what make equipment? If it's a WW2 the lower boom prox is on the pass. side inside the gantry panel. As you face the gantry with the panel off it is on the right side, down low, closer to the face of the gantry. there will be a bunch of counting tabs sticking out that the boom uses to locate itself and the boom prox will be below those tabs. Wave a piece of metal in front of it to see if it lights up. And look for a tin can or piece of tinfoil laying around in there that could be blowing around and tripping it. If the prox is on it's because it has detected something in front of it, or it's shorted out. My experience with prox switches has been that they stop working as opposed to staying "on"