I thought I’d send a follow-up, as I managed to solve the problem and the unexpected root cause may be helpful to anyone else who has a similar issue in the future…
A post on the “Cruisers Forum” led to a person writing back who had a similar problem that was solved by a mechanic in Martinque and ended up being burned out glow plugs. Then last week I had a very difficult engine start that made it clear, as did some subsequent testing with a multimeter, that the glow plugs were not functioning and were not being supplied any power. I initially thought the problem was with the “black box” CPU for the EVC system (an $850 item), as it has a non-servicable relay in it that sends power to the plugs. Turns out though that the EVC unit actually tests the circuit on startup, and if it detects an open circuit, it triggers the vague “system fault” alarm (and presumably will not then send power to the plugs when the button is depressed even after the alarm is silenced). How though, you might ask, could all 3 glow plugs burn out simultaneously to lead to this situation? Well the answer, I think, is that they did not. Rather they likely failed sequentially over months or even years (and I had noticed the cold weather starts were getting more difficult), but only when the 3rd and final one burned out did the mysterious alarm go off. One way to test this theory is simply to ground the current bar (supplying the positive power) to the glow plugs when a system fault alarm goes off, and when I did this the alarm silenced (as the open circuit was eliminated). And so with this presumptive diagnosis in mind I went ahead and pulled all the glow plugs and discovered that they all had infinite resistance on ohmmeter testing, and none of them lit up when hooked directly to 12v power.
So 3 new glow plugs have been ordered, and the problem is hopefully solved! If things STILL aren’t working after this, I may need to replace that EVC black box after all, but hopefully not. Needless to say, even the electrical workshop manual for the Volvo EVC system does not explicitly explain the possibility of what I’ve described above. Gotta love all that fancy electronics – it’s wonderful until something goes wrong and then the troubleshooting can be quite problematic to say the least!
If you’re interested, below is a link to the reply I received on the Cruisers Forum about this issue (it has some additional details that may be helpful, and the ultimate issue which was the same as mine is revealed at the end of the thread):