what's wrong with my transmission?

im driving a 1981 300sd turbo diesel sedan. it has an automatic transmission. it goes from 1st to 2nd gear fine.. but when it tries to shift from 2 to 3 it revs high then engages and then the engine revs low again. it does the same when it tries to get from 3 to 4th gear too. the only time when it doesn't do the high rev shift is when i really lightly push on the pedals and acceleration is really snow. what could be wrong with the transmission? i'm planning on fixing it myself instead of taking it to a mechanic.

thanks in advance

I'd start off by making sure all the vacuum hoses are hooked up to the transmission. Sometimes/hopefully that's all it is. There's not much else you can do in your driveway with the excepetion of the vacuum hoses, changing the fluid, or installing a new valve body, unless you're fully trained to rebuild transmissions.

The condition you’re experiencing is called “flaring” and is common to MB diesels of this era. You need to shorten the throttle linkage and adjust the vacuum modulator to the desired “late” shifts the diesels need. It worked wonders for my 81 300 D with 384,000 miles on an original tranny. If you can’t do it a mechanic can for less than 100 bucks if that much.

Make sure the right modulator is in there. Also, unless someone has modified your trans, the omnly way to get these to start in 1ST gear is to manually shift it from low gear up through the gears. Also, check to be sure the right amount of fluid is in there.