Never worked on Volvo engines -Cat Cummins and little Detroit. First thing i always ask are you getting illumination of the check engine and stop engine light when key is turned on if so this would indicate the Ecm is turning on. Then Observe stacks when cranking, Do you see any smoke, this will not apply to a engine with a DPF. If you see smoke that indicates you are getting fuel, so injectors are firing, Possible air in fuel or low fuel pressure. If no smoke. A voltage detector like the ones electricians use for check outlets and such works great if held up to the valve cover harness while cranking or even better on the top of the injector while cranking will verify the ecm is firing the injectors. If the Ecm is not turning on. I would start at the batteries usually they use 2 fuses 5 amp to feed the ecm. Many of the oem,s start with one wire then splice it into 2 which kinda defeats the purpose of having 2 wires. Pull lightly on all connections and splices to verify the are connected 10 lb pull test. I have seen where ecms just fail out of nowhere but you will not see the check engine and stop engine lights. Don't forget to check you grounds as well the Ecm will use several grounds for the circuity as well as the external ground strap you can see. also realize the key circuit will power the ecm up most likely through a relay. The ecm will have unswitched power and switched power