Help diagnosing alternator fault please

[badgerfax]

Hope someone can help,

My charge warning light came on today so pulled over to make sure still had a fan belt, this was ok.

I Made it home and checked with meter the battery voltage which was 12.2 with the engine off.

when the engines running i'm getting a steady 11.3 directly off the alternator connection. I don't think the alternators charging at all.

I have read about changing brushes and voltage regulator but not sure if this will cure the problem.

There is a very thin wire coming from the same sleeve as the thick one can't tell what colour the insulation is as most of it is absent. this is in a very poor state. what is its purpose? could this be the cause?

Are there any other checks i can make to correctly diagnose what to replace.

[itchyfeet]

Yes i would change the brushpack as the first port of call after checking connections are good
assuming its a bosch origional its this

http://www.brickwerks.co.uk/index.php/t ... 3-all.html" onclick="window.open(this.href);return false;

is the thin wire blue? If so thats the charge light wire and needs to be connected, im unsure if its needed on this alternator to generate power( i read it is on some), i dont think it is, its just the light but somebody will confirm im sure

if you want i have an old regulator i can pop in the post for you to try, just post it back if works and buy one

[Ralf85]

Sounds spot on. Took my old regulator pack out recently as it was not charging properly. Hardly anything left of the old brushes. Replaced it, just two screws to undo and now back to full charge.

[ghost123uk]

the thin wire blue? If so thats the charge light wire and needs to be connected, im unsure if its needed on this alternator to generate power( i read it is on some), i dont think it is, its just the light but somebody will confirm im sure

I would love to "put this one to bed". Are the usual alternators fitted to our vans "self exciting" or not. I am fairly sure that they are not and they do need the blue wire to be connected up properly. Next time I have the lid off my engine I am going to test mine by disconnecting that blue wire and firing it up, just so I know for sure

Oh, and worth remembering that the blue wire is also normally used to fire up the split charge relay as well. So a failure in the blue wire will stop that relay from working.

[Oldiebut goodie]

It is simple to work out really - are there 3 wires or just a single wire from the alternator. Only the single wire ones are self exciting. If there is a single wire self exciting alternator there shouldn't be a charge light on the dash.

[itchyfeet]

OK so it sounds like the blue wire is the alternator light but also exites the alternator so if its broken as implied then the alternator won't generate power

so clean up and remake that wire, cut back the sleeve and join a new piece of wire on if it's now too short.

[itchyfeet]

It is simple to work out really - are there 3 wires or just a single wire from the alternator. Only the single wire ones are self exciting. If there is a single wire self exciting alternator there shouldn't be a charge light on the dash.

three wires or three connections?

My Bosch has only two wires but three connections as the body is the ground? so is it self exciting or does the blue wire exite it?

[badgerfax]

Thanks for the advice guys, really helpful and i'm learning loads which is always nice

Heres some pics of where were at:



I now know this to be a blue wire but could not tell yesterday, hangin in there but pooly


Heres the repair, I have solder joined then heat shrink then heat proof sleeve

The brushes here look very short to me??


Dont know what this is for or where it goes


or what this is or does

[ghost123uk]

OK, those brushes are WELL shot !! Get some new ones (Ebay = about £15 or less, or of course Brickwerks)

Round plug is for the VW main dealers "computer" to measure TDC and not used these days = ignore it.

The "other" thing on the alternator is a suppressor, designed to cut out any interference from the alternator which sometimes shows up as a whine on the radio. Usually only a problem on A.M. ("medium wave") not our more normal F.M. stations.

[Oldiebut goodie]

The "other" thing on the alternator is a suppressor, designed to cut out any interference from the alternator which sometimes shows up as a whine on the radio. Usually only a problem on A.M. ("medium wave") not our more normal F.M. stations.

Gives loads of sparklies and false traces on your echo sounders also.

[ghost123uk]

Oh yes, I forgot about that, often a problem that is...

[Oldiebut goodie]

It is simple to work out really - are there 3 wires or just a single wire from the alternator. Only the single wire ones are self exciting. If there is a single wire self exciting alternator there shouldn't be a charge light on the dash.

three wires or three connections?

My Bosch has only two wires but three connections as the body is the ground? so is it self exciting or does the blue wire exite it?

There should be ideally a sense wire that detects the battery voltage, an exciter wire (in our case the blue one) to start the alternator and the heavy output wire. Some alternators use the output as a sense wire which defeats the object and doesn't always sense the battery voltage correctly. It will charge but not necessarily at the correct voltage as it thinks that the output, at say 14.2v, is the battery voltage whereas the battery may be sitting at - say 13v. Earthing is carried out by the body. I don't know how the single wire ones manage to sense the battery voltage correctly though! With the long cable runs on your petrol setups via the starter solenoid there is scope for maybe 0.5 - 1v drop over the length. The diesel ones are so short that it really has no effect. (appx one foot cable)
The problem is that most of our vans have had alternators replaced over 20+ years so what was true at the time they were assembled is all up in the air and each alternator needs research as to exactly what type is currently fitted.
Mine has a Lucas one with the blue exciter and two heavy wires direct to the battery being a diesel, one being the sense and the other the output.

[Oldiebut goodie]

Oh yes, I forgot about that, often a problem that is...

Now that is dedication to VW!

[CJH]

Mine has a Lucas one with the blue exciter and two heavy wires direct to the battery being a diesel, one being the sense and the other the output.

Cable losses aside, won't the sense wire always see the output voltage, since they're both connected to the same battery terminal?

[Oldiebut goodie]

Ideally yes, but with the petrol setup no, it is because of these losses otherwise the sense would be connected directly to the output at the alternator. That's the way it works best! That's why you need to check exactly what alternator you have as there are differing methods with differing results.