Further to this thread I am still having problems.
When I turn the ignition key on the Volvo one click power appears on the starter panel. Turning the key one further notch to the start position produces a click from the engine and then all goes dead. Turning the key back to off then on again...everything remains dead. If I then cycle the battery switches from ON to OFF to ON then power is restored to the start panel and the whole sequence starts again. After 5 or 6 "resets" of the battery switch I have usually been able to get the engine going. In the mean time (in addition to the replacements mentioned earlier) I have replaced the 2 Volvo relays (12V 40A pn 841177) behind the starter panel and in the black box on the engine. I have just been away from the boat for 2 weeks and could not get the engine to start at all this morning even after multiple battery resets.
By chance during the no power part of the cycle I tried starting the Onan to discover that it too would not start. I cycled the battery switches one more time to restore the power and hey presto the Onan started! I ran this for a few minutes and tried the Volvo again and it started too. So I am now thinking this is a 12V system problem not purely a Volvo one. The 12V battery was replaced 8 months ago on spec because the previous one which had worked perfectly for the 9 years of my ownership seemed too good to be true and I didn't want it to fail somewhere awkward. When Volvo or Onan is running there is 13+V across the starter battery so I am assuming that the battery is being charged ok. It shows 12.6V after being left for some time and the green charge indicator on the top has always indicated fully charged. I will check tomorrow for stray currents on the new battery, but may be I have just been unlucky and bought a dud? It is a maintenance free DELKOR Royal 102 AH 12V CCA625. What size of batts do the rest of you have in your 12V circuits? Any one experienced anything like this before?
If any one has any suggestions I would love to hear from you, this is starting to "do my head in" !!!