My hunter zone 7 works fine independently but also stays on when zones 8 and and 9 are running. It does not go on with zones 1-6. If I disconnect the zone 7 wire from the controller it doesn’t activate at all (as expected). I checked for debris in the valves and found none. This just started this summer. I’m wondering if this a downstream wiring issue or a solenoid issue?
While zone 8 is running, disconnect the zone 7 wire. If zone 7 stops, it’s a controller hardware issue. If it’s still under warranty, I’d get it replaced. Or, if you have an unused zone, try connecting the valve there.
If zone 7 does not stop, check voltage between the zone 7 wire and common. A small voltage (less than 1 V) is normal, because of drop in the common line. More than one volt points to a wiring fault.
If it’s neither a controller or wiring issue, I suspect that there is an issue with the solenoid or diaphragm, so the valve doesn’t close when de-energized, but after zones 8 and 9 shut off, the increased water pressure is enough to close the stuck valve. Try swapping solenoids with another zone to see whether it’s the solenoid or valve body. Possibly, some additional cleaning will help.