1989 GM Light truck th400 won't shift after rebuild

billr

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I am hoping BP or GRC can lead you to easy diagnosis/fix with this. They are real pros, I am just a very advanced DIYer.

As I have already demonstrated, I am not familiar with the TH400, but am trying to follow along and learn. The trans was in production so long and so widely used that there seem to be many variants. Does yours have that "variable stator" thing?

I suggested before doing the air-checks, and still feel that may be necessary or beneficial to do. In fact, a sophisticated air-check setup should be able to activate the clutches such that you could rotate the input shaft (TC removed) and count tailshaft revolutions to verify the trans innards can mechanically provide all proper gears. That would pretty much eliminate all but the VB.

In doing searches for TH400 info, I find many articles at a site called "yellow bullet" (or similar). I hate to direct folks away from batauto, but you should peek at that "yellow bullet" site. I also see that the TH400 is old enough now that a reman VB is no longer a common item. Plenty of high-priced aftermarket stuff, but no $200-300 simple remans? Last comment: I see some indication that the TH400 and the more modern 3L80 are the same thing. Can anybody confirm? Maybe more chance of finding a VB for a 3L80???
 

bp042665

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i dont believe it to be vb related more towards govenor or kick down solenoid issue
 

Old soul Rich

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Well I have a list to go through over the holidays. Last one is dropping the trans. Governor has been gone through already, unless I did not seat it correctly so I am leaning towards the detent solenoid also.
I appreciate everyone’s replies here very much. I will be checking in with results Monday.
 

Old soul Rich

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I have a very good one plus my manuals from my tranny class in college. Haha. So damn old it has this trans in it. (1980)
 

billr

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So, do you have the air-check info?

Old??? the first auto trans I worked on was a "Hydra-matic", the design GM introduced in 1939. There is another member here who is in his 90's and still an active DIYer!
 

bp042665

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That’s Awsum. I am going to beat this thing I swear.
So, do you have the air-check info?

Old??? the first auto trans I worked on was a "Hydra-matic", the design GM introduced in 1939. There is another member here who is in his 90's and still an active DIYer!
heck i got that beat Bill im Working on a Dyna Flow right now thats the first 1 gm ever did
 

billr

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I gotta disagree:

The Buick Dynaflow didn't come out until '47 (for '48 MY); the Olds Hydra-Matic started in '39 ('40 MY).

That you are presently working on a "Dynaslow" is rather unusual, but it wasn't the first from GM (or anybody else, I think).
 

billr

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Yes, it is a 2-speed, with lots of monkey-motion in the convertor. It was intended for smoothness in the Caddie-wannabe Buicks. Performance was akin to the Nash Ramblers of that era. Flooring the throttle produced different sound, but little acceleration.

I think the Chevy Powerglides of that era were also 2-speeds.
 

bp042665

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Yes, it is a 2-speed, with lots of monkey-motion in the convertor. It was intended for smoothness in the Caddie-wannabe Buicks. Performance was akin to the Nash Ramblers of that era. Flooring the throttle produced different sound, but little acceleration.

I think the Chevy Powerglides of that era were also 2-speeds.
it is a forward and reverse with a manual low and yea you got me i for got the year i was thinking of the chrysler i did a month ago
 

billr

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I gotta ask... where do you get parts for a Dynaflow now??? Both the 'Glide and Hydra-Matic were so popular with racers that aftermarket parts will probably be available as long as necessary, but the Dynaflow was an orphan right from the beginning!
 
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