Volvo Repair: 86 760 turbo overdrive, overdrive system, throttle linkage

Question
Hello Mr. Frolich
My 1986 760 turbo's overdrive system isn't engaging properly.  At highway speed, the overdrive engages, but then immediatly disengages again, though the rpm, ground,etc. seem to be appropriate for the overdrive gear.
Clogged internal oil filter? Are they hard to clean?
Possibly the non-return valve? Again, very difficult to service? Can I access these with the overdrive unit on the car, or does it have to be removed?
Thanks.

Answer
Manual or automatic transmission?Does the arrow in the instrument cluster come on when it disengages?Have you checked the fluid level and condition(does it smell burnt?)Check the kickdown cable adjustment(pull on the cable at the throttle linkage and let it snap back,you should hear it click)