Ok one way you can figure this out...
Find the starter solenoid on the starter. It will have three wires connected to terminals. One very thick red wire is the positive cable from the battery. The other smaller wire ( probably black) goes from the starter around the engine to the alternator. The thin yellow/red stripe wire is the one that sends current from the assist solenoid to the starter solenoid on the engine. If you can follow this wire bsck from the starter solenoid it should lead you to the assist solenoid.
http://forums.iboats.com/forum/engine-r ... ing-systemScroll down to the schematic, where the Merc diagram has a slave solenoid, Volvo uses a relay, you're looking for something that looks like an aux light relay for an automotive application. Follow back the yellow/red stripe wire to it from the starter solenoid. Check for voltage as per the procedure in this thread.
BTW note that if you have wingnuts holding the battery terminals on the battery, this alone can cause this problem because they get loose. Either use stainless nyloc nuts, or make up new cables with marine quality automotive clamps. That's what I did when I did the dual battery system about 8 years ago.
The reason why they use a slave solenoid or relay is to prevent the voltage drop you would have from running the 12V from the battery all the way up to the ignition switch back to the starter. This way the ignition switch only has to supply enough current to turn on the assist solenoid.
You need to know your model # and go to
www.volvopentastore.comand look up under the right model # for the engine the part.
I can't decode Volvo's incredibly arcane model numbering system, but this looks like it, starter relay part #6...not saying this is yours but probably similar. On my OMC I replaced the assist solenoid and the starter solenoid and never had another problem.
http://www.volvopentastore.com/Engine-H ... _id.790289