TH400 issues-slams into gear

[Kenny78]

Ok, so I my TH400 in a K20, slams into gear hard from park. I have checked for a vacuum leak from the brake booster, none. The tranny shifts fine when driving. It does have a small leak (in the process of fixing it) but I have checked the fluid before driving which is very little since I just brought the truck Tuesday. I was reading something about check balls being in the incorrect position but IDK. Any ideas?

Thanks in advance

 

[thatK30guy]

Modulator?

 

[gotblazers]

vaccum modulator would be my guess as well. I replaced mine on my 76 k5 and it shifts much more firmly now. You can adjust it to make it shift earlier or later, Be prepared to deal with a bitch though on mine its right next to the linkages and tranny crossmember and i had to nearly drop the cross member all the way down to get to it which is fine... If i didnt spend 2 hours trying to do it without the crossmember because my dad insisted i didnt need to drop it lol. Anyways gl man

 

[Fordum]

First thing, what is your idle rpm in park. If its too high, that will give you a slam. Also, make sure the vacuum line to the modulator is hooked up to the right port.
Many years ago, I saw one that got hooked to a temprature controlled vacuum source. Transmission slammed until the engine warmed up.

Plus, when you take the line loose going to the modulator, any transmission fluid inside means a bad modulator.

Another friend called me up bitching because the line going to his tranny kept coming off. It was a hard line down to the modulator, and then a short piece of hose.

When the transmission started shifting wrong, he would climb under, and either cut off a small piece or replace it, because it kept swelling up and falling off.
Plus he kept having to add transmission fluid and could not find the leak.

J.

 

[Kenny78]

Update, kinda. My wife posted this up for me today. We just got this truck monday and have been busy fixing the little things. The leak appears to be between the trans and adapter, wanted to tear into that today but was stuck at work. I havent checked the idle speedbut will tomorrow. This truck is the first truck in the fleet without glasspacks so it is deceptively quiet I hooked my vac. gauge inline with the modulator line(which is hooked to manifold tee behind the carb.

Thanks for the tips and will check back