When does the alternator charge???

[saturn]

Hi all,
I'm trying to get some more info on the logic behind when the alternator cuts in to charge the battery. The car in question is my partners 1.4 2003 Ibiza 1.4 sport.

I have noticed in the past that it only seems to cut in once the engine has been revved past about 2100-2200rpm. I'm sure I remember reading somewhere on here that this is normal? Thereafter it charges at any rpm, its just after each initial start that it doesn't.

Does anyone know the reasoning behind this?

Is also a bit annoying, had to go jumpstart the car this morning as she left it running in the car part at work to "defrost" for 20mins, whereapon the battery (brand new by the way) went flat!

Would have thought that it would at least be clever enough design to ensure the alternator cuts in before the battery goes flat?.... that is unless there is something wrong with my car.

Any ideas?

 

[krussel]

I would have thought the alternator is charging as soon as the engine is running. My MR2 has an ammeter and that goes straight over to 14v once you start up.

A battery will lose a fair bit of power in this freezing weather and may be slower at spinning the motor over but a new battery should not run flat with the motor running.

It sounds like your alternator is faulty and not charging or (more likley) your belt is loose and slipping..

If you have a multi/volt meter put it on the battery with the engine running and you should see at least 13v or more like 14v. If you get anything less than 12v your getting no charge.

 

[Seat Adam]

No the light should go out straight away unless there is a fault. Have you checked all the connections for tightness and make sure they are all clean?

 

[Seat Adam]

Oh yeah. The alternator should be kicking out around 14.4V if you check the output.

 

[saturn]

Had a good look around and can't see anything out of place....

With a multimeter on, the battery is at 12v pretty much (engine off) As soon as you start it it drops to 11.5v and holds that pretty consistently.

As soon as you rev the engine over 2300rpm it starts charging and nominal voltage is about 14v. It will then hold that even at idle.

Belt is not slipping, seems to be an electronic restriction of some sort.... can't figure it out though. There are no fault lights and no faults showing in vagcom (except after the battery went flat which is understandable)

 

[RUM4MO]

You say the car is a 1.4 - is it a petrol engine or DERV? I've not heard of this happening with petrol engines, but the rough rocking of the DERV engines can break the excitation lead to the alternator which will cause this problem. It has been said that at higher engine revs (maybe 2000+RPM), the alternator will "self excite" so start supplying enough voltage to charge the battery. The lead is normally broken near where there is a plug and socket arrangement below the engine/gearbox joining point - maybe check out that area and splice in a new section of wire which might mean you needing to bypass this plug/socket - the lead colour might be yellow or blue, in any case repair any wires you find broken or partially broken and if your problems do not disappear then I'd think that you will need to sort the alternator - either slip rings or brushes or diode pack or control module - all these bits are inside the alternator so it might be cheaper just to trade it in against a re-built one. If you do this make very sure you replace like for like so that you do not need to change any wiring - some (non VAG) places will just hand you out a "suitable" one - which might not end up being very helpful. Do not hand over your old one until you are happy that the new one is same as old one and it works.

I would have thought that if you have checked it with VAG COM, and any leads are broken, then you would have found a fault code - if only a warning that the supply voltage was low.

And yes, after you have started to get the alternator to self excite, it will remain charging even at idle speeds - until you stop/start it again.

 

[saturn]

Its a 1.4 petrol.
Now that you mention it, when it was last mot'd they did some work and said they noticed two wire that looked broken down in front of the engine block. They couldn't tell me where they came from and at the time until now have not had a problem.

Its possible they could be connected to the problem you mentioned. Will be worth having a look at as well as the wiring that you mentioned if its different.

Thanks for the inid. will let you know how I get on