I have an '87 760 GLE V6 with the auto tranny. 4th gear does not seem to be engaging, regardless of whether the overdrive light on the dash is on (little orange arrow pointing up). This puts me at about 3500-4000 rpm on the highway, which cannot be good for fuel economy.

Is this a problem internal to the auto trans, or do these cars have an external transfer case that engages when the auto tranny is in 3rd to provide the 4th gear? What are some common issues that cause these to go out? Thanks!

Pete