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Ok, having some issues with the alternator/wiring in my car. The alternator isn't charging the battery (it's a refurb unit and was working fine for many months before the engine swap) - the battery light is on even with the key out of the ignition. Ground (brown, ring connector), B+ (red, rubber sheathed ring connector) and D+ (blue - spade connector) are properly connected as far as I can see. However, there is another brown wire with a ring connector coming from the same part of the loom as the temp sensor - can't remember where this went on my old engine. The car only seems to want to fire if this brown wire is connected to the same alternator earthing post (on the alternator housing) as the existing alternator earth wire so this is where we connected it - however from memory there should only be three wires to the alternator, not four, and the Hayne's manual confirms this.

Does anyone know what this brown wire is and where it's meant to go? I believe this is the cause of my alternator/charging issues. Attempting to ground this wire elsewhere doesn't seem to help - I'm not even sure if it is ground.

brown wire?...is it a quite thick one about 150mm long with a small loop on the end and a larger loop that is bolted to the block via the alternator mount ? ..if so! the small loop connects to the case of the alternator to provide a earth (normally on the end of one of the thin bolts that hold the alternator together)as the alternator is mounted on rubber bushes

No, but I know the wire you're talking about (alternator earth: alternator case to alternator mount bracket) - I already have that connected as you describe. This is a thin brown wire with a smallish ring connector on the end that comes from the loom, sharing a path (for some distance at least) with the wire for the temp sensor. The car will only start (turns over but doesn't fire) if this brown wire is connected to the same case bolt as the alternator ground - but the battery won't charge and the battery light is on.

AFAIK there should be only three connection to the alternator: earth, B+ and D+. This is an extra wire which I'm certain it shouldn't be connected to the alternator. I'm guessing it's an earth since it's brown, but where is it supposed to earth to and what is it for? Why would earthing something to the alternator case allow the car to start but stop the alternator from charging? Unless I can identify it I'm going to have to cut the loom open and follow it, which I don't particularly want to do! My electrical knowledge is non existent but I could probably grab a cheapo voltmeter if it helps diagnose..

Dit you swap or at least check the brushes and the voltage regulator? The 'extra' wire sounds like a ground indeed, if you provided a ground from alternator housing to engine it doesn't really matter where you connect it. This wire may have nothing to do with your problem?

Remembered something about battery light permanently on. Found this...

http://www.bmwe21.net/forum/viewtopic.p ... on#p134941

Think Gonzo got a new alternator

I suppose it could be unrelated. Just seems strange that it's a newish unit and was working fine when I swapped it from one engine to another. I do have two old spares though, guess I'll try them after work tomorrow.

Jason, obviously it seems something really straightforward and I wouldn't normally recommend an auto-electrician - seeing as many charge more than an A-list film star as an hourly rate! However, I know a bunch of autoelectricans in levenshulme who will have it sorted for £10, maybe less.
On my owners gallery thread

http://www.bmwe21.net/forum/viewtopic.p ... a&start=90

It's next to the Asda there. Their called Prime Assist Auto Services Ltd 0161-225 1915715 Stockport Rd, Manchester, M19 3AG.

Thanks Arshad. Good to know for the future. You're right about auto electricians though - the one near me charge £50 just for diagnosis! I wouldn't be able to get the car there though. Besides, come this far with the engine swap now I'm not letting a stinking alternator beat me.

To top matters off my trickle charger says the battery is full when it certainly isn't - showing 10.3v on the voltmeter!

Then could it be your battery is just up for replacement?!

Again, it's fairly new and was working fine before the swap - but it has been very cold lately. How can so many things fail at once! First clutch slave and master, now alternator and battery haha. Voltmeter confirmed alternator is putting out no charge (and the battery light being lit with ignition off indicates a problem with the charge circuit iirc).

Easy swap: try a different voltage regulator and see what happens

Suppose that's quicker than swapping the whole alternator. If I can bring myself to brave the snow and sub zero temps after work tonight, I'll report back with my findings.. Is it just those two screws and it pops out?

Yep. Just make sure to disconnect the battery.

Didn't get out last night but did get hold of a multimeter. Battery was showing 10.7v while in car, after a few hours on the trickle charger it's showing 12.45 steady - so I'm confident it's accepting charge, so rulign that out for now. There are no fuses for the charging circuit in the fusebox, right?

I hear what you're saying but I would give another battery a try anyway.