Hi all.
I rarely start the Disco and leave it some minutes idling at the driveway to soon hit the road. It was a very cold morning and had extra time before taking off.

Four blocks away, engine and everything misbehaved much worse than limping until died. Battery read ~9V. Recharged the battery and it recovered fine. Tried to reproduce the flaw the same way... yes, died at the driveway. Suspect was a frozen battery. Another battery, behaved the same !

Now the suspicion moved to the alternator. Prompted me to post a nearby thread to diagnose its components.
Tested/probed the alternator guts, saw nothing failed. Cleaned terminals, put together back in its place.

Started the engine the same fashion stretching arm and turn key from outside. Ran perfect. Red 'bat' icon light on !. Measured voltage at battery : 12.3V = not charging.

Jumped in, pressed throttle and the 'bat' icon went off; battery voltage rose to 14V. All good.

----> Operator error? Starting when very cold the alternator does not generate enough at idle ? The battery was 'consumed' in 10-15 minutes idling and driving at near idle speed 4 blocks. Needs to hit the throttle for the 'batt' icon to go off and start charging. Was not like that before. Idling was enough for the alternator to work.

Any new developments will be reported...

 

Some alternators need to see a certain spike in revs to get the “exciter” to commence charging. I see it on old BMWs all the time and beyond the nuisance factor it doesn’t really have any effect on anything else. It can mean it’s time to replace the brush pack. Late 70s early 80s cars ran this circuit through the lamp in the cluster, so if that burned out your alternator wouldn’t charge and with no battery light you wouldn’t know it until it was a bigger problem. Anyway, I’ve never seen any if this behavior on any of my Discos, maybe a failing alternator? Or the battery is cold and unable to take a charge so the circuit is somehow interrupted until a serious output voltage wales it up? Interested to hear what you find out.

 

Where are you measuring voltage, you should check 4 locations :

• Positive to Alternator positive - negative to Alternator casing
• Positive to Alternator positive - negative to Big Black Negative cable at the engine block
• Positive to Alternator positive - negative to Big Black Negative cable at midpoint Engine fuse block
• Finally at the two connecters to the battery

There might be a bit of a drop, I go from 14.3 at the 1st to 14 at the battery

 

Are you sure brushes are not end of life? They typically function intermittently at end of travel.

 

I think the alternators that need a spike in RPM are the one wire alternators. They need to spin fast enough to excite themselves. My D2 needed a spike for a time. It turned out that the exciter wire had broken.