Battery light staying on

Hi people, wondering if anyone can give me some advice. I've got a 1.8t in my mk1 and all has been ok until I broke the 2 pin plug in the back of the alternator, wires were plugged into alternator but the pins were touching as the engine was running. So I replaced the plug and started the car up and the battery light is staying on and the battery is not charging. I've checked the blue exciter wire back to the fusebox and all ok. I tried unplugging the blue wire and started up the car and battery light remains on, I even plugged in a spare instrument cluster and still no joy. Does it seem like the connection being bridged has killed the regulator on the alternator? Hope someone has an answer for me. TIA

Well, try this.  When the Engine is runnnig, touch a +12V wire from the battery to the alternators exciter wire at the plug (blue colored wire), if the voltage goes from 11.5-12 something to 13.8 Hurrah, you have a bad exciter wire.  If not I would take the alternator out and have it tested as you could have possibly shorted out the voltage regulator.

As you state that you have a furbared plug,  The plug should of been 3 pin with one of the pins being the exciter, and the other two going back to the battery +
One wire is blue the other two red.

As there are repair connector ends out there.  It could be that your exciter wire got broken at the connector or a few inches back.

Do the 12V jumper first if that doesn't turn the battery light out, then you need to have it tested.  

Thanks Briano, just went to carry the suggested checks. First thing I done, was to start the car with the exciter wire already unplugged, result was that the alternator was charging at 14.2 volts. Turned off the car and plugged the exciter wire into the alternator and re-started the engine, now the alternator is not charging.
I've checked the resistance of the exciter wire from the alternator right back to the fuse box and there is 0.2 ohm resistance, this would rule out the wire to this point. 
Only thing is, regardless of it the exciter wire is connected or not, the battery light stays on as soon as ignition is switch on, also when engine is running. 
I've tried 2 instrument clusters and they both have the batter lights staying on. 
Any other ideas, or could I have issues with both clusters I'm using? 

What is the voltage on the exciter wire when it is unplugged, with the key in the run not started position?

0 bolts at the exciter, also 0 at the back of fuse box. Shall I be checking the voltage from the blue wire direct from the cluster?

I checked the alternator wire direct from the cluster and it was showing 0 volts, I'm thinking both clusters I have must be faulty, so I'll get them checked now and then update, just strange as there was no battery light issue up until I changed the loom from megasquirt to Bosch me 7.5

If the voltage is 0, which it shouldn't be.  Then is it 0 Ohms to ground as if that is the case then connecting the exciter to the alternator is applying 0 volts to the alternator and the idiot light will turn on.

I would test it for continuity to ground.

The exciter wires starts at the cluster and runs to the alternator.

since the main 12Vb+ comes in to the cluster on pin 14 black, then it goes through the LED parallel resistors and then the diode, so if you don't have the exciter plugged into the alternator, then what is the voltage on the exciter plug on the alternator at this time.

So I unplugged the exciter wire, tested the side coming from  the dash/fusebox. I checked continuity to ground and it has got 0.2 ohms to ground. The exciter wire coming out of the alternator (2 pin d plug connection), is showing me 14.2 volts. 

Also just to confirm, the battery light on the cluster is always on when ignition is on or engine running, that's with the exciter connected to the alternator and also disconnected. 

The Alternator is not self starting normally that is it can't produce voltage until the Field coils are excited to start production.

So years ago when the Alternator was added to cars, they (used to be generators).  They are cheaper to produce. The major differences between them are Alternators produce Alternating current, they use a voltage regulator/bridge rectifier to take AC and convert it to DC.

Since we know that the idiot light or battery light in the cluster gets it power to light from the battery input on the black wire from the cluster.  Battery power comes up through led through the 2 parallel resistors and the Diode in the cluster, then goes over to the Alternator.
to tickle the field coils and starts them to generate 13.75-14 VDC.

Since the battery is at 12V current flows through the led and diode as it is forward biased turning on the light.

When the alternator has started to generate power, the Exciter post goes more positive, as in 14V.  Since this is more than the 12 that lights the led (battery light) the Diode in the circuit becomes reversed biased, and turns off the led, normally which isn't happening in your car.

With the car ignition switch in the Run not started position, what is the voltage on the Blue wire in the cluster (disconnect it from the alternator for this test).

The whole circuit for the battery light, doesn't have a ground reference to it.  It uses the Alternator as a negative source to allow current flow through the led, and diode in the circuit to light the LED since this is feeding the alternator 12V to start it charging, then as the Alternator starts producing a greater voltage 14V it reverse biases the diode at the top of the led in the circuit not allowing current to pass turning off the light.

Diodes are represents as a single line and a arrow pointing to the line or triangle.  The way that it points is in the direction of the flow or the negative side of the
diode.    12v and go up the diode (looking at the schematic as up the page turning it on as the exciter side of the alternator normally isn't at 14v or 12, it is negative to the led so that is why current flows through it.

I would suggest at this point as you have used 2 clusters, both showing the same thing.  That you take your Alternator off the car and have it tested, as I suspect that your voltage regulator or Bridge Rectum Fryer (old joke in electrics) has some issues as in one side of the Bridge may be bad.

If it tests ok, this would leave the Fuse box as the issue
if you have Water migration may have caused corrosion and Bridged something to the exciter. circuit as the traces in them are really close.


Dang we are getting too deep in this.
Do you have the Bosch flat voltage regulator with the Plug or the external mounted regulator with 2 wires.

Old type internal regulator Alternator (motorola over hear on this side of the pond) 2 styles that I have seen






External mounted Voltage regulator no plug just 2 lugs.
Most latter Cabriolets used these, but early Golf, Rabbit, and Cabriolet as in 77-82 early over here use the plugs or (internal regulators).



Take the Alternator off and have it tested.

So with the exciter disconnected from alternator and ignition on, I tested the wire pin12 from cluster, it's reading 0volts, I've also just come back from having both clocks tested and they are ok. 
Alternator along with engine, is from a 2000 1.8t mk4 golf. 
Fusebox is NOS and completely mint, I've got a spare so will throw that on and see what happens. Appreciate your help, will let you know what I find. 

Just over at a friends place, same car and engine (1.8t). Unplugged his exciter wire from the spade connector near battery and battery light stays on plug it back in and the light goes out. 
I then unplugged it again and gave it 12v feed and again the light goes out. Now the strange thing, on my car when I give the exciter wire 12v with it disconnected from the alternator, my wipers started to operate 😳. 
Checked both blue and yellow plugs on fusebox and can confirm that the blue exciter wires are in the correct locations. I'm stumped!

Sounds like a loom issue what year of Golf?

Hey, it's a 1983. 

I will have to peruse my Bentley's for a 83 Schematic to see where the issue may be…. Be a day or two, I have moved and things aren't settled.

Problem found, Luckily I had another loom, I found the plug on the white loom was slightly damaged, the pin 12 was contacting 12 and 4. Replaced and all good. Thanks for all your insight 👍🏽

Mk1dubber said

Problem found, Luckily I had another loom, I found the plug on the white loom was slightly damaged, the pin 12 was contacting 12 and 4. Replaced and all good. Thanks for all your insight 👍🏽

Good find, glad to be of an assist.