Alternator wiring help - simple answer for an expert :)

I noticed my battery was dying when the car was sat for a day or two so have tested as follows
Charged battery fully over a course of a day or two on trickle charge. Battery now reads at 12.5v

The alternator has two cables attached. One Large cable which goes directly to battery + and one small blue cable which doesnt go anywhere.

When i start the car, battery voltage doesnt increase much. If i touch this loose blue wire to the battery + then all of a sudden the battery starts reading at 14 - 14.5v indicating the alternator is working.

If i leave this blue wire connected to battery +, as soon as the car is switched off the battery voltage starts dropping quite rapidly. Disconnect the blue wire and battery sits stable.
Obviously this blue wire is required in order to get the alternator to charge the battery, but im guessing it needs some sort of ign + feed?

Some help here would be hugely appreciated - hopefully the last piece of the puzzle before the cab is road ready

blue wire should be plugged into the matching blue wire connector near the battery +ve terminal. its for the battery warning light in the dash and when disconnected the alternator wont charge properly

Blue wire is the field exciter. Basically it enables the alternator to generate voltage.

It's on a  switched positive so it only excites the alternator when the ignition is on otherwise the battery will be drained when the engine is not running. As above it's also part of the warning light.

Cheers for the pointers guys
Alternator is now wired up correctly and charging, and more importantly the car isnt dying over night!!

The wiring is something else.
The fuel pump gets its ignition live from……… the heater/fan switch inside the car!
The radiator fan has been disconnected. The front lights are wired in circuit?? as in if the passenger light goes so will the driver side..

On the plus side having it in sucha mess is helping me learn how everything works as i fix it all.

Sadly ive spent a few hundred pounds now and only bought the boring stuff.
New HT LEads, Plugs, Filters, Dizzy, Air Box…
Also rewired all earths, and replaced the main power leads.

Very boring but hopefully will be worth it

Guys, a minor resurrection on this thread. If the plug at the alternator is unplugged; should I see 12V at the blue wire with ignition on and engine not running? 1983 1600 gti. Thanks 

I don't have a red warning light on the dash: with engine running. But new loom hasn't resolved everything. Yet 

Yes blue wire should be ignition live, if you earth it the battery light should.come on

There's a spade plug near battery live on the blue wire check there as well. It's unsealed so corrosion can build

Thanks RJ. Checked continuity right through from alternator plug to instrument cluster pins. All tested out clear. (Which also included the connection through the fuse box). I referenced the wiring diagram and can't see where it picks up the ignition live. It's an aftermarket fuse box that was already fitted. 
Is the ignition live from an internal fuse box connection? There are two points on the wiring diagram I don't understand. 

It's not tapped directly to an ignition live, it's actually the battery warning circuit earth just if you probe it you'll see ignition live as the bulb circuit will earth thru your multimeter

The alternator circuit in the fusebox is also common to plug D pin 4 and G spade 4 but not actually used in MK1 golf wiring

Ah. Understood. But I still have the issue of no red light on the dash. Wiring tests out ok, new dash loom, new clocks, new alternator but no warning light? No warning light at all, whether engine is running or not. And zero volts at unplugged alternator plug with ignition on. I'm a little stumped. ?? 

Bulbs go bad in your cluster, I would test it.
This will cause the alternator to not charge.

Battery B+ 12.5V from the battery through the ignition to pin 14 of your cluster (black wire) through the cluster loom (mylar) to the bulb diode and resistor, through those and back out to the blue wire from the cluster to the small post on your alternator.  

It is the exciter circuit that starts the field coils to start charging.

If the Lamp on the Cluster is toast or the diode, then the b+ fro the battery out to the alternator isn't making it.

Key on, engine not started, the blue wire on your alternator (on the small post or the separate pin on the 3 pin motorola connector should have 12.5V b + on it.

If not then look towards the cluster.

When the alternator is working it provides 13.5V to the Alternator lamp reverse biasing the diode and turning off the light.

When the alternator isn't producing 13.5 V properly, the lamp in the cluster starts to glow.

So if your alternator lamps doesn't come on with key in, engine off, then you need to look at why it isn't coming on.  Usually bad wire or mylar loom, but then I have seen those bulbs go bad.

The bulb isn't actually needed to trigger the alternator, it's wired in parallel to the circuit required to turn it on. It's just a warning device to tell you if alternator is charging or not. The fact you get no power on the blue wire suggests either there's a break somewhere (but you tested it out as good) or damage to the blue foil/loose contact at the cluster pin

rubjonny said

The bulb isn't actually needed to trigger the alternator, it's wired in parallel to the circuit required to turn it on. It's just a warning device to tell you if alternator is charging or not. The fact you get no power on the blue wire suggests either there's a break somewhere (but you tested it out as good) or damage to the blue foil/loose contact at the cluster pin

If you tie the blue line from the battery positive to the Alternator exciter D+, then you don't need a Lamp in the dash, just a voltmeter to let you know if the battery is not being charged.  

But over here if I didn't hook the blue exciter wire from the Cluster to the Alternator the Alternator never produced a charge.  I did have the blue exciter break up into the vinyl tubing about 12 inches up…

Old wires get brittle with age and the shielding breaks. It is usually more than solid enough to break and cut the smaller gauge wires at the break (Crimps go bad as well).

Oh yeah I'm not saying the blue wire isn't required to be hooked up, just that the bulb specifically doesn't need to be in the cluster for it to work. so if the led is blown but the rest of the circuit is otherwise good, the alternator will continue to charge

Thanks for the input guys. 
Engine is 'new' (fully rebuilt) and freshly painted. Alternator is new. Alternator bracket is new. Bracket has an anti vibration mount that also fully insulates alternator from engine. Good idea, but I don't recall seeing this before. 
Alternator isn't earthed due to nylon/rubber spacers on all securing bolts. Add a little earth wire to the alternator and all is well with the battery light. Happy boy 
Now the oil pressure warning light is another matter…

I always used a earth on a case nut to the engine block to bond all.  Sometimes I forget that. Good one.