Engine ground problems ?

[goingcoastal]

Not sure I'm posting this in the right spot, but here goes. The power tilt sensor indicator on my Mercury 115 quit working, so I replaced the sensor because it was corroded and coming apart. Checked the gauge prior to install to make sure it was working, and it was. After install,gauge still does not register. thought I might have a ground problem, so I ran a direct ground from the battery to the sensor ground and low and behold , it works. My question is why doesn't the sensor wire ground when fastened to the mounting screw ? Shouldn't the transom mount be a ground if the engine is grounded ? The wire coming off the sensor is only about 3 " long, and the original was grounded in the same fashion. Sorry I'd post pics, but don't have any way to do it. Any help would be appreciated.