Alternator issue? Or something else to check first...?

Hi all,

I've had my N Reg 300tdi Disco for just over a month now and I love it!

I've had a few little niggles that I've sorted out, but had been mythed by flickering/dimming lights. I've found another useful thread about this that helped somewhat, but my problem is slightly different. I'm a newb when it comes to electrics, so please bare with me.

The lights had pretty much always flickered (mainly on full beam) and eventually the Landy just didn't start. Checked the battery and it was flat, so I put it on charge. When I jumped started it to get home I checked the alternator with a multi-meter and it was reading about 3 volts - does this suggest the alternator has died?

Now the battery is charged (seems perfect) it obviously starts etc, but when I disconnect the positive terminal on the battery it cuts straight out. Should it not continue to run via the alternator? Would this confirm it is the alternator or is there other things I should check before?

The two wires connected to the alternator appear to be on there fine, and I've seen people mention checking the earth. Very newby question, but where am I likely to find this earth?

Need the Landy sorted for next week, so I'm aiming to sort her out tomorrow if anyone could shed any light that would be greatly appreciated!

There should be 3 "connections" to an alternator; a thin wire which is the excitation circuit, a thick wire which is the high current output wire and the earth which is usually through the body and mounting of the alternator itself. The excitation wire should have 12 volts on it through the charge indicator light circuit on the dashboard.
As a general rule, with the engine running just above tickover the voltage across the battery should be in the area of 14.5 volts (give or take). Anything much less than that, say below 13.5 volts and I'd say the alternator is goosed.

Alternator may just need new brushes and controller. Depends if pickup rings are badly worn or not.

brian47 said:

There should be 3 "connections" to an alternator; a thin wire which is the excitation circuit, a thick wire which is the high current output wire and the earth which is usually through the body and mounting of the alternator itself. The excitation wire should have 12 volts on it through the charge indicator light circuit on the dashboard.
As a general rule, with the engine running just above tickover the voltage across the battery should be in the area of 14.5 volts (give or take). Anything much less than that, say below 13.5 volts and I'd say the alternator is goosed.

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Don't forget the "W" terminal connection for the rev counter.

OP - is your rev counter still working ??

Don't run the engine with the battery disconnected to try and prove the alternator, this is one of the easiest ways to destroy it!
You cant have 3 V at the alternator with a fully charged battery unless you have a wiring fault, the battery +ve is effectively connected to the alternator (via the starter) by a heavy lead, the battery -ve is connected to the chassis and also to the engine crankcase. If you have 12v at the battery you should also have 12v at the alternator (even if it is duff). Fully charge the battery and measure it, then start it, does the battery voltage get higher? Also is the battery light on when you turn the ignition on and does it go out when it is started?

Hi chaps,

Thank you for all of the responses, much appreciated. Brian47, I will have a look for the three wires (from the top of my head I can only remember seeing two?).

Disco1BFG - I forgot to mention that the rev counter initially flickered a bit, but has now completely stopped working!

Kwakerman - have you any 'common' wires I should check for before replacing the alternator? I don't believe I have a battery light, which is odd.

Thanks again chaps

Brookstain said:

Thank you for all of the responses, much appreciated. Brian47, I will have a look for the three wires (from the top of my head I can only remember seeing two?).

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If you read my post again you'll see that I said 3 "connections", not 3 wires.
2 wires, a thin one for excitation and a thick one for the output current. The third connection, the earth is by the bolts holding the alternator to the engine.
As @Disco1BFG has already said, there will also be a wire from the alternator to feed both the Rev counter and EGR valve control module on the diesel.

You need to have the battery light working, the power it uses excites (starts) the alternator producing power. this may be the cause of your fault and you should look at this area first. Look at the dash (use a torch if need be to see the legends on the warning panel) and try and find the one that looks like a battery (haven't got a disco but most cars use a battery symbol). It should light with the ignition on and engine off. if it doesn't (and you only have one thin wire going to the alternator) take the thin wire off of the alternator and short the wire to a clean bit of metal on the engine block. Does it light with ignition on and engine off? If yes your alternator regulator / rectifier is duff, if no you need to look at the bulb in the dash in the first instance.

