300dt won't shift up to fourth
 

83 300dt with approximately 305k miles. last week suddenly from nowhere would not shift up. today i replaced the kick down switch. another story. in any case the bugger still will not upshift into 4th. although playing around - letting up on the accelerator pedal - it did so once. but only once. would appreciate your help. thank you.

Pull the modulator line off at the modulator, and look for oil in the line.

fuse #14 still good?

might be time to give it a fluid and filter change. Then, look carefully in the pan and also in the filter for particulates. Look for flakes of friction material in the filter and steel fines in the pan. Better yet, snap a few pictures and post them here.

gentlemen. thank you. status in reverse order.

fluid and filter replaced in last 10k miles. pan looked fine.

changed out #14 fuze.

still no unshift.

modulator suggestion. sorry i'm kinda dumb. where is the modulator?

again thank you all.

left side of trans with a vacuum hose attached. Turning modulator adjustment clockwise may be in order. However, you COULD still drop the pan and change the filter, cutting it open to look for debris. After 10K it should be pretty clean. If it's not, try adding a quart of trans-x. Often it will loosen varnish on the clutch plates that causes slipping or loosen a stuck spool in the valve body. But if the frictions or drum seals are shot, you'll need to do some disassembly.

wont shift into fourth
 

I had this problem on my 85 300 TD also. Not being much of a mechanic, I took it to a local shop that fixed it for $250. They told me the vaccuum lines were screwed up ( I think that's a technical term ). Any way it has worked fine since they worked on the vaccuum lines so that may be the source of your problem as well. Good luck.