DESPERATE: I need a transmission EXPERT

Hi everyone I REALLY need help with my transmission problem (400 Turbo) I know this isn't old car
related BUT.... I'm helping my nephew who is 17 build his first vehicle. It's an '81
Chevy 4x4. We bought the truck with a blown transmission (only having reverse). We
replaced it with one that was supposed to work but it didn't, it only had reverse
and first. With no other used transmissions available and the only transmission
shop being 100 miles away and wanting $1000.00 to rebuild it, we opted to rebuild
it ourselves (my first transmsiion rebuild). With the rebuild complete
and the transmssion in place it doesn't work, IT too, now only has reverse and first.
I cant believe I messed up on the rebuild just enough for it to work just like to other.
Now having 2 transmissions that have first and reverse one rebuilt and one not. I need
an expert to lead me in the right direction. Hopefully I can get this fixed soon, my
nephew is starting to think this project is a BUST. He had hopes of building my 31 Willys
for his second project but his glimer of light is fading fast. PLEASE HELP.... THANKS....

 

What did you have for instruction when you rebuilt the transmission?

I'm guessing there are some worn parts you didn't notice. It's easy to overlook things if you don't have any experience with them, you don't know what to look for.

 

Something does not seem right with the same symptom 3 times in a row. Maybe the transmission is not the problem.

 

I had a Haynes HOW TO book

 

is your tranny cooler clogged ?
400t is pretty tough trans and should only be half what that shop is charging you for full rebuild bench job.

 

My guess is the Haynes manual doesn't do a good job of showing you what to look for as far as wear and such.

If you're game to try again, you might want to get the Ron Sessions book on the TH400, and also post a bunch of pictures of parts showing how much wear they have, how things were assembled, etc. on here. You have a great resource in the HAMB, there are several guys who know this stuff and will take the time to look over your pictures and read your posts and help you figure it out.

 

They are pretty easy but, once I put the sprag in backwards and that'll mess you right up...

 

I have checked for vacumm leaks and I have checked the shifting linkage 2 times to see if that was it. I have replaced the governor with 3 different ones, changed modulators with two other ones I had. I even checked the pump pressure.

 

Check the sprag...

 

Oh I am game for trying again. This is something I would like to do for myself and my friends. I spent around $250.00 on tools to do this and I'm not going to give up on this tranny till it works.

 

When you say you only have first and reverse, I assume you mean that the trans doesn't upshift, even with the selector in Drive. Have you checked the vacuum modulator and hose to it? If the vacuum modulator is not working properly or does not see manifold vacuum, the trans thinks the engine is at wide open throttle and will upshift at 4500-5500 RPM, depending on how it was built. If you can get the trans to upshift by standing on the throttle, the problem is the modulator.

 

Be careful when "standing on the throttle" to test the vac modulator (if it even has one, I know the TH350 has them but I don't have any experience with TH400. If I remember correctly, the TH350 only needs the vac modulator for 3rd gear, first and second should still work). I saw a customer years ago get a car towed to the shop for a slipping trans. After it was off the tow truck, buddy started it up and attempted to drive it into the shop ... and blew the engine.

 

A TH400 with a bad modulator will have only 1st and reverse. You said you've tried a couple different ones, so, two things leap into my old, addled brain:

1. Check your vacuum source for good MANIFOLD vacuum.

2. If memory serves me, the business end of the modulator has a hole in it, I believe there is about a 2" long "pin" that plugs into that hole, then goes to the valve body. Could you be missing it?

 

Is your kickdown wire connected to connector on trans? If it is, try it with wire off.

 

PS: Just to be more clear- "manifold" vacuum is high with the throttle closed- like at idle and deceleration. It's source is BELOW the throttle plate. "Ported" vacuum has it's source ABOVE the throttle plate and is zero (or near zero) at idle (with the throttle closed) and increases with engine speed. As you see, they are nearly opposite.
I cannot tell you how many times I've seen them mis-applied.

Good luck!

 

I have tried the kick down wire on and off the trans and neither change my situation.
The vacuum is connected directly to the manifold and doesn't seen to have any obstructions. But I don't have a vacuum gauge to check it, but I can borrow one.
Does anyone know how much vacuum there should be to activate the modulator? I'll check on the pin. I'm sure I put it in, and I'll check to see if it is stuck. Thanks for the replies, keep 'em coming, I'll keep fiddling with it.

