Battery ignition light staying on

[pjnixon]

Hi there, does anyone know what to check when the battery ignition light remains on when the engine starts? Assume it means the alternator isn't charging the battery which does appear to be the case. Multimeter across the battery when off reads 12V, but only 11.5V with the engine running, also connecting the multi to the big red lead coming out the back of the alternator reads 2.5V??? I assume this is not correct.

Anyone know any "fixes" I could try before phoning the garage? (Alternator replacement is beyond my capabilities I'm afraid)

Thanks.
Phil

[ukkid35]

I would check the 'big fuse' first.

[TimJM]

ukkid35 said:

I would check the 'big fuse' first.

^ Big fuse. Most are swapped out for the 125amp connector over the 100amp one now and these tend to be more reliable.

[TimJM]

Oh - the big fuse is down the back of the engine...from another post:

Ireland said:

It's down here



and it's bolted inside a black plastic holder.

It's an absolute pig to get at it.

[Supateg]

Check the voltage at the alternator to be sure, not difficult with a standard probe.
If it's ok, suspect big fuse then corrosion on ring terminals.

[TimJM]

pjnixon said:

...also connecting the multi to the big red lead coming out the back of the alternator reads 2.5V??? I assume this is not correct.

Did you have the revs above 2000RPM or at a low idle when testing? I thought alternators had regulators and you have to rev an engine to 2000+ to get a correct read of the output. I may be wrong of course but my understanding is that an alternator only outputs when necessary and with a health battery and an engine at low idle it isn't necessary.

[pjnixon]

Just taken BIG fuse out (only lost a 10mm strip of skin along forefinger knuckle, not bad for a first attempt), and there is indeed no reading on a resistance test (or an infinite reading to be precise I guess). Took it out and it just fell into 2 parts. Have ordered a 125A one off ebay.

Didn't realise that info re the alternator, makes sense really, I just assumed it was always pumping out 14V when it was spinning (was going by GCSE physics there, didn't think of a regulator).

So fingers crossed that's all it is.

Thanks guys, saved me an expensive trip to the garage. :-)