Battery Light

[Wurzel 16]

Hi,

 

Just got back from my hols and thought I'd get the old motor ready for the MOT.

 

I've noticed on starting the battery light remains on after engine start. I've done a bit of prodding about with a multi meter, here is what I found:

 

Battery voltage (engine stopped) 12.5V

 

Ignition switched on (engine still not started), voltage at alternator excitation wire 11.4V

 

Battery Terminal voltage when engine started 13.4V At this stage, the battery light is on faintly but increases in brightness as engine revs rise.

 

When I put a wire directly from the battery posotive terminal to the alternator excitation wire spade, the battery terminal voltage rises to 14.5V and the dash battery light extinguishes.

 

Am I right in thinking that the excitation voltage of 11.4V is obviously not enough to fully excite the alternator therefore the cable is dropping voltage OR should this be sufficient to fully excite the alternator and therefore the alternator is at fault?

 

My money is on the fact that the cable is dropping too much voltage due to some corrosion at a terminal or similar but wouldn't mind a confirmation/correction

 

Sorry to those who PM'd me over the past three months or so. I didn't have the chance to reply in most occasions

[Rob Thomson 6]

Yep - I'd agree your wiring is at fault.

 

Exactly how the light/excitation circuit works is a bit hard to get my head around (ie. can it work the warning light correctly even though it's other role is suspect) but if connecting a decent supply to the spade terminal gives you 14+ volts then I don't think there's anything wrong with the alternator.

 

The excitation wire goes through the good ol' Brown MultiPlug, so look there before doing anything else.

Edited May 15, 2007 by Rob Thomson

[Anthony 1,003]

Give the engine a rev to around 4000rpm or so - the alternator should "self-excite" (oh-er) and provide full charge, regardless of the wiring between the light and the alternator

 

Certainly I have to do this with my Passat because there's a wiring fault somewhere that I can't be bothered to trace, and yet still provides charge just fine after giving it a rev (and has been for the past 30k miles atleast)

[Wurzel 16]

The good old brown multi plug is my next port of call. One day I'll get around to rewiring that whole loom getting rid of the plugs altogether.

 

Cheers

[Anton green 12 1 Cars]

Did you have any luck Wurzel?

[Wurzel 16]

I've done a lot of prodding around with the multimeter etc.

 

If I run a length of cable from the multiplug inside the car directly to the alternator excitation/battery light feed, there is no difference to the fact that the batt light comes on dimmly at idle, increasing in brightness when engine revved.

 

Measuring at the battery, it seems now after a little time, the voltage rises to just over 14V when revved to around 4k using all original wiring. I have to say though, that the voltage fluctuates from 13.5 to 14.5 volts quite quickly. It's only when I feed the alternator directly from the battery that I get a constant 14.5 Volts battery charge.

 

From what I can tell, despite there being some voltage drop at the alternator excitation terminal compared to a switched earth from under the dash (both around 11.6 ish volts), the wiring is fine.

 

The battery is still charging when driven, despite the batt light being on, but obviously not very well. I'm going to try another alternator when I can source one

[Anton green 12 1 Cars]

I have the same problem and no luck sorting it even though I have been at it all today and yesterday

 

No idea how to fix it and I dont really want to knack another battery or stop driving my 205 regularly. At the moment my pug starts just about ok, but after I have driven it the battery is too flat to restart it (which makes petrol stations difficult!)

 

One thing I have found from all this though, is I have another female spade connector by the exitation spade connector for the alternator which isnt being used??-Dont suppose this could be something related to the problems could it?

[Wurzel 16]

I think the two spade connectors are connected internally so either is fine. Not entirely sure on that but something in the back of my mind says it's so.

I had a different fault where the battery light was on bright constantly some time ago. Classic, duff wire or alternator fault. It turned out that the wire sheathing had been punctured by a sharp point on my throttle body, grounding it to earth! Just goes to show that the obvious answers aren't always right.

 

I'm sure you'll sort it out in the end.

[rocky 0]

Hi Guys,

 

My Mi16 is doing a similar thing.

 

It starts ok, but when if revs are below 4000rpm it will just die. However above 4000 rpm seems to keep the car from dying.

 

My alternator is reconditoned, my connections to the battery are tight and are not corroded. But my battery lost alot of fluid (when old alternator was overcharging) so i think it might just be a bad battery which needs changing.

[rocky 0]

Hi Guys,

 

My Mi16 is doing a similar thing.

 

It starts ok, but when if revs are below 4000rpm it will just die. However above 4000 rpm seems to keep the car from dying.

 

My alternator is reconditoned, my connections to the battery are tight and are not corroded. But my battery lost alot of fluid (when old alternator was overcharging) so i think it might just be a bad battery which needs changing.

 

Just an update, brand new battery, and the car still needs 4000 rpm to keep the battery charged. If i dont do this eventually it will cut out and fail to restart.

 

I am off to a friends now, to get some voltage readings. (if he can find his voltmeter!)

[Wurzel 16]

Just for a conclusion more than a request for answers:

 

Finally got around to fitting a new alternator yesterday. Problems solved I knew it was this all along, should have had more courage in my convictions but at least it's made for a good future search thread.

 

Cheers all for your answers/help

[Beastie 1]

How are you measuring the voltage at the exitation wire? If you are connecting a volt meter between it and earth with the ignition switched on then you have connected a voltmeter in series with the bulb which has a resistance. Expect the voltage to be lower than battery voltage in this case.