DESPERATE: I need a transmission EXPERT

Man, this is weird. It can also be something very simple. Try removing the modulator bracket and check the modulator presure. With the bracket off, push the modulator in by hand into trans case as far in as it will go. It should spring back about 1/8 inch. If it springs back more than that, your modulator valve is not completely seated in case. That would raise your presure. If that checks ok, try replacing the kickdown solenoid. Get one w/rubber insert, no gasket needed. If it still does not shift,pull valve body and seperator plate and air check intermediate clutch. Apply air (about 30 psi) to center support bolt, which is hollow, and listen for thud. If you hear air escaping you have internal problems. Hope that helps you. Good luck.

 

Well I got it to shift, but it was around 4500 rpm. I pulled it out of the garage put it on the ice and stepped on the throttle. It turned up to around 4500 rpm shifted firmly into second and it even shifted into third. It had around 18 in of vacuum when it shifted into second. I have checked the vacuum line more than once, but I am going to replace the whole thing tomorrow. I'm also thinking of buying a new modulator on Monday. Any other info on getting it to shift at a lower rpm will help. Thanks

 

4500 is exactly where squirrel said it would shift at with a bad modulator, if you recall.

 

Take the governer out and test it like masracingtd1167 said in post #58.
My guess is that the valve in it is stuck.

 

Ya squirrel was right on. I can't remember if I heard it, or read it somewhere. But aren't the modulators adjustable? Is there anyway to test a particular modulator for leaks? Cause I have three of them to try.

 

I tought you said you tested multiple vacuum modulators. And since you also confirmed to have vacuum at the modulator i don't think it's the modulator.

If i were you i would unbolt the governer cover (passenger side of transmission) and test the governer valve.

The governer also effects shift timing and when stuck will cause late or early shift points.

 

Shine a penlight into the tube where the vacuum hose connects. See a screw head?

 

if you put the sprag in backwards, when it shifts to 2nd it will lock up and slide the tires.ask me how i know. also the accumulator/servo cover inside the pan , if the bolts are left loose it wont shift make sure there tight. most 400's i have dealt with will shift by themselves out of 1st gear at about 4500 rpm's i had a 69 el camino once that shifted @6200 rpm's but i'm uncertain as to whether the govenor had been set up to do this or not. hope this helps hot rust

 

Well I have three governers and three modulators and I've tried them all in the trans. I have checked all the governers to see if they are sticking and they all move freely. The governer that is in the trans now I had the gear off of it and the valve out cleaned and reassembled.

 

Even though they're in the business of selling transmissions, maybe one of the technicians at Gearstar in Akron, OH would be willing to help you? Just a thought.

 

Has anyone rented videos from here? http://smartflix.com/store/video/3098/GM-Turbo-400-Automatic-Transmission-Rebuilding-Series
I rented the TH350 video before I rebuilt mine for a refresher since I hadn't touched a tranny in almost 20 yrs. Very good and thorough video. Plus they have a pretty good selection and its cheaper than buying them.

 

Hi, I'll throw in my 2 cents,

All automatic trans require 2 inputs: governor pressure (road speed), and TV (throttle valve = engine demand = throttle opening) transmitted via cable, linkage, or in the case of the 400, manifold vacuum. To make an automatic upshift, the shift valves in the valve body have to move, or "snap over" against spring pressure. When the shift valve moves, it directs the oil pressure to apply a set of clutches, or a band etc. GOV oil acts on the large end of the valve, and TV acts on the return spring side. The more TV pressure (throttle opening) acting on the spring side of the shift valve, the later and firmer the upshift. So, no automatic upshift is either caused by too much TV or no GOV pressure.

Possible causes include: No vacuum at mod, defective mod, stuck mod valve in case, stripped gov gear, stuck gov valve, plugged gov screen (in valve body, where gov tube goes in), loose or stuck kickdown solenoid.

