Trim gauge wires

[BQUICK]

OK, I'm working on two 88 150s. One has a 4 wire trim sensoe (pink, orange, black, gray). The other 3 wire (pink, orange, black). The boat I'm rigging has the 4 wire gauge (Yamaha old style long tube style). Tried the 3 wire sensor and won't work. Put the 4 wire sensor on and works fine.
Only difference is gray wire. I've read that it is for overheat but that can't be. Gray wire goes from sensor all the way INSIDE the trim-only gauge. What is it?

Also, instructions say to mount the trim gauge at a 50-80 degree angle. The dash is 90 deg and besides the angle changes all the time when running the boat.

[boats.net]

There are two gray wires and two pink wires on older engines, ones coming from trim sender are only for trim gauge connection.

[BQUICK]

Mine only has one gray and one pink. Without the gray connected it doesn't work.