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the factory speedo gets signal form the gbox then form back of the speedo there is an output speed signal that goes to the ecu
now if yr speedo doesnt have this then the signal might be too weak to run both
but being a square wave it should not loose strength so i think the signal might be slightly wrong

now every time i wire up an au v8 auto well the ecu needs the speed signal so it changes gears took me awhile but i figured if i ran factory speed signal from gbox to ecu it still isnt good i had to run a signal booster to amplify the signal form speed sensor on box to ecu and it fixed the problem

not sure if this will help but it works on all the au autos i have done

this unit will actually take a weak input signal and amplify it and it also has a small led to show u the pulses coming in

i can sell u one and if it doesnt work send it back
u just have to pay for freight or drive car to sydney and i can fix it
or find someone down yr way