I've been having some issues with my 2005 Cooper S, the electrical system has completely shutdown on me in the past including power steering. It happened again, but the electrics came back on so I decided to check out the voltage with the OBC monitor. Also I've had the radio turn itself on randomly, and wouldn't let me change the station or turn it off for a while as if the car has a little ghost inside.

At idle with little to no accessories voltage readout: 13.5-13.9 volts

With AC on at full blast at idle in park: 12.5-12.9 volts

Voltage dips even further when in reverse when the revs are low, especially when parking to about 12.3 volts

With fan on full blast: ~13 volts steady

The battery when checked with a multi-meter seems to hold just over 12 volts, and when starting voltage dips to 11.5 then climbs back up. So, maybe my alternator is just starting to crap itself after so long or the battery is faulty?

EDIT: Maybe I've been making too many short trips with the AC running and radio on? In the city, the trips are often under 20-15 minutes and often in heavy traffic depending on time of day.

 

The DME reads the electrical load on the alternator and adjusts idle speed accordingly to keep the voltage up. It sounds like it's not getting this signal, either because the alternator isn't sending it or there's a bad connection on the signal wire.

 

I was driving it yesterday and kept an eye on the voltage level, had accessories turned down to a minimum apart from the radio and the voltage would go between 13.2 - 13.8 most of the time.

At one point I watched it dip significantly for no reason to 12.5, then jump back up.

Not sure how to go about diagnosing this if it’s a bad wire or the voltage regulator or the alternator just crapped out? Might have to take it for a diagnostic.

 

Your voltages are really not too bad, though the target is about 14.2V. You may have a loose/corroded connection or ground somewhere that you could check & clean. But I think the likelihood is that the alternator is going bad.
Mike

 

Unfortunately I think you might be right...

The engine ground strap is quite clean, I’ve taken it off a few times cause it was in the way for service. I’ll re-check the battery ground as well, but the alternator might be on its last legs.

When the radiator fan turns on based on the ECT the voltage has a significant dip, sends it down to 12.2 and perhaps having other accessories running puts too much strain on the alternator.

Should I work on replacing that alternator soon? Or you think it will last for at least another year?

 

Mine just suddenly died one day without any warning!
It could go on forever just the way it is - there's really no way to tell.
I do find the A/C is great on the highway, but running around in town the fan & the compressor cause voltage changes.

 

I had all kinds of issues with a faulty battery. Made the mini run rough. Stuff would cut out. Mechanic couldn't find anything. But I replaced the battery, and it's ran right ever since. Well, besides the fan, the coil going bad, etc. lol.

 

Yeah maybe I'll try a battery replacement before I go looking at buying a brand new alternator...

I'm also wondering if there's an issue with the belt, because in stop and go traffic the voltage can dip sometimes to 11.5 volts when the revs are <=1,000. As soon as I get on the gas it starts going up again to 13.5, or maybe that's just the way it is.

 

Just rebuild the one you have: replace the voltage regulator, rotor, bearings. Parts can be found here: https://www.maniacelectricmotors.com/ (or, you can get already rebuilt one from there).
I did rebuild both - starter and alternator: