How do you replace a mercury vapor light sensor?
Most Mercury Vapor Lamp Fixtures have a PLUG IN module on top of the fixture. Most are about the size of an inverted "shot glass" and have a photocell "eye" somewhere on the outside surface. Typically, you turn the Module counterclockwise to "unlock" the module from the socket and pull the module UP and off the top of the lamp fixture.
Failure mode for these Light Sensors are twofold: Either [ 1 ] the electric eye's lens gets cloudy and the light stays on unless it is VERY sunny....or [ 2 ] the Module, being mounted ABOVE the light fixture gets HOT...and over the years the module gets COOKED and fails. Take the light sensor with you to an electrical supply house and match it up with a new one !!
BYW: Inspect the socket and plug of the Light Sensor module & lamp fixture. Often the "cooked module" will fail because of socket/plug failure. The plug/socket Phenolic materials break down from the heat over the years . Or the contacts corrode.
NOTE: It is important , when installing the Light fixture...that the plug-in Light Sensor' photocell lens is pointing AWAY from the building or other mounting structure. If the "electric eye' is facing the structure..the reflected light will confuse the light sensor and the lamp will come on DIM[*] , flicker or turn off & on constantly.
[ * ] Do not confuse the " comes on DIM " statement with the normal DIM warmup cycle of the lamp.
It is also advised to have the High Output Merury Vapor LIght on a dedicated electrical circuit. If you are using an existing circuit for outside-the-house loads..... such things like pool pumps etc will extinguish the lamp when the pump kicks in.. The pump's heavy inductive load in-rush current on startup will cause the lamp to go out..... in a few minutes the lamp will restart, warm up and come back at full intensity.