Just work backwards from the pump to the power supply and you will find your problem. It sounds like a bad connection, the load of the pump is pulling the supply voltage down. This happens because the bad connection is effectively a resistance. The Ecu seems to be switching the relay as you can hear a click from it so look at the 12 Volts going in to the relay. You need to check it when the pump is trying to run, otherwise there may appear to be 12V there but it will kind of disappear as the pump demands current. Check at the pump terminal, at the relay output terminal, relay input terminal, fuse in and fuse out. I would bet you will find at one of those points there will be a bad connection. The fact that you see some voltage suggests the problem is not an open circuit.
Hope that helps,
Dave.
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