When using a L6 the voltage should be 5v at closed and goes down when opening throttle

on a L8 its the opposite

If you are using a 2wd loom and a L8 ecu, you need to swith the outer wires around, and ideally use a pf09 tps

I'm using a 2wd loom, L8 and a pf01 tps , with the wires swithed and it works

Its all here:
www.bigturbo.co.uk