I don't think my alternator is working. When I fire up the car, there's no noticeable change in voltage across the battery. I have cleaned all terminals on the multiplug and all fuses are fine. The battery light doesn't light up on the first turn of the key either. Any ideas?

Anyone?

battery light should come on with ignition, something must be wrong there! unless your clocks are dodgy. haven't you got a spare alt off of that mk2 your breaking? (obv its a pain in the ass to change though)

I'm not getting the increase in voltage across the battery though. I know the light is supposed to be on, that what made me think the alternator is broken

id double check the connections ect before replacing the alt though, has your other mk2 still got the engine in to compare with?

It does yeah, I'll have a look. The alternator has different fittings from the earlier loon though.

I don't think its the alternator thats faulty as it was pretty new on the car when I had it.

Any way I can test the output voltage at the alternator rather than at the battery to eliminate the wiring as a potential problem?

I'm not sure mate, you need nige or caretaker to help with that really, lol

wiring shouldnt be TOO much different!

There's a multi plug on the earlier ones whereas the later ones have bolt on connections. I had a similar problem before and it was a poor earth at the alternator.

I was hoping for one of those two to pop up!

the light will not come on if the wire isnt going to the alternator. check D+ wire. this is what makes the alternator "turn on" without it, wont work. use multimeter to check continuity from the dials to the D+ wire

This is what I'm hoping, that its an issue with the wiring. All of the contacts are there, there are 3 of them, a blue one and 2 thickish red ones. Which is the D+?

Edited 16 July, 201214 yr by prankstar2003

If you rev it do you see a voltage increase? The alternator light/wire excites the field windings but sometimes increased RPM self excites the alternator and it will output

Nope, theres no change in voltage at all, it reads 12 all the time, which is making me think its a wiring issue. I hope. Dont want to replace the alternator as it looks nice!

If you start the car you should be able to put a small wire from the D+ to the battery positive, without a light. This should excite the windings and you'll have 14.4 at the battery.

It'll go when you turn the car off.

Could the bulb be gone?

I've heard that if the bulb goes, then this stops the alternator working, correct?

Which is the D+ on the alternator? The blue one or one of the red ones? Both red ones seem to be equal thickness

Yeah, that's correct.

Basically, when the field on the alt isn't excited, it has 0 volts across it. The field is just a coil of wire. One end goes to ground, the other goes to D+ (minus some electric regs and stuff).

D+ goes to a bulb, which goes to battery positive. So when the ignition is on, there's 12v over the bulb because the alt isn't putting out power, so D+ goes to ground.

Once running, the alt makes power, D+ goes to 12v, which means there is no potential difference over the bulb, so it goes out..

The blue one sounds like the one that makes the power tbf.

The blue one is thin and connects to the loom just above the starter. I'll have to have a look later in the week and get some pics etc,

Is there any way I can test the alternator without going 'through' the battery? E.g. connecting to the alternator directly?

Yeah, that's the one IIRC.. You should be able to put a wire from that blue one onto any live feed. Starter, the alt one, anything..

Once the engine is running, if you can.. Remove the blue wire from the rest of the car if you can, just incase.

Juat in case what?

If i put a live to the blue wire, what should i see change?

Actually, ignore that, it'll be fine.

If the engine is running and you feed the blue wire with 12v, the alt should produce charge.

Right I see. Where does the blue wire usually get its 12v from then?

Ignition somewhere I guess

Thanks Harris. I'm kind of looking for certain answers here. I spent the best part of three days chasing my fog light feed due to hunches! I'll trace the blue wire back as far as i can though and see if your right

The bulb.

Like this..

So if i put 12v to the blue wire with engine OFF the lamp will light

If I put 12v to the blue with the engine running, the lamp should go out AND I should get roughly 14v at the battery with revs?

What am i actually testing here? wiring, lamp? what? damn I'm rubbish at electrics!

I can't see that, the blue wire is only observational.

With the engine running, remove the live wire from the battery. The battery is now completely disconnected from the car.

The engine should still be running!

If not, you have an issue with the main feed.

Another thing to check - the old alternators have a removable cartridge for the bushes, withdraw it and check the bushes. Maybe swap with the one in the other alternator.