Thanks for your reply. The starting battery (12 volt) has a small solar panel to trickel charge it while on the hard. Even so, for the few days we were in the yard I charged it with a 12 volt battery charger and so it was topped up. It is about two years old and I haven't run a load test on it yet (just brought down from the states a load tester). Will have to try that out. The brushes on the starter were in good shape but dirty, as was the commutator, both of which I cleaned up. When I pre-heat, you can hear the fuel pump clicking and the voltage does drop a bit. I will try disconnecting the fuel pump and see if I get a voltage drop when only the glow plug is in line. The starting battery does turn over the Yanmar easily and we have tried running the Yanmar at 2,000 rpm, in neutral gear, to boost the battery voltage during starting attemps. I measure 13.8 volts on the input side of the starter during start attempts so I think this isn't a battery/voltage issue. The negative side contactor (relay) was replaced 4 years ago. I haven't measured the voltage drop across it yet.
Nevertheless, I am going to clean up all the remaining grounds on the engine block, then check the stop solenoid to make sure it is fully retracting, and lastly I am going to remove the exhaust elbow to see if if is plugged. I have a friend on another Amel headed to San Juan to get parts and he is going to pick up an exhaust gasket (if it is available).
Hope all is well with you Olivier, our family will never forget your kindnesses to us in July 2001 when you trained us on the boat.
All the best,