How to test alternator?

[Dan_GTi8v]

My battery warning light is on all the time with engine running, thought it was alternator so changed it but its still on How can I test the alternator to check I havnt bought a duff one? Also what else could I check?

Cheers

[dom]

Whack a multimeter on the battery with the engine/all electrics off and it should read around 12v, start the engine and if the alternator is ok the reading should go up to about 14v. If it doesn't then the alternator is more than likely at fault

[Dan_GTi8v]

Cheers I got a feeling its gonna be something else but would be good to rule out alternator. I'm guessing if it was a wiring fault this method wouldnt help me though?

[nikkiy9]

Could it not be a dodgy wire, had it before on an older car and it was just a snapped wire

[boneybradley]

many car spares shops offer battery and alty testing and should use the correct machine... to test the wires is more conplicated and needs a multimeter accross the plug

[trog_nfs]

If there's a Halfords near you I know they can do alternator tests. I used to do them after all 

[Dan_GTi8v]

I know the positive wire works as the light comes on if you ground it with a screw driver or something (think thats right) other wire just goes to the ecu I think? I may be able to get a multimeter on it tonight, is there a way to test the alternator itself without using the battery so I can rule out wiring or whatever?