I'm having the same problem. It happened when I replace my alternator and serpentine belt. The backlight is fine. Earlier today the temperature gauge was reading 136+. So how did you fix the problem.

__________________
1995 E420 152k
1991 300D 2.5 348116 daily driver
1992 300E 2.6 196k-work in progress(head gasket)
2002 BMW 745Li 93k