TH400 problem.. could use some advice

[6872xtc]

Thanks in advance for helping. We put a used 400/203 combo in my dad's '72 back in 1992. It has worked very well until the past few years. It got retired to a hunting truck about 6yrs. ago so it seems to only get used for 2 weeks every november. The problem is that first thing in the A.M. the trans seems to fails to upshift to second after releasing first. He found out that if he shifts it manually back down to first, it will then upshift to second. After that all is well. He doesn't want to pull the trans for a rebuild because he's 73 and doesn't think the truck is worth the money. I am having a hard time with the thought of helping him install a used TH350 from a friend of his. At the same time, I am not confident in my own skills to fix the problem even if we did a complete build. Does anyone have some insight for this problem? My thoughts are weak seal ring on 2nd that are helped by manual shifting? He has checked the S-link at the valvebody and thinks it is okay.

 

[76zimmer]

I have come up with a couple things after checking my 85 toubleshooting manual, could be one of these or something else I'm not aware of, but...could be a govenor control valve...under a side cover towards the rear of the trans. remove cover and make sure the valve moves smoothly in its bore, and that there are not broken parts in the gov assy.
The vacuum modulator might be a culprit too, check the vacuum line to the engine, make sure there is no fluid in it. If there is the diaphragm is shot in the modulator.
Remove the modulator and there will be a valve in it, make sure it moves freely in its bore. This valve should only be installed one way, so make sure you know which end is which before you remove it from its bore, be careful, sometimes it will stay in the body and not come out with the modulator.
Anything more than this, you might have to do some line pressure checks to determine what the problem is internally.

Also a TH350 isn't a direct swap...it will probably have a 27 spline output, and a different t/c adapter than the 400 has.

 

[Greg Ducato]

Sounds like a sticking shift valve or governor, may free itself up with continued use

 

[Chaddy]

I would rebuild a 400 before even thinking about swapping it with a 350.
a 400 is a far better trans.

 

[6872xtc]

Thanks guys! He already checked for fluid in the modulator, but I couldn't remember anything about the valve under it. I never gave the governor a thought!!!?? I've had one hang up on a 700R4 before,, DOH! He has a complete TH350/203 combo since I had told him about the differences. Granted this truck hasn't towed more than a 4x8 trailer in 20+ years, but I don't think he should step down either.

 

[LgDude77]

How can you tell if you have a TH350 or a 400?

 

[GMCSuburban2500]

Easiest way to tell AFAIK is by looking at the oil pan... TH400 has an oil pan that's described as looking somewhat like the state of texas... TH350 is square save for one rear corner being cut off.

I'd post a pic but I don't have a high enough post count yet... But if you google gm transmission pan identification you'll see.

 

[LgDude77]

Thanks. I cant post pics either.

 

[LgDude77]

As far as these different Tranny's go, is one better than the other or is it a question of application? Should I be crossing my fingers for one model more than another? (the trucks in the shop right now otherwise i'd already know which one I have)

 

[76zimmer]

the 400 is the heavier duty model of GM auto's.

The 400 has 6 bolts on the tailshaft/t-c adapter, the 350 has 4

Hope for a 400 of the two, pretty stout in stock form.