SM-S-105DM Relay Insufficient Control Voltage

To determine if the control signal is the root cause, set your multimeter to DC Voltage mode. Measure the voltage across the coil pins while the device is attempting to trigger the relay. A functional system must provide approximately 5V DC. If the measured voltage is significantly lower than 5V or is completely absent, the control circuit board is failing to provide the necessary power to activate the relay.

Sources