 

Since nobodys asked this yet, what did this rebuild consist of? Just new clutches, or did you get a new pump and valve body? Did either get disassembled and cleaned? My guess is there is crap stuck in the valve body.

 

The kit included clutchs, o-rings, seals, gaskets I think that was it. I had the pump apart checked and cleaned and I had the valve body apart checked and cleaned. Everything was cleaned with a lint free towel and then blown clean with air and ATF added to all the parts.

 

Did you replace any bushings? sealing rings? how's the center support fit in the case splines? are the drums worn where the sealing rings ride? how's the end play? you used the correct year valve body gaskets? checked clutch pack clearance? Did you replace the governor gear and make sure it's valve is free? governor tubes are seated properly? Kickdown solenoid gasket is the correct one (only use the metal gasket if you have the metal can solenoid)? kickdown solenoid orifice is in place? how are the pump gears and crescent?

 

Holy crap .... sounds to me like squirrel is the man you want to talk to. I don't even know half the stuff he just mentioned.

 

There are other guys on here who know more about the 400 trans than I do. These are just general things that can getcha. I've learned a few things keeping the 400 in my 55 alive....blown big blocks are fun. Especially trying to make the trans act mellow most of the time, and hold up to the occasional full throttle blast. With the added fun of a funky vacuum signal.

 

Just to clarify, is this true: The trans that came with the car wouldn't upshift and reverse worked?
The SECOND transmisson did the same thing?
AFTER rebuilding, it STILL did the same thing?

 

No didn't replace any bushings the fit seemed fine. I replaced the sealing rings. The center suport seemed fine. The end play is good I bought a new selective washer and a new shim for the pump. I bought a dial indicator just so I could get those right.
I also bought a set of good calipers to check sizing on other parts. Check and double check on the valve body gasket, and I changed the gear on the governor had it all apart and cleaned. I will double check the governor tubes but they seemed to seat in fine. Yup used the metal gasket on the kick down solenoid. I tried to cover all the bases.....

 

Been there and done that. It was stupid, the th400 would rev to the moon before shifting and then fall out of gear at idle

Any chance the line is ubstructed and its not pulling vacuum from the motor? Something in the line? I know it sounds dumb..

 

The trans that was in the truck when we got it didn't have any forward gears but it had reverse. That was taken out. We put in a used trans but this one only had first and reverse. Then I took the trans that was in the truck origanally and rebuilt IT, now this one has first and reverse.

 

At this point nothing sounds dumb, LOL

 

Ok. I know it's tough to use cold, hard Logic when it's your own car (at least it is for me).
But to me, it seems highly unlikely that you'd get the same problem from two different transmissions. Of course it's possible.

It seems much more likely that the same, existing problem is causing the same symptoms in two different transmissions.

That being said, try to not question your rebuild as much as question the things which have NOT been changed.
The 1st thing I would do is buy a cheap vacuum guage, 8' of 5/32" vacuum hose and a 5/32" vacuum "T". Plumb the guage into your vacuum modulator line, run a line to the guage and set it on the seat beside you and drive it. You should be seeing 14" of vacuum at idle. it should drop to zero when you punch it, then slowly climb back up at partial throttle.

If that's ok, check that pin that plugs into the vacuum modulator.

 

espo35 I agree it seems highly impossible that I ended up with 2 transmissions that had the same internal problems, with one of them being rebuilt and one not. I'll try the vacuum tomorrow and check the pin.

 

Right. Doesn't seem logical or likely.

As far as the pin goes, something in my memory tells me there is more than one common length. Do you still have the second trans to check the part against?

 

Okay, my diesel Suburban ('85) came with a bad TH400 - no reverse and they told me no high either, apparently they use the same band. I would think if you have R you'd have high gear, although you might not be able to get going enough to get to it. In R it would only go very, very slowly on a flat surface and any obstruction or incline stopped it. I just went and got a used 700R4 to replace it.

Beyond that, once I saw the guts of a couple of automatic transmissions I wanted no part of trying to fix them.