It depends on how the voltage is being tested. When you first turn the key on, the ECU tries to run the pump briefly by actuating the relay and then turns off. If pump doesn't run, it could be a problem with the pump, or the relay, or the ECU and it's inputs.

I would try disconnecting the pump or just removing the pump relay then apply 12v to the pump itself. That will tell you if the pump is faulty or not.

I would also check that you have spark - get that easy check out of the way.

It could be a completely different problem. Maybe the crankshaft position sensor? You can check that by watching rpm with an OBD2 meter or app while cranking. You should see the cranking rpm - maybe about 200rpm.

I went through something like this on our e320. Turned out to be an ecu related problem.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5