"Lift" the common and the valve wire from the controller.
"Earth Ground" the common wire at the controller.
Go to a known VB and "Lift" the common at the value.
Attach the transmitter to the common and "earth ground" it, trace and flag the path towards the controller.
Once done, attach the transmitter to the common that continues to the field, trace and flag.
When that is done remove the 521 and reconnect the wiring in the VB.
Take the 521 to the controller, lift the common from ground and attach the transmitter to the common and valve wire (Closed Loop), follow the marked path until you find the valve.
Earth Grounding the far end of a conductor really improves a signal and is a common practice for utility locators. Our main problem is that we don't always know where the last valve is so we have to cut into the wire path beyond the interference and work both ways.
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