Ok - here's an update. Checking elearn there is a small wire from the alternator that goes to the body computer that is used to determine if the alternator is charging. According to elearn, it comes into play at 700rpm plus.
So, I disconnected the wire and started the engine - the battery alarm went off immediately.
So I'm going to guess there is something failing on the sense circuitry of the alternator that produces the signal for the body computer. Thoughts?
So maybe the charging circuitry is ok and the problem is in the sense circuitry. Whether that is a portent for future problems its not clear.
But with the voltmeter connected in the interim, I should be able to at least monitor the charging in the near term. Hopefully, its not urgent to replace. Pity the circuitry on the alternator can't be changed without taking the alternator out (I think the turbo is too close)- because a replacement rectifier is only $40. OF course - unless someone has done it in situ and can tell me how they did it! I'm all ears if you can advise!
Here is the elearn extract:
With the alternator still with the ignition key in the ON position, the Body Computer lights up the warning light in the instrument panel and sends a power supply to the voltage regulator built into the alternator via terminal D+.
In these conditions the energizing circuit (rotor) is enabled to earth by the regulator electronics.
With the alternator rotating through the effect of the variation of the rpm and the magnetic field, a three-phase alternating voltage is produced in the electrical circuit (stator) which rectified by the diode bridge can exit terminal B+.
When the upper fixed calibration level (13.7 - 14.2 V) is reached it charges the battery and supplies the system.
The Body Computer checks the efficiency of the alternator recharging system by detecting two parameters: the voltage signal coming from terminal D+ of the actual alternator and the engine rpm signal received on the CAN from the engine management control unit
At the key-on as long as the voltage is below around 5.5 V, the Body Computer signals the insufficient recharging state; when the voltage exceeds 5.5 V, the warning light goes out; if, on the other hand, with the engine moving (speed above 700 rpm), the voltage decreases below the level of 4.5 V, then the warning light comes on constantly accompanied by the display of a message.
Any further thoughts?
Last edited by m282; 12-05-19 at 05:10.