Battery or Alternator, how do you tell?

[MPowerMark]

Hi guys, My sister is having some issues with her Passat. Various indications have come up on the dash, airbag light, traction control failure etc etc.....all the symptoms of a flat battery. Then the car wouldn't start and needed to be jumped. The battery was last changed around 4 or 5 years ago so could obviously need a new battery but also could be the alternator not charging the battery. Any idea how you tell which one is at fault? I haven't taken a reading across the battery yet but im guessing anything below 10 volts DC is too low?
any tips on how to determine whats at fault, would be a shame to spend £80 on a bettery and it turn out to be the alternator.

Cheers.

[Boosted LS1]

I'm no expert on electrics but do the lights (if dim) get brighter when you up the revs from idle?

[PeteG]

Funny you should mention this, my 1.8T Skoda is going through the same.

With engine running (after jump starting if necessary), voltage across the battery should be over 13.5V, ideally 13.9 or more.

[Dogwatch]

Alternator has to generate more than 12v in order to push charge into the battery.

[XG332]

Get you multimeter out, put it across the battery should be getting 12v+
Start it up and you should get 14v+
Rev it up a little and you should get a rough 16v+

If the voltage doesn't change and is low then the alternator is gone, if it is low then it may have blown a diode, if it shows as expected then its ok.



Why do i know this?
I have my final exam next week, and it just so happens to be electrical.

Edited by XG332 on Friday 14th May 19:19

[Plotloss]

Take the positive off the battery when the car is running.

If the engine stops the alternator is fked.

[davepoth]

If you've jumped it a couple of times the battery will be knackered anyway. What does it do when you leave it running for a bit? if it conks out then it'll be the alternator, if not it was the battery.

[Sf_Manta]

If you've got a multimeter, start the car and then take a voltage reading across the battery itself.

If you've got anything over 12.5 to 13 volts, the alternator's fine, if not then it's fubared.

Turn the ignition completely off and repeat again, if the voltage drops below 11 volts on the battery, then the plates have collapsed inside and the thing's as good as dead and time for a new one.

Hope this helps

[saaby93]

There's the battery light on the dash too
That should come on when you turn on the ignition but before you start the engine, and go out once the engine is running
If it doesnt do either of those its dud alternator

[ranjan]

to check the alternator start engine put a multi meter on dc to battery check if it is giveing out 14.5 volts put everything on see what it is reading

[Rich_W]

XG332 said:

Get you multimeter out, put it across the battery should be getting 12v+
Start it up and you should get 14v+
Rev it up a little and you should get a rough 16v+

If the voltage doesn't change and is low then the alternator is gone, if it is low then it may have blown a diode, if it shows as expected then its ok.



Why do i know this?
I have my final exam next week, and it just so happens to be electrical.

Edited by XG332 on Friday 14th May 19:19

Based on what you wrote above. Prepare to fail then!

If your Alternator is bunging out 16V it's screwed! 13.6-14.4 as a guide. Obviously nearer to 14.4 the better. On a side note, depending on how old the alternator is it might have the freewheeling pulley. These can seize solid and kill the alternators when they shear off! these type have a plastic cap on the pulley and if you remove that, you'll see a multi spline (female) fitting inside. If it's a regular hex bolt it's the older type.

(Slip belt off, stick a insulated screwdriver in the alternator to lock the armature. Then see if the pulley free wheels one way and locks the other.)

[750turbo]

Rich_W said:

XG332 said:

Get you multimeter out, put it across the battery should be getting 12v+
Start it up and you should get 14v+
Rev it up a little and you should get a rough 16v+

If the voltage doesn't change and is low then the alternator is gone, if it is low then it may have blown a diode, if it shows as expected then its ok.



Why do i know this?
I have my final exam next week, and it just so happens to be electrical.

Edited by XG332 on Friday 14th May 19:19

Based on what you wrote above. Prepare to fail then!

If your Alternator is bunging out 16V it's screwed! 13.6-14.4 as a guide.

Yep

[eltax91]

Has it actually died and failed to start? The wife's A3 TDi shows similar symptoms and it turned out to be an earth point that has gone green/ rusty because the rear washer pipe has split and leaked all over the earth point.

[XG332]

750turbo said:

Rich_W said:

XG332 said:

Get you multimeter out, put it across the battery should be getting 12v+
Start it up and you should get 14v+
Rev it up a little and you should get a rough 16v+

If the voltage doesn't change and is low then the alternator is gone, if it is low then it may have blown a diode, if it shows as expected then its ok.



Why do i know this?
I have my final exam next week, and it just so happens to be electrical.

Edited by XG332 on Friday 14th May 19:19

Based on what you wrote above. Prepare to fail then!

If your Alternator is bunging out 16V it's screwed! 13.6-14.4 as a guide.

Yep

This is the one i couldn't remember, tho it does vary. Depends on the rectifiers cut off is. woops

[Nolar Dog]

XG332 said:

Rev it up a little and you should get a rough 16v+

Glad you are not fixing my car.

[Jasandjules]

Uh, my Passat had a little thing on the computer which said Engine Failure - stop. That was when the alternator went. As above, stick a multimeter across the battery and see if it's pumping out 13v or so. Or I would stick an multimeter across the alternator and see if it's charging at all..

[XG332]

Nolar Dog said:

XG332 said:

Rev it up a little and you should get a rough 16v+

Glad you are not fixing my car.

[Kevin]Alright, stop going on about it. man[/kevin]