1979 300SD weird trans problem
 

I hope Stretch will chime in on this one. On a 1979 300 SD, the trans goes into neutral after stopping. It shifts the rest of the gears great, but I have to put it in neutral and then back into drive after stopping. If I shift it down into second before stopping, it seems to start in first ok. I adjusted the modulator pressure a little tighter thinking it has low pressure, but that did not help. The modulator holds vacuum, and the VCV tests good (full vac idling, gradually decreasing to zero at a little before full throttle). The fluid level seems a tad over full cold, have not checked it warm (have not had the car long enough). The previous owner said it has been like this for many thousands of miles, and he just dealt with the hassle. Any ideas?

Shifter bushings?

I don't believe so. Something is happening hydraulically causing the problem, but what exactly, I have no clue.

Stretch? Anyone?

Normally I am about the last to offer transmission advice, but seeing as you seem to be getting an echo....

I have no idea if this behaviour is actually linked to the operating pressure, but I'll go with that for now.

You say you adjusted it a bit, did that make any difference to the shifting otherwise? If it did not, I wonder if the modulator is set way too low. How does it shift otherwise, firm or what I call slithering through the shifts (I hate overmodulated setups)?

If the modulator way off the correct setting, you will need many turns. You can either go ahead and just try one turn at a time, to see if anything changes. Or you can try to put a gauge on to test the operating pressure, to see if modulator adjustment is indeed the tree you need to bark up. To do that you will need to confirm which is the correct test port (there are probably three or more choices and they are apparently different on the later trans, so beware hooking up a gauge based on info from archive posts).

Indeed a strange problem. An incorrect vacuum can cause all kinds of shifting weirdness, but I never heard of a transmission slipping out of gear because of it.

First start with the basics: when was the fluid and filter last changed? The fluid level should be below the marks when cold. Best way to check fluid level is when the transmission is warm.

I believe the 1979 300SD has a shift rod to control the shift points and the first gear take-off. Is the linkage adjusted according to specs?

A small investment in a pressure gauge and pressure line should be made so that you can measure the oil pressures inside the transmission. In that way you can correctly set the modulator.

Does the modulator hold vacuum?

You can also use a T piece and a long vacuum line routed to the cabin and connected to a vacuum gauge to measure the vacuum to the transmission during driving.

Yes the modulator holds vacuum and VCV working properly (explained in the first post). I don't believe the problem is with a mechanical linkage in any way (control rod, shifter bushings etc). Like I said, it just goes into neutral when stopping.

I did remove the cap, adjusted the "T" in about a turn and a half, replaced the O ring, and put the cap back on. It is possible that I should have put 3 or 4 turns on the T, but it does shift firm in all the gears. I don't want it to slam into gear, but maybe more turns on the T tighter will make this weird problem go away, it may be worth a try. I am not sure when the fluid was changed last, but it looks newish....Rich