Disco 1 Alternator does not charge, warning light does not come on

Hi guys,

My alternator recently started to behave in a weird way. The voltage went from about 14V to around 12V and after some time (a few minutes) went back to below 14V. There was no pattern here, just random switching from charching to not charging. Eventually it stopped charging at all.

I noticed that my alternator charging light never comes on - not even before starting the engine when ignition is on.

I took a picture of my alternator and I have a few questions regarding cables 1, 2 and 3.

1) The brown-yellow cable which is cut. Where should it be connected? Is it responsible for the charging light and it's the reason why the light never comes on?
2) Why is this cable not connected anywhere on one side? On the other side it's connected to another white cable and together they are connected to the alternator at the bottom.
3) I also have this hanging just like that but I have no idea what this is... So what can it be?

I tried to "activate" the alternator (sorry, not sure what the proper word in English is) by pressing the accelerator pedal but the RPM needle goes up for just a second and then goes down and never comes up again. The voltage is about 12V - measured both at the battery and at the alternator (plus to the thick brown cable, minus to alternator's body).

tomke92z said:

1) The brown-yellow cable which is cut. Where should it be connected? Is it responsible for the charging light and it's the reason why the light never comes on?

Click to expand...

Yes, this wire is the warning lamp's circuit if it's not connected to the alternator's dedicated port the lamp does not come on

tomke92z said:

2) Why is this cable not connected anywhere on one side? On the other side it's connected to another white cable and together they are connected to the alternator at the bottom.
3) I also have this hanging just like that but I have no idea what this is... So what can it be?

Click to expand...

The tachometer works with a white/grey wire from the alternator so one of those hanging ones might be for that, this pic is for 200tdi so might not be the correct one

Thanks! I will connect the yellow-brown wire and see if the light comes on. But that won't solve my charging problem, right? Is there anything else I can check before taking the alternator out and disassembling it?

Measure voltage directly on the alternator's output wire and if you dont get around 14V there with engine running then the alternator is fubar

tomke92z said:

Thanks! I will connect the yellow-brown wire and see if the light comes on. But that won't solve my charging problem, right? Is there anything else I can check before taking the alternator out and disassembling it?

Click to expand...

You need the light working as it is this what causes the alt to start charging, no light = no charge.

lynall said:

You need the light working as it is this what causes the alt to start charging, no light = no charge.

Click to expand...

+1 for that ^^^, i wasnt clear enough to say that you have to measure voltage on the alternator AFTER the warning light circuit was restored cos that's the "exciter" for it

lynall said:

You need the light working as it is this what causes the alt to start charging, no light = no charge.

Click to expand...

That's what I thought too BUT I have a video of a cold start of my Disco from December where the light did not come on at all but the alternator was charging (I could tell my the voltage which was over 14V). This probably means that someone changed something in the original wiring?

Proof:


There is no sound. Sorry about that but you can see that when the oil light comes off the engine is already on and the alternator light did not come on for even a second.

Diode pack?

tomke92z said:

That's what I thought too BUT I have a video of a cold start of my Disco from December where the light did not come on at all but the alternator was charging (I could tell my the voltage which was over 14V). This probably means that someone changed something in the original wiring?

Proof:


There is no sound. Sorry about that but you can see that when the oil light comes off the engine is already on and the alternator light did not come on for even a second.

Click to expand...

Once you have had a look at the wiring, it is always worth popping the brush box out for a look see, easy enough to do, when refitting make sure you dont damage the brushs, new boxes about a tenner off ebay.

tomke92z said:

That's what I thought too BUT I have a video of a cold start of my Disco from December where the light did not come on at all but the alternator was charging (I could tell my the voltage which was over 14V). This probably means that someone changed something in the original wiring?

Click to expand...

The warning lamp circuit is the exciter which makes the alternator generate right away once the engine is started, it will generate voltage on it's own too after some spinning eventually. Here'sfrom the D1's WsM:

sierrafery said:

The warning lamp circuit is the exciter which makes the alternator generate right away once the engine is started, it will generate voltage on it's own too after some spinning eventually.

Click to expand...

Ok, thanks for all the answers! Let me summarize it to make sure that I got it. With the warning light wire (brown-yellow) connected to the alternator, the alternator would get "excited" right after starting the engine. When the wire is disconnected, however, the light never comes on and I need to manually excite the alternator by giving it some gas but the alternator normally still charges the battery. Correct?

So my charging problem is not related to the wiring - I need to check the alternator itself to find out why it doesn't charge. Maybe it's the diode pack or the brushes or whatever else.

tomke92z said:

Ok, thanks for all the answers! Let me summarize it to make sure that I got it. With the warning light wire (brown-yellow) connected to the alternator, the alternator would get "excited" right after starting the engine. When the wire is disconnected, however, the light never comes on and I need to manually excite the alternator by giving it some gas but the alternator normally still charges the battery. Correct?

Click to expand...

Yes, seems correct to me but that's valid for a D2 which i tested, without the exciter the alternator still delivers voltage but at lower power(or even normally if the alternator is fitted with a voltage regulator) and on a long run the battery will suffer, i dont have personal experience with the D1 alternator but based on your video and presuming that at that point the wire was also disconnected i'd say that if the alternator doesnt deliver voltage on the output then it's fubar... worth a check directly on it with the battery wire disconnected cos then it should deliver higher voltage... a basic explanation here https://itstillruns.com/happens-bad-battery-terminal-7241227.html

Thanks a lot. I'm leaving for a couple of days now but once I'm back I'll figure out what the problem is and let you all know here. Cheers!

I believe its residual magnetism that can make the alternator work without the warning lamp connected, but this can fade away and will need the wire connected again. Fix this first before taking anything to pieces. If you turn ignition on and touch the wire to ground the warning lamp should illuminate. If it doesn't check the bulb !. The ignition supplies battery voltage to bulb this then goes to alternator which takes it to ground (and excites alternator) the alternator then raises this terminal ton 12v(14.x) and as there is 12v on both sides of bulb it goes out.

I take it you have checked the basics that the Alternator has a good earth connection

mystery said:

I take it you have checked the basics that the Alternator has a good earth connection

Click to expand...

Isnt it "earth via fixing"?

I don't suppose it would do any harm to check all earths from batt to body and engine.

I checked the earths, they all are ok.

I took off the heat cover and got a better look at the alternator. I noticed that the one white wire which was hanging is coming out of the D+ slot so I figured this is where the yellow-brown wire should be connected.

I connected it as in the picture below but the warning light still does not come on when ignition is on...

Is there any fuse that could be broken? Or any other switch or whatever that could be faulty between the alternator and the dashboard? Or it has to be the warning light itself?

Looking at circuit on rave the power to this circuit is derived from Fascia use box fuse F14 and exits at connector C208/pin 15 going to instrument cluster Connector C222 pin 2 inside it goes
to the lamp and an unknown component and exits at connector C221/pin 5 which is Brown/yellow wire. This then continues through 2x connectors C215/24 and C217/4 then to alternator terminal D+. Inside it goes to the voltage regulator module. The best course of action is to disconnect the brown/yellow wire at alternator and turn ignition on and test for voltage if its there then chances are that voltage regulator in alternator is fried. If no voltage is present check Fuse 14 then unfortunately you will need to remove instrument to check voltage in and out then on to connectors also check lamp while instrument panel is out as a blown bulb could also mimic a blown fuse. Sorry I can't supply diagrams of connectors or instrument cluster, I have to run Rave in a virtual machine as its not supported on Win11 Maybe someone can point you to a location of the pdf files