Kwakerman said:

this may be the cause of your fault and you should look at this area first.

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+1 - wot he says !! excitation is essential

Kwakerman said:

If yes your alternator regulator / rectifier is duff,

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If the rev counter "flickered" then either the rectifier is duff, or the wiring to it is broken. ( essentially, the rev counter actually counts the pulses from the half wave of the rectifier output - hence the name "W" terminal ).

as @wammers said above, you can replace the "controller" within the alternator ( rectifier & regulator), but it'll only be worth doing if the slip rings have some life left in them .

On our D1's (1997) - the battery symbol is just above the fuel gauge.

Good morning all.

Your help has solved my issue, so thank you very much.

I checked all wiring, cleaned all connections and still had no luck. I didn't have the battery light come up, and located where it should be above the fuel gauge.

31% off on a Lucas Alternator, so for £80 I thought I'd give it a go.

Had a bit of an issue with the thread on the belt tensioner, but with a bit of fiddling it's all on and working perfectly. (Is there a specific way to do the tension?).

The battery light now comes on before I start the engine, and the battery is reading 14.7 volts.

Thanks for your help guys!

To remove the belt tension on a 300 TDI you use a long spanner (15mm IIRC) on the bolt head that holds the tensioner pulley on the tensioner arm, lever the spanner in an anticlockwise and the tensioner arm will move and relieve the tension then slip the belt off the pulley. There is no need to remove anything from the tensioner as it is spring loaded, shouldn't take more than 10 / 15 seconds to remove the belt tension. Watch this

Kwakerman, I did try that but the pulley didn't move at all. So I therefore removed the tensioner altogether and replaced it. It seems as tight as it was before... hopefully I've not caused any issues

The tensioner should be moveable as is shown in the video. Although I normally use a 15mm socket and a long breaker bar to make it easy to rotate it. I have seen someone once try to move the tensioner pulley away from the belt by trying to turn the the tensioner fixing nut
, oops, gave me a giggle.

If it is not moveable , then the tensioner is knackered and is not doing its job correctly. They are fairly cheap items to replace. Make sure you use the correct torque to bolt it in as well.

Cheers

You do need a fair amount of force to get the tensioner to move (probably more than you think) , as neilly says if it doesn't move then it is knackered.

... and if it is knackered, and particularly if its too tight, it won't be doing your nice shiny new alternator bearings any good at all!

@Brookstain - Is you rev counter back working too ??

I couldn't work out the mechanism inside the tensioner, but I'll have a look online for a new tensioner and change it anyway! Does anyone have a link they could send me by any chance? Tell me to stop being lazy if you want.... ;-)

The rev counter is indeed back Disco1BFG!

Here is a selection for you. you can probably get it cheaper in other places.

https://www.lrdirect.com/ERR4708-Tensioner-Polyvee-Belt-300Tdi/

Also you may find this site useful for part number searches.

http://allbrit.de/UNI.cfm?PAGE=801306&SPRACHE=EN

Double check it is the right one for your motor.


Cheers

Did you get the old one to rotate about the pivot point at all?

Cheers

Neilly that's ace, thank you. The part number search will be very handy going forward!

I did indeed get the old one to move around the pivot. I couldn't understand (or see) the mechanism of it, so when I reattached the tensioner I had a mate to hold tension on it as I did it up. The belt seems tight enough, and hopefully not too tight. My lack of understanding as to how it works may explain the fact that the tensioners not working?

It seems to be working fine though.. I've ran the Landy for about 20 minutes or so since, and the belt is still as tight and isn't squeezing.

Any bright idea on how I could check if it works? Will it just give up suddenly?

I'm going away off-roading for the weekend on Friday, so don't want to risk the tensioner failing!

It is basically a Big spring in the pivot point which makes the pulley apply pressure to the belt.

If you can get it rotate as shown in the video link given before then it is probably all ok. One thing is , did you check the bearing in the pulley, to make sure it is in good condition?

Cheers