Ok, first thing is to download/print the wiring diagram and legend that I have uploaded for you. You will first note that Volvo does not believe in labeling the components with their names, instead using numbers for them. The instrument cluster is 5/1 and the cluster harness connector is 24/13. The legend is on the last page, so you have to constantly refer to it.
The first we do is to check for power at the cluster. This will tell us if it is the cluster or not. If not, then we will need to check for power from the sending unit. First, this makes sure you dont have a defective one and we spend hours chasing our tails because we did not check it. If power is good, then we will check the ecm.
So first, on the green with red striped wire, pin 4 on the instrument cluster connector with key on, check for at least 10v. If present, cause found - replace cluster or send out for repair.
If not present, then we will go from there, but one thing at a time