There have been a few posts relating to Volvo
MD series engines turing over on their own with the ignition off. This happened to me last year. This year my starter quit working all together (on its own or otherwise) so it was time to dig a little deeper...
The problem with the starter turned out to be a bad solenoid, but the related ignition wiring
was also badly cooked. The ignition circuit from the panel is protected by a 15 amp fuse which had not blown & clearly way more than 15 amps had flowed thru this circuit in order to bake the wiring
so badly...so the current
got into this circuit from some other path. Other wires in the harness were fine except for some minor damage from where they were in contact with the ignition circuit.
Reviewing the circuit diagram reveals that the Red wire in the ignition circuit is connected to the B+ terminal on the starter (the same one connected to a nice fat cable to carry high amps for the starter), then to the start relay, then to the solenoid via a Red/Yellow wire. Turning the ignition closes the start relay and current
flows to the solenoid from the starter B+ lug via the above circuit. So, if the solenoid fails so that it creates a short to ground or otherwise draws big amps then this modest ignition circuit will turn into a heating
element with no fuse protection. Over a span of 14 years I've had this circuit get baked on both engines. To trace this circuit start at the starter B+ connection (30 on the diagram), follow the Red wire (R 2.5) to the start relay (6), to the ignition connection on the starter (50) via Red/Yellow wire (R/Y 2.5). Imagine 50 shorted to ground, with the start relay closed, and you get the picture...straight connection to B+ with no fuse protection.
One handy thing about a cat is when you have a problem on one side then you know the other is probably not far behind. So, when the first one baked I bought a replacement harness section and a spare. This is just the section that runs from the engine
mounted relay box to the engine
. The first one baked early in the boat's life and I've been carrying around this spare harness section for about a decade now...and finally got to use it, but first I added an inline fuse (don't like em, but its the easiest fit for this mod) in this ignition circuit...it should only blow about once a decade. Now if the start solenoid fails it wont damage the wiring harness.
I think this is related to the mystery start issue, but haven't back tracked and verified yet.
Related pics attached. The first pick shows the related ignition circuit wires (R & R/Y). They are not only badly burned, but actually fused together in the middle. The second pic is detail of the pigtail (Red wire with square plastic end fitting) that connects to the inginition fuse block which is part of this same circuit. Notice that it is just fine...so the amperage did not come thru it.
If you have an MD series engine (and I think a few others too) then inspect this circuit.