Thank you all for your input to this issue. I finally traced the problem to a faulty -ve battery isolation switch, with intermittent contact. I am now trying to replace both the +ve and -ve 2 pole switches. The originals are by Soderep Ecans in France and have been priced at €185 each! A scan on the Webb has thrown up another 2 pole marine switch by Durite. Identical ratings and dimensions, but only £75 each. Is the French switch really twice as good as the Durite? Any one any experience of changing these switches?
It would appear that I should also be fitting a ground starter solenoid to the Volvo starter circuit as per that on the Onan. I can find absolutely no evidence of bolt holes or a bracket on the main block where this might have been fitted. I have owned the boat for 10 years now and everything in the galvanic corrosion are would appear normal at this stage. Better safe than sorry though, so can someone with a Volvo with a ground starter solenoid post a picture of their installation for me? Any info on type/rating for the solenoid would also be bell received.
Thanks again for the advice,
SM 140 SULA