To test a Voltage Sensitive Relay (VSR), start by checking the voltage on the main battery with a multimeter. Start your engine and observe the voltage rise; when it reaches the VSR’s activation point (typically around 13.3V), the relay should engage, and you should see voltage on the auxiliary battery as it begins charging. If the VSR doesn’t activate as expected, inspect the wiring connections and ensure there are no blown fuses. If everything checks out but the VSR still fails to engage, it might be faulty and in need of replacement.