First, it's an intermittent problem - and it goes like this:
Bosch 12v 55A alternator
with 'W' output to drive engine rpm
Works OK when engine
first started but these symptoms can occur as soon as half an hour after starting.
The charge voltage (measured with a Fluke digital meter) initially at the batteries is 13.8v or 14.2v which I imagine depends on the state of charge of the batts at first.
THEN the following symptoms occur very briefly, over maybe 2-3 seconds - the engine note changes as the rpm
drops slightly; the rpm tacho drops to zero or almost zero.
THEN normal operation resumes but the charge voltage (again measured as above) is down to 13.5 0r 13.6v. If I load it with nav lights, deck
lights etc to take around 20A it drops still further, maybe to 13.4v. I can't load it much more without running the anchor windlass
Once these symptoms have occurred the charge voltage stays low and drag-downable but the rpm indicator is good and the engine revs are normal.... then it'll happen again - we notice an engine note change and the tacho flutters down but recovers almost right away.
The fanbelt is correctly tensioned and not slipping. The high current
leads are good and secure all round. The alt has been tested locally by a Bosch dealer on his test bench- he showed me that it was giving 14.2v but I don't know if it was loaded and it was a short 30 sec test.
What could be happening here? I can't get a replacement alt locally (Spain) for under 250 euros (shiiped in).
Suggestions for further testing or possible causes are welcome! Or a cheaper diesel
car type alt to fit?