A common problem that most car wash owners will eventually encounter is with vacuums that won’t shut off.
The nonstop vac problem will always be one of two possibilities:
1) The timer is bad or malfunctioning, or
2) The mercury contactor is bad
To discover which part is bad, test for voltage with a VOM on the coil of the mercury contactor. If there is no voltage across the coil, then the contactor is bad.
If the mercury contactor has voltage, then the timer is the culprit. You can then proceed with the repair.