The pressure port on the case is line pressure. Should be about 90 psi in D, 150+psi in L1, L2 and R. One simple way to check if your shift valves are free. Pull Gov out, take two small pieces of vacuum hose and block the Gov weights OUT away from the stem. Reinstall, and if you should be able to manually shift your trans, after making sure your TV system is operational.

 

Well the roads finnally cleared up enough to try for another ride. We now have a tach hooked up. Ok we are headed down the road in first picking up rpms. The trans shifts into second on its own at 3500 rpm. Still accelerating the trans shifts into third. But when you let off the throttle it seems like it goes into neutrel. Then the trans stays that way until you are going slow enough to manually shift it back into first. This is where I stand now after checking everything up to as far as post #60.

 

[Quote:Mr4Speed] When you had the pump apart did you check the pressure regulator and boost valve?

It's also possible that the kick down solenoid is stuck open!


Well when I had the pump apart I took everything apart checked and cleaned. The
kick down solenoid, I put 12v to it to make sure everything worked. I also have been test driving the truck with the kick down wire unhooked.

 

Bingo. Many TH400 modulators are adjustable. If yours is set too high, this could be the problem.

 

What is it doing when you manualy shift it through the gears?
It won't downshift in to second?

 

It won't shift manually into second till the rpms are way up and shifting into third doesn't seem to be a problem. The timing between second and third shift seems to be right. As far as downshifting into second manually or automatically it doesn't. It seems like it goes into neutrel.

 

Maybe a stuck valve in the valve body or a cross leak somewhere.
Did you check if the check balls in the drums and/or pistons were free?
How many and whish check balls did you use under the valve body?
Did you use a shift kit?

 

I used all the check balls and double checked they were in the right place, and no shift kit. I didn't want to do anything other than stock in case I had to trouble shoot something. LOL

 

See if you can get a pressure tester with a long hose and take it for a test drive.

 

Vacuum tester or hydraulic tester??

 

Hydraulic, use one with a range of at least 0-300 psi.

 

I don't know if I can borrow one like that. I'm not even sure where I could buy one around here.

 

Do a google search for "transmission and engine oil pressure tester".
Shouldn't be that hard for you to find locally! i would think?

 

How sure are you about the linkage adjustment?

How about lifting the rear wheel onto jackstands, disconnect the linkage and put it into Drive by hand?

Perhaps you're in between gears.

 

Well I'm pretty sure about the linkage. The truck has a removable tunnel, we have been running it without it and have shifted it though the floor by hand once without the linkage.

 

Well its pouring today. I don't think I'll be test driving today. First thing tomorrow afternoon I'm going to try the vacuum line pieces holding the governor weights open. Hopefully I will get some insight from that. I can't give up on it for my nephews sake. Thanks again for all the help

 

[QUOTE : potshot] One simple way to check if your shift valves are free. Pull Gov out, take two small pieces of vacuum hose and block the Gov weights OUT away from the stem. Reinstall, and if you should be able to manually shift your trans, after making sure your TV system is operational.


Well I tried the vacuum line pieces wedged in the governor weights. Now when I put it into D it goes into first gear, shifts into second on its own at 2000 rpms has engine breaking in first and second but I lost third. Even if you try to manually shift it. I don't know if that is supposed to happen with this sort of test (not having third)? I'm going to pull the pan and check the governor passages, screen, and tubes for leaks or blockage. I'm not really sure what else to check while to pan is off? Any ideas????

 

Did you put the vacuum line pieces behind both weights or only the outer?
Normally the inner weights have the largest effect on WOT and the outer weights have the most effect on part throttle 1-2.

Does it have a deep oilpan? If so does it also have the long filter intake pipe and long spacer and bolt?
Something i always do on the filter intake pipe is use two o-rings where it goes in the case to make sure it has a good seal.

Although the kick down solenoid seamed good it's possible it's not closing 100% i would try a other one.

I would also take the vacuum modulator valve out and compare it to a other one to make sure it has not been "modified".