Mercedes: Transmission, modulator valve, shift points

Question
I recently purchased a 1990 Mercedes 300 SEL; and, just noticed that car is not shifted to fourth gear.  So, my question is two-fold: (1) when should the vehicle shift to fourth gear (mph); and, (2) is this a sign that there is transmission problems?

Answer
The shift points are pretty high on this vehicle. What speed are you expecting 4th to come in? With caution on an open road, from 2nd gear,kick it down to 1st and hold it, count the shifts which should be in the 4,000 rpm range. Fourth gear would be coming on at about 80 mph.
If fourth is still not there, you certainly may a transmission issue. It could be in the the modulator valve.
An independent transmission shop that specializes in Mercedes could offer some good advice.
Good Luck
Rob