brown wire (brown with yello codeline) runs to the red ignition controll light at the dash (the other wire at this light is a white one from the ignition/starter switch) and is connected to the same terminal on the alternator as the thick brown wire (running to the permanent + at the solenoyd).
It can (should) be connected but there will be no illfunction if it stays isolatad. The blub indicates poor charging and workes like a balance between the voltage of the alternator and that of the batterys.

BTW, in the end of the week temperatures should rise again according to the forecarst!!!



Hi again ralph,

I know the brown/yellow wire you refer to.

This is a completely brown wire, half the thickness of the main brown alternator wire.

I just don't know where it goes.

Regards, Liam

Read this.
Its about the MGB wiring and what the wires are.

But Ralph is right, it's the alternater light that comes on if it is not charging.

Where you put it depends on how many terminals the new alternator has. (and what each alternator terminal is supposed to do).

If you give us more information, we can help. Like what kind of alternator are you putting in?

Anyway, I bet the link will answer the question.
Richard Morris

Hi Ralph and Richard,

This is what I have.

Car: November 1972 GT, much played with loom by PO.

Engine/Starter : SD1 Series 2 V8, with the later distributor - no separate ballast resistor, and has the little box of tricks on the side of the distributor itself which has a thin white wire and a thin black/white wire coming from it.

Alternator : New, type CA305IR. Has 'W' post, B+ post, D+ spade terminal, 2 x B+ spade terminals. Puts out about 70 amps.


Liam H


W is for a tachometer for diesel engines and should not be used in your installation.

B+ is for charging the batteries
D+ is for the ignition light at the dash
B+ spade terminals are mentioned for voltmeter and relais for low oil pressure starting inhabitor, fuel pump cut off, engine hour counter etc.

With the dizzy you should be careful to keep it running as cold as possible. So try to route the radiator hose behind the dizzy or change its internal bits to the newtronic upgrade, as the Lucas electronic is prone to failiure due to heat in the narrow engine compartment of the B.

With the additional brown wire, chech whether it is running to the original loom. If so, there should be a connection to the fuse box and you should sort out which electrical components are served from there.

If it does not enter a loom, check out what the PO has conneted to this wire, radio or alarm?




I believe it was on the alternator + terminal. Brown is an unfused hot(12V) wire. It is feeding 12V to something, so it would be a good idea to trace it back.

I don't know what a "sense" wire is, either. Sensor wire? A sensor wire should use a different color code.
Carl Floyd

Thanks Ralph,

I will trace the route of the brown wire, it does go into the loom, but my photos suggest it does not re-appear at the fuse box.

Also, so can I run my brown yellow wire from any +ve terminal on the alternator?


Sorry Carl, I wasn't ignoring you! our posts crossed.

On every diagram I have (the Bentley ones), where there is a 2nd brown wire from the alternator it goes back to the starter solenoid.

However, when reading through the archives here, some refered to it as a sense wire and seemed to imply it wasn't just another +ve out wire. Why have another wire on a standard MGB that runs more current to the solenoid, when one seems to be enough otherwise?

I might just connect it to alntr +ve and see what happens.


you can put the brown/yellow wire to the D+ terminal.
This is only used for the ignition warning light at the dash.

With the brown wire, there was some confusion as it was only used on 1970 and 1971 production cars (may be yours is a 1971 car with first registration in 1972?) and this is routed to the solenoid too, just a parallel line to that from B+ (thick brown wire).
It is shown in the official BL workshop manual AKD 3773, chapter Nb, diagram No. 10.

I think you should disconnect the doubious wire and check it out with a multimeter to be sure it can be connected right this way.


My '79 has 2 brown wire from the alternator to the starter. They are both the same size. I figured that they thought that 2 smaller wires were easier to deal with in the harness than one big fat wire.
Carl Floyd

This thread was discussed between 23/01/2007 and 26/01/2007

MG MGB GT V8 Factory Originals Technical index

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