alternator on the way out?

[HairyScreech]

cars been cranking lazily for the last few days, went out yesterday and it was dead.

charged it up and had a nice 12.7v at the battery.

fired her up and still had 12.7v across the terminals, wouldnt rise to 13v unless it was held at 1500rpm or above.

left it an hour and it dropped .2 of a volt down to 12.5v

havent tested for amp draw with everything off yet, but thats the next step.

just thinking im probably looking at an alternator diode pack packing up.
what your thoughts?

[e301988325i]

Yep should be charging at 14.4V once the revs are up, but I'd check the alternator to engine block voltage to rule out a dud earth. HTH

[HairyScreech]

good plan and that what i was thinking, she runs ok when its running, but the cranking speed just fades away over a couple of days.
the instant i saw no voltage increase even at idle i though, "hmmm".

if the rectifier/diodes are dead i expect ill just remote mount a generic module, as the alternator itself is mechanically fine.


EDIT - just looked and a bosche rectifier is £30, didnt think to check if they were replaceable on the e30. doh.

[ucpsale]

Sounds like it needs a new set of brushes.

[daimlerman]

First check really should be the engine/chassis earth...the little buggers can look fine,even check fine,but a couple of quid on a nice shiny new one can restore cranking speed and cure all manner of electrical gemlins!

[HairyScreech]

yes, its something i saw mentioned on here while looking at its rough idle.

perhaps the two are connected?

(or not in the case of a bad earth, budum tsh.)

oh and i must point out it is clearly discharging over about 4 days as well, thus the diodes are most likely suspect as well.

[spiny]

i have a similar problem with a Mondeo (it's my parents car, about 6 years old ...) and the battery keeps going flat on that.
I thought it was just because it doesn't get used much (my dad has been ill for a while) but I fitted a new battery a few months back and even with it being charged over a weekend, it's flat again in a few days , soooo

could this be a duff alternator too ?

I've been meaning to go round with my meter and check all is well, but just haven't got round to it yet

[daimlerman]

i have a similar problem with a Mondeo (it's my parents car, about 6 years old ...) and the battery keeps going flat on that.
I thought it was just because it doesn't get used much (my dad has been ill for a while) but I fitted a new battery a few months back and even with it being charged over a weekend, it's flat again in a few days , soooo

could this be a duff alternator too ?

I've been meaning to go round with my meter and check all is well, but just haven't got round to it yet

That sounds more like something is switched on that should'nt be...boot light for example....or a radio fit thats drawing current...

[HairyScreech]

general diagnosis method is:

charge the battery, leave it an hour to settle and go and check the voltage across the battery - ~12.5v

fire it up and see what voltage you get running ~14v - otherwise dead regulator

leave it for a few hours and see if the battery has lost power - check for power drain

disconnect the positive terminal and connect multi meter in series and check for amps drawn when car is off.

if amps drawn, reconnect the positive, disconnect the positive on the alternator and repeat test between the cable and the alternator pin. - if current flow through there replace diode pack.

if nothing there start looking elsewhere.

i just just couldnt be arsed to go through the whole procedure today on account of having 4 stitches in my leg.

and i think i might have a 2l zetec mondeo alternator at home.

[HairyScreech]

i have a similar problem with a Mondeo (it's my parents car, about 6 years old ...) and the battery keeps going flat on that.
I thought it was just because it doesn't get used much (my dad has been ill for a while) but I fitted a new battery a few months back and even with it being charged over a weekend, it's flat again in a few days , soooo

could this be a duff alternator too ?

I've been meaning to go round with my meter and check all is well, but just haven't got round to it yet

That sounds more like something is switched on that should'nt be...boot light for example....or a radio fit thats drawing current...

the diode pack in the alternator has two functions, firstly it is a Wheatstone bridge, secondly it prevents back drain from energizing the coils and the alternator behaving like a big resistor.

[cypriot_boy_2k7]

yes, its something i saw mentioned on here while looking at its rough idle.

perhaps the two are connected?

(or not in the case of a bad earth, budum tsh.)

oh and i must point out it is clearly discharging over about 4 days as well, thus the diodes are most likely suspect as well.

I also read this and my brother said this

[spiny]

>if amps drawn, reconnect the positive, disconnect the positive on the alternator and repeat test between the cable and the alternator pin. - if current flow through there replace diode pack.

cool, didn't know that bit.
I was going to do the 'connect in series then remove fuses one by one until no current flows' method ...

[HairyScreech]

that also works, but only do that AFTER checking the alternator, as i generally find alternators to be the most common drain as they wear out, which a switch just wont.