Two things to check:
1. make sure your transformer outputs AC voltage, not DC. it’s sometimes confusing what ‘ac adapter’ means. strictly speaking, sprinkler transformers are ac-ac transformers (i.e. outputs AC voltage), whereas most other adapters (like USB, laptop adapters) are ac-dc transformers.
2. can you connect an actual sprinkler solenoid to test? where there is no load, the multimeter may pick up stray voltage (which appears as if the station is active). the stray voltage should be gone if an actual load is connected.