Transmission shift problem
 

I have a 85 500SEC with about 200K miles. The transmission started to skip 3rd gear and shift from 2nd to 4th. Now it may stay in 2nd or 3rd and not shift into 4th. What are the possible causes? The fluid level is OK and the fluid looks and smells OK. I hope I don't need a new trans. Any help?

gas engine
 

I do not know very much about a gas engine...

but on my diesel 83 300sd it has a vac control for shifting..

and this vacuum control bleeds down to zero as you reach 4th gear..
and if it is broken and has to much suchion then it will stay in say 3rd gear or skip a gear....

:D

That’s good you check the condition and level of the fluid. Next, check the Bowden cable on the throttle linkage on the engine; make sure the cable is not frayed or binding.
If the cable looks good; with that type of erratic shifting I would say valve body.