Need help with THM400

[Peter J. Bierman]

My th400 trans has severe slide shifts on 1-2.
Checked line pressures and are ok
replaced the vacuum modulator and valve a wile back.
Took out the governor but seems to be fine.
When playing with vacuum on both modulator and valve problem gats worse with more vacuum.
With no vacuum trans shift high RPM but seems to let go when shifting and then grabs second gear firm, does the same but less at 2-3.
ATF is fine, no burn smell or black collor.
I am cueless other then drop the valve body and look for??????????? :(

Peter

PS I am going to put in the 4L80E but would be nice to have a good 400 on the shelf :D

[86burb]

Check the ID plate on the side of the trans.,The year/model/serial# . The model for 1985 will be 'VR','VS','VV'. Some of these models have a harsh 1-2 and 2-3 shift problem sometimes requiring the replacement of the control valve and/or modulator assemblies. The 1986 'LRA','LSA' models were built with the new control valve (P/N:8655160) and modulator assembly (3030643). Hope this helps.

[Peter J. Bierman]

It's kinda oppossit from harsh, shifting is very soft when normal driving but let's go inbetween gears completly when putting on some real power.
I'll check on ID anyway.

Thanks, Peter

[Peter J. Bierman]

I know these are complicated things but I know that there are some fine technicians out here too!

Anybody? :(

Peter

[N Wilkens]

Having gone through three THM400 transmissions in 2.5 years, I can speak from experience that the slipping is probably a bad sign. I would recommend dropping the transmission pan and looking for metal flakes. It is likely that there is also a black scum at the bottom of the pan that represents the remnants of the transmission clutch plates. This goo is likely plugging everything up, giving you the hard or no shift condition. As for burnt fluid, I drove my military 6.2 Blazer from Atlanta to Phoenix, with the clutch plates nearly destroyed (and super hard, high RPM shifts) without a sign of burnt ATF fluid.
I should make the point that my first tranny died because of abuse (stump pulling) from the previous owner. The second two died because the transmission shop was inept. I still stand by the THM400 as the strongest automatic tranny on the market.

[Peter J. Bierman]

I had the pan off a few times lately but did not find anything suspicious in there.
That's the weird thin about it.
Beside that I can't tell when this started so it's hard to say if there is a connection between things like replacing the filter and this problem.
But in order to drop the valve body, I'll have the pan off any way and see what's inside.

Peter

[Peter J. Bierman]

Got the pan off so all ATF could drip out before I have to work on it.
In the pan was a little blackish stuff, I think its kinda normal concidering that it slips a lot between gears.
No metal flake's or chips or what so ever.
I guess it's in the valve body somewere, so I start looking for any thing that looks unusual like dammaged gasket or missing or misplaced checkball.
Any body has a clue?

Peter