M20-knackered diode?

[eko]

Had a quick look at a mates 320i earlier which has recently started to suffer flat battery.
Used a jump pack to start it earlier then took it on a half hour run.Switched off after and it didnt have enough power to crank the engine although ignition lights lit up fine.
Battery is new heavy duty thing.
Fan belt is correctly adjusted.
Engine earth good and cleaned.
Battery reads 13.5v when engine is running,12.4v when switched off.
Pulled fuses and no noticable power drain.
battery light on dash goes out immediately once the engine is running.

Could this be down to knackered diode pack in the alternator?

[eko]

[Brianmoooore]

If your voltage readings are accurate, then that should be enough to crank OK. How bright are the lights (switch on interior light) when it's turning over?
If it started OK on the jump pack though, then this does to an inadequate on board battery for some reason.
Scrape an area of metal clean on the alternator casing and check that there's no appreciable voltage between this and an earth point on the body. It's not unknown for the alternator (even ones without the rubber mounts) not to earth properly to the engine block, even though all bolts are tight.
If this all checks OK, then you might well be right about a diode. Needs a 'scope to check for sure.

[makman]

Take the cables off the alternator and tape them up s as not to arch out then try it..you could try bridging the possitve of the starter to the exciter wire which sends 12v to activate the starter..do tthis with the ignition off..if its off and it whirls over then uno that something se in the system is drawing power from cranking. Could also be a 'lazy' starter too? Sometimes when there nocking on abit they need that extra amps from a boost pack to make them work. Been working on a loading shovel this week dling this lol

[parkin10]

Had a quick look at a mates 320i earlier which has recently started to suffer flat battery.
Used a jump pack to start it earlier then took it on a half hour run.Switched off after and it didnt have enough power to crank the engine although ignition lights lit up fine.
Battery is new heavy duty thing.
Fan belt is correctly adjusted.
Engine earth good and cleaned.
Battery reads 13.5v when engine is running,12.4v when switched off.
Pulled fuses and no noticable power drain.
battery light on dash goes out immediately once the engine is running.

Could this be down to knackered diode pack in the alternator?

i had this problem on mine
so i took the car to an auto electrician
it was what brian said,the alternator was not earthing on the engine
he tested it by
volt meter,red probe on the alternator output,black probe on the alternator body if its good 13.5v
then black probe on the engine to see if theres any loss and then the body to see what loss there is should be the same reading or there abouts
costs nothing to test

[eko]

All sorted now.
Alternator swapped for known good one.
Battery now reads 12.9v when switched off and 14.4v when running.
Car left stood in freezing cold for 2 days and started first time earlier.

Thanks for the help.