97 XK8: Battery charges to 13.4V with external charger. When starting cold voltage will show about 12.7 at the dash voltmeter for a few mins settling at 12.6V. Full lights and stereo blasting will show 12.5 for a while. When everything is properly warm voltage sinks to 12.2 A 25 mile drive will see it below 12V indicated. Drive on for a few miles and headlights go off, limp home mode kicks in and after another few miles battery is drained and everything stops.

With engine running and 12.0V showing at the dash my multimeter shows 10.75V at the big terminal nut by the left hand wing (fender) in the engine compartment.

My diagnosis is a dying alternator, either diode(s) failing or winding insulation breaking down as it gets hot. I cannot see the regulator or brushes giving this behaviour.

But strangely no charging fault message. The battery/charging warning lamp goes on when switching on the ignition, but goes out after a few secs even if I do not start the engine. Does it only monitor voltage instead of actually checking charging status?

Anybody have any view on this?
​​

 

The voltage should be near 14V when the alt is operating.

bob

 

Right, alternator dying then.

 

Further checks today showed that a partially recharged battery with 12.4V at the terminals and partially cooled down engine (alternator) settled at 12.9V at the battery and at the aforementioned terminal in the engine compartment. From this I conclude the alternator was charging the battery and the voltage drop from battery to terminal in engine compartment is negligible as it should be. That imho confirms the alternator to be the most likely culprit.

 

Reading about the common problems related to battery and charging cirquits I checked all the connections easily available. Lo and behold, one of the nuts in the main fuseholder by the spare wheel was loose. Now the car charges happily at 13.0V on the dash instrument. :-)

 

Yep, those damn loose connections, they'll get ya every time. Glad it was an easy fix. TM

 

Well done. We have all eventually learned to check the tightness of the electrical connections in the trunk periodically (along with the spare tire pressure)....