The only other change is the "fifth wire" added after 1987 to connect the display to the speedometer so the display won't update when the car is standing still. This keeps the display from reading super-high temps when the car is stopped in traffic on a hot summer day. This fifth wire was not added to W126 cars.

That explains a lot! Thought the display was pretty useless.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car