Tranny Question and expert advice

[jack avant]

1984 300sd. My transmission is driving me nuts. I will try to explain what is going on. I have read several post and looked at the DYI information, and tried several adjustments. I can be driving down the road after car warms up at approx. 55 mph and the car will downshift to 3rd gear. If I shift to neautral and let the rpms lower the transmission will shift. I can let the car sit and cool down and it shifts fine for a while. I have adjusted the vacuum to the modulator several times. The only time it will shift is when I disconnect the vacuum line from the transmission. I have also adjusted the bowden cable but that has not helped. Does any one out there have some advice on what I need to do to solve this problem?

[DubMutant]

I'd put it right in the valve cover and light the fuse and then wait 30 seconds.

That should help.

But seriously. Have you completed the normal routine maintenance? Filter kit and fresh ATF to the proper level? (10mm below the bottom line when cold) Also inspect the vacuum paddles in the black box on top of the valve cover. there will be 2 and if they are worn down there will be issues.

I would also inspect all vacuum lines from the hard brake booster line from the vacuum pump. I had a similar issue 30 minutes ago where my car refused to shift out of second. I popped the hood and discovered there was a loose vacuum line, just not pushed all the way in.