[MotorHead]

I haven't used the Ultrasonic yet, so I can't comment on its setup. I do have a problem with your logic. The way you currently are configured is: If distance >= to 40 then back up with y =-.4, If distance < 40, stop and activate solenoid.

The problem with this is you do not have a deadzone in which to stop. Your arrangement can cause the robot to excessively hunt about the setpoint of 40. Look again at the vi I posted. There is a nested case structure to test if you are above or below the setpoint or if you are within the acceptable deadzone, in which case, do nothing.