Time for a new altenator?

[cmsslee]

Hi all, the battery in my M40 engined convertible was draining to flat in a very short space of time, with it being an old battery i replaced it thinking this would solve the problem.

However after changing the battery I noticed that the battery light was illuminated on the dash cluster without the key in the ignition.

Is this indicative of a failed alternator or could it just be the regulator pack? I want to narrow it down before I spend £££ so any help appreciated. Thanks

[Brianmoooore]

For the battery light to be illuminated with the car off, power from the battery, on the alternator B+ terminal, must be getting through to the smaller D+ terminal, which connects to the battery light.
There are diodes between the two terminals, connected "back to back", so that power can't get through, so the chances are that one of the diodes between the windings and the B+ terminal has failed.
The diode pack can be changed, but it's not really a DIY job, so a second hand alternator would be the way to go, checking the state of the brushes in the regulator pack before you fit it, and possibly using the one from your existing unit.

[cmsslee]

Thanks Brian. Looking in the wiki i see there were different sizes fitted. 65amp for my model or does it not matter?

[Yeti]

take the earth off the battery,put a multimeter in between the negative terminal of the battery and the chassis to see what the drain is. then get someone to pull a fuse out 1 at a time to see when the drain goes to 0 as there shouldnt be much more than 0.5 of an amp. that will show what circuit has an issue.

[Brianmoooore]

Thanks Brian. Looking in the wiki i see there were different sizes fitted. 65amp for my model or does it not matter?

Get one that came from an M40, because it will have the correct pulley fitted. 65A or 80A won't make any practical difference.

take the earth off the battery,put a multimeter in between the negative terminal of the battery and the chassis to see what the drain is. then get someone to pull a fuse out 1 at a time to see when the drain goes to 0 as there shouldnt be much more than 0.5 of an amp. that will show what circuit has an issue.

Since the battery warning light it lit with the engine turned off, we already know where a significant drain is going. If, after the alternator has been replaced, and the glowing light sorted, there is still an issue with the battery going flat, then this is the way to go.

[cmsslee]

Thank you all

[cmsslee]

Fitted a replacement alternator from Mick today and everything seems sweet. Thanks again for the advice. Is it worth getting the diode pack exchanged on the old one to keep as a spare? Any ideas who does it and how much it costs?

[Brianmoooore]

Keep the regulator pack and bin the rest. Diode pack will cost more than a second hand complete alternator, and is far from easy to change.