th400 tranny filter change & shift questions

Hey guys,
I searched for about half an hour to find some answers but didn't find what I was looking for. I just bought a 1982 Chevy K20 w/ a 6.2L and what I believe I've determined to be a th400 (stock auto tranny w/out OD). My questions are as follows:

1. How do I change the tranny filter? Sounds basic but I've never kept a vehicle long enough to do it or had a shop take care of it. When I got the truck it was very, very low on fluid so it has had a fair amount of new put in. I'm afraid to do a full tranny drain because of the cleaning effect of new fluid on an old tranny, worried I may have a problem (happened in the past). I just want to change out the filter. From what I understand the filter is attached to the tranny pan or directly above it? What can i expect when i drop the pan? Anything special to look for? What do I torque tranny pan bolts to when re-installing (with new gasket of course)?

2. My second question involves downshifting. I noticed when I drove the truck today that I had to either manually downshift or put the pedal to the floor and wait for a little bit to maybe get the tranny to downshift. Is downshifting controlled by vaccum or other means? What should I be checking? If it is vaccum what is the best way to check the pump and how much vaccum pressure should I have?


Thanks for the help all.
James

Howdy

Let see now, guess I will answer things in the order you asked them.
The filter is held on by a little bolt in the center of the filter.
The filter is fairly long and plugs onto a long plastic tube where it goes into the case of the tranny.
Be sure to change the Oring on the case end of the filter tube so you dont get an air leak.
Changing out all the fluid by flushing is not harmful at all.
You cant drain the converter as in older trannies but you can remove the return line from the cooler and pump the stuff into a bucket all the while adding new until it comes out pink and then do the filter change and refill.

When you take the pan off look for large amounts of black flakey stuff
(Clutch, band material) and metal. It will have some sooty goop in the bottom and this is fairly normal. Just wash it out. If there is not any great amount of crud then your probably in great shape

Use a composite cork pan gasket and snug the bolts down with a short ratchet. No need to twist them too tight, just good and snug is great. Be sure the last person did not warp the pan rail between the holes and if they are turn the pan upside down and tap it out straight with a small hammer.

The TH 400 uses two forms of control. The vacuum modulator on the RH front of the case at the pan rail controls the upshifts and part throttle downshifts.
This is controlled by a vacuum switch on the Injector pump that controls vacuum from the vacuum pump.

If the tranny upshifts about right its probably ok, if its late and hard then there is issues with the system.
The vacuum pump should provide about 26"hg at the pump and it should be steady, if the gauge is wiggling all over the pump is shot.

The vacuum at the modulator should be about 18-20' hg with the engine Idling. maybe a bit more or less.

As the throttle is rolled on this will drop to allow the modulator to compensate for more throttle and make the shift firmer and later which corresponds to engine load.

The second control is the downshift solenoid.
This is inside the tranny and is an electric solenoid that is controled by a switch on the throttle pedal bracket. The switch has a little rod that sticks out and as the pedal is depressed to the floor the switch is activated.

The control voltage to the solenoid is transmitted through a single wire that connects to the LH side of the tranny above the pan rail with a small plug.

Hope this helps and if you have any other ???? just ask :)

Robyn