I am replacing an old light fixture. The junction box has no ground wire coming out of it. The instructions say that if this is the case to attach the ground wire from the light fixture to the green screw on the mounting plate.
Here is a picture of the old light where they have apparently done exactly that:
So how does this work? The ground has to lead back to the breaker box in order to work right? Is this mounting bracket connected to a wire leading to the breaker box?
Update:
Here is a pic of the inside of the box:
It does actually appear that the box has an attached ground wire leading out of it, so that's good news.
Best Answer
So long as there's no corrosion, the metal screws that attach the light's mounting bracket to the metal box are what provides continuity from the grounding wire attached to the metal box through to the ground screw and wire attached to the light fixture itself.
If your new light has this same set up (which is very likely), then you can continue to use the same system for your grounding.