1989 GM Light truck th400 won't shift after rebuild

Old soul Rich

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Make
GM
Model
LIGHT TRUCK
Year
1989
Miles
N/A
Engine
327 CI GEN 1 SBC
Hello all, I just rebuilt my first auto trans. TH400 out of a '89 light truck. I am 64/yrs old
I am not a driveway mechanic, have been in the auto industry for over 35 years and have built or restored plenty of cars but never an Auto trans.
I had no history on the trans, bought a kit that included new bands - fibre plates etc. New B&M Street converter and deep pan.
Installed behind a Gen 1 SBC 327 with aprox 380hp. Vacuum to modulator at 17#'s, governor was removed, new gear installed and spool valve moves with weights.
I have been unable to get it to shift out of first gear at all, in D or manually. WHERE would you go next??
I do not have the electric downshift solenoid connected as I was told it should not matter?????????
I am kinda anal about cleaning things, everything that came out of that trans got cleaned, polished wire wheeled etc.
ANY input greatly appreciated.
8-5-23 trans pan - dust shield-2.jpgIMG_3857.jpeg
 

billr

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Did you do air-checks on the clutches/bands before putting the VB on? How about line-pressure, has that been checked?

I assume the governor has been put back in, was just removed for inspection and the new gear; correct?

Was the VB opened up at all? Did this trans or VB work correctly before the overhaul?
 

Old soul Rich

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Did you do air-checks on the clutches/bands before putting the VB on? How about line-pressure, has that been checked?

I assume the governor has been put back in, was just removed for inspection and the new gear; correct?

Was the VB opened up at all? Did this trans or VB work correctly before the overhaul?
I did not, I did not know you could do that and waiting for a pressure gauge to arrive as I did not have one that went high enough.
Governor is back in with no change to condition and yes the VB was"carefully" disassembled along with photos of everything, every ball and spool valve. I did not test the detente valve though, thoughts on it being stuck in the wrong position?
Guy I got it from had no history on the trans.
appreciate your time rl
 

billr

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You don't need a gauge for the air-checks, but you will need a diagram of where to apply the air. I don't have that, but somebody may provide it tomorrow morning. Also, look and see what TH400 info there is at <charm.li> and on the Sonnax website.

Even being as careful as possible, the VB is complicated enough and has enough small pieces in it that you may have made a mistake. Besides, you don't know if the VB was working and even had all its bits when you got it. I think the VB (including detented manual spool) is the prime suspect. Be ready to post lots of pix if you want to keep trying to save it.

Anybody know if the TH400 case and innards are similar enough to a 4L60E that I can get some useful clues from my 4L60E manual?

A cheap gauge, maybe 0-300 psi, is as good as you need for checking line pressure. We just need to know the LP isn't grossly low. I'm thinking you can get a gauge for about $25 or less... Isn't that a pittance compared to the rest of this project???
 

Old soul Rich

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Gauge will be here tomorrow.
I am definitely gonna save it. It was spotless inside, pan was clean nothing scorched. Looked like low miles to me. I put it together step by step using a manual too.
I am sure it will be my error. At least it goes forward, backwards and has a good park.
Again. Thx for your time Bill
 

billr

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I understand about saving the trans, but replacing the VB might be a practical choice. VBs tend to be "wear" items that can be difficult to diagnose and may only be repairable with special (expensive) tools.

Besides the VB, I am wondering if you put in a sprag-clutch backwards. That would be easy to do and hard to spot visually
 

bp042665

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ok i have had the same issue a few times and it gets over looked but the down shift solenoid can leak past and dump the line pressure for a down shift so i would try to double check that it the button is still on it or maybe just replace it if it dont seal off right you will only have first gear and reverse also this is a steel vb witch didnt give much trouble as for wear and bill this is not like a 4L60 so i am leaning to a problem with that down shift solenoid i had 1 come to my shop that it wasn't even put back on by another shop and they couldn't figure it out so i put 1 on it and shifted fine
 
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billr

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I was unaware the TH400 ever had electric control. What year did that come in?
 

Old soul Rich

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ok i have had the same issue a few times and it gets over looked but the down shift solenoid can leak past and dump the line pressure for a down shift so i would try to double check that it the button is still on it or maybe just replace it if it dont seal off right you will only have first gear and reverse also this is a steel vb witch didnt give much trouble as for wear and bill this is not like a 4L60 so i am leaning to a problem with that down shift solenoid i had 1 come to my shop that it wasn't even put back on by another shop and they couldn't figure it out so i put 1 on it and shifted fine
Appreciate the info for sure. I will do pressure test tomorrow and pull and check over the modulator too. Then it’s a pan drop, and that solenoid is getting replaced. It’s not even hooked up yet.
 

Old soul Rich

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Ok, unfortunately I started my weekend putting down the wife’s horse Friday so did not get as much done as I wanted;
1. 12v to detent solenoid and heard the click.
2. Pulled the new modulator, removed the spool valve and gave it a polish. Modulator held vacuum
3. Pressure test, going to try to upload video.
Too large it says. I will review, edit and update
 

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billr

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I'm not sure what I am seeing in the video, but borrowed the specs below from another forum. How do they compare with what you are seeing for LP? I can not be sure the specs below are correct, but expect BP will chime in tomorrow morning if they are bogus!

Factory specifications for the TH400 line pressure are as follows:
In neutral, with brakes applied, at idle rpm: 55-70 psi
In Drive, at idle (idle "set to spec"): 60-85 psi
In Drive, brakes applied, at 1000 RPM: 60-90 psi
Intermediate drum applied or in low, brakes applied, engine at 1000 RPM 135-160 psi
In Reverse, brakes applied, at 1000 RPM: 95-150 psi
In Drive, brakes applied, at 1000 RPM, downshift switch activated: 90-110 psi
 

Old soul Rich

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I'm not sure what I am seeing in the video, but borrowed the specs below from another forum. How do they compare with what you are seeing for LP? I can not be sure the specs below are correct, but expect BP will chime in tomorrow morning if they are bogus!

Factory specifications for the TH400 line pressure are as follows:
In neutral, with brakes applied, at idle rpm: 55-70 psi
In Drive, at idle (idle "set to spec"): 60-85 psi
In Drive, brakes applied, at 1000 RPM: 60-90 psi
Intermediate drum applied or in low, brakes applied, engine at 1000 RPM 135-160 psi
In Reverse, brakes applied, at 1000 RPM: 95-150 psi
In Drive, brakes applied, at 1000 RPM, downshift switch activated: 90-110 psi
Great info, I will review and compare.
Thx Bill as always
 

Old soul Rich

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Great info, I will review and compare.
Thx Bill as always
So my numbers are right at these pressures as far as at idle. My high numbers also are the same except mine are running through the gears with the car up on stands not 1000rpm and brakes on
 
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