The odo most likely has a slipping gear. Take it out and you'll probably be able to spot it. Usually some CA glue will do the trick.

If you apply 30 ohms resistance across the brown and other two wires at the sender, one of them will have the guage reading about 2/3 full.

I'm not sure what the normal resistance range is.

Bruce McCreary
'85 300D, '83 300CD, '85 300D