05 VNL won't start - clicks once won't click again

12.5 volts should be good. If it was low voltage it would try to turn the engine. I am thinking it might be the starter. You may be able to jump it manually at the starter to bypass all previous wiring, relays etc., but you've gotta know exactly what you're doing to try this. (And make sure it's not in gear). Some trucks have a soleniod on the firewall that sends current down to the starter, but I'm not sure that Volvo does. I know I haven't been a ton of help, but thought I'd give you these ideas since nobody else replied.

 

Let's look at it like this, in order to simplify things. There are 3 parts to the picture, batteries, starter, and all the wiring, switches, relays etc. in between.
The first 2 are the easiest to check, if the problem is in the 3rd it can be the toughest. I would verify your batteries first. Load test them if you can. Turn on lights and heaters, does the voltage hold up or drag down. Do specific gravity tests on the cells if needed. Once you know batteries are good, verify the starter next. What kind, how old is it (starters are typically good for a couple to a few years, depending on how often they are used, and under what conditions. I've had a rebuilt Delco 42 MT that didn't last a year once). If you are good with electric diagnosis, you can do an ohm test on the starter and check for open, shorted, or normal windings. If you are sure batteries and starter are both good, then is the time to start the process of checking everything in between. If you did pick up another starter, and found it that wasn't the problem, you'd have a spare. Not the worst thing, you'll need it eventually. Anyway, good luck with this. It bloody sucks when a guys truck won't start doesn't it.

 

Would swap the starter solenoid first then starter after batteries passed a load test.

 

I have a Volvo Vnl d16 and I don't know where the crankshaft portion sensor is located. Does anyone know where it is located?