the "downshift linkage" does control the shift points,and the firmness of the shifts, and it is very important for it to be adjusted properly. not enough trhottle pressure causes the tranny to shift too soon.ths can cause lugging,and excess heat,which is very bad for the tranny. too much throttle pressure causes the tranny tothink youre accelerating faster than you are,and delay the upshift. ths causes higher rpm shift points when accelerating lightly,high rpms create tranny heat,and are also bad for the engine.
if youve allready adjusted the throttle linkage according to the manual,then the problem could well be elsewhere.you should first change the fluid and filter,and check it well for metal,clean out the pan and adjust the bands.
if the problem still exists,youve got worn clutches,or some other internal problem.does it slip in any other gear? reverse? first in D or 1? second in D or 2?
in 3rd gear,both the front and rear clutches are applied,and no bands.if youre only slippage/noise is in 3rd gear,the rear clutches are slipping. if it also slips in reverse,the front clutch is slipping. it may be time for an overhaul.
you can use air pressure to check the clutches and servos with the tranny in the car,but youll have to remove the valve body. or you could have a reputable tranny shop check it out. you could still save money by pulling it yourself if it needs rebuilt and youre not comfortable doing the rebuild yourself.
hope this helps.
85 grand wagoneer
258/904/twin stick dana 300/dana 44/amc 20
38x15.5 gumbo mudders
snorkel/dual batteries/onboard air/"custom" convertable
3 inch body lift/mostly stock suspension/"modified" fender openings
custom front/rear bumpers and brushgaurd