928Forever!

After driving the 928 5.0 for about 15 miles the battery light comes on and the car starts to judder and loose power, then after about 5 more miles it picks up again and the light goes out, 15 miles later it goes on again, etc...

It's getting very annoying!!

I'm guessing the alternator ? It is the original one so would it be worth changing ?

I can't think of anything else that would cause that!

... Apart from the dreaded ECU failure...

SeanR

I'd start by checking grounds, and testing the amount of volts at the battery jump post as it runs.

Mrmerlin

from your post I would also include a regulator brush inspection.
If the alt is original then there is a very good chance your brushes are worn to the point that they need to be replaced.

3 ways to fix it.
1 buy new brushes and solder them into the regulator, about 6.00 for brushes at the rebuilder shop
2 buy a new regulator it has new brushes about 75.00 for a new Bosch regulator
3 Buy a new alternator it has a new regulator as well as new bearings and new armature slip rings, about 275.00

928Forever!

Well I'm going to see if I can get a reconditioned alternator in the next week or so. Until then I'll have to just keep driving the car and hope the alternator doesn't completely pack in until I get a new one!

Kiln_Red

Be careful not to rush to judgement here. Sounds like your car is charging intermittently to me. Not a typical symptom for a failing alternator. Take Sean's advice and check your connections first; before you go buying parts that you don't need.

928Forever!

Yeah it's charging intermittently. I know not the usual symptom of a bad alternator but can happen! I'll check the connections and grounds tomorrow then go for a run and see if it works better.

Mrmerlin

you need to make some inspections before you replace anything, after checking the connections as Sean suggested, pull the alternator off its mounts, and remove the rear cover then the brush holder ( 2 screws) .
I will bet they are worn.
disconnect the battery first,
Then hang the alternator with a coat hanger so you dont have any weight on the wire harness it can stay connected while you remove the brush holder

928Forever!

I'll go and check that soon, it is getting pretty urgent. It turns out the alternator is hardly charging the battery at all, even when the light is out. As I went out to start it this morning (about 6am) and it barely turned over! And it wasn't like I parked and shut down while it was showing the light, I waited till it settled down again before turning it off.

Until I buy new brushes (I assume that is the problem, but will check first), I will have to drive my Xantia, thank God for back-up cars eh!?

Alan

Well it isn't a "battery light" its more of a generator light. As you correctly surmise it mans the alternator isn't generating (therefore it isn't charging). This also means the battery is discharging any time you run with this light on.

However its doesn't necessarily mean there is a problem with the alernator - thats one of several possbilities. Charge the battery, test the jump post voltage when runiing and evaluate the exciter circuit. Its quite likely that you have an alternator regulator problem of some kind. Its also possiblke you have an interittent short in the exciter line which may cause such symptoms even with a new alternator - do a bit of investigative work first...

Alan