Poor ground cable to Battery, bad starter or bad solinoid on starter, or poor ground to starter.

One day after a rain when the carrosion swelled up from the moisture---that's all it took to give me a poor ground. Check it out. Also one other cable to the Air mass meter was caroded and after I cleaned my off it all started off very well.

What 740 says. The solder used to fix the ends on the high amperage cables degrades. It may not even be solder, but rather some sort of lingonberry compound. Pull on the cables. They may well come right out of the ends. You can clean them up and resolder, crimp or replace.

The click is the solenoid working so it's not some start inhibit function. The problem is the contacts in the solenoid, which connect the battery cable to the starter motor, are burned out. You can replace the solenoid or get just the contacts from a Volvo dealer or a starter rebuilding shop.

I had the same problem once.
My neighbor said it might be the battery cables where they connect to the terminals.I said;"no,that's not the problem." After spending hours trying everything else,(starter solenoid,etc.).I checked the battery terminal ends, and even though they looked fine, there was unseen corrosion.Replaced the terminal ends, and the problem was solved...I had to sheepishly tell my neighbor he was right,,and luckily, he was kind enough to not rub it in with an;"I told ya so!".

best wishes

If you are sure the battery is OK, hit the solenoid with a stick and see if that does anything. It sounds like a dead battery to me.
Dan