TH400 stutters under acceleration? HELP

[rockspider]

Hallo,
I've a CUCV M1028 with 6.2, TH400, NP205.
When starting from a stop the trasmission stutters like crazy for the first few meters on flat surface, the heavier the throttle or the load, the more it stutters. As it gains some speed things suddenly get normal, and it upshift and run perfectly.
If starting on an incline it stutters 'till it can gain enough speed, but if speed cannot be achieved it stutters for the whole climb.
It's as bad as it sounds.
Engine is ok.
The rig is lifted 6" but beaing a lwb don't think a driveshaft vibration or worn/loose U-joint should give this kind of behaviour, or at least not only when starting.
So back to the TH400: could it be a faulty converter (faulty one-way stator bearing inside)?
Or a first gear clutch discs? But then it should not stutters if I put it in 1st instead of D, as the tranny uses now also 1st gear bands...
Could it be a pressure related thing, some leaky passage gasket or piston inside the valvebody? I know the tranny has a Transgo kit stage 1 installed.
Help!

 

[nvrenuf]

Please better define "stutter"?

Slip?
Noise?
Requires throttle to build pressure before tranny applies?

 

[k20]

Tranny fluid correct? Smell burned? Dark color? Slivers of metal floating in the fluid? When did it start doing this, did you change anything before it started?

 

[JanneB]

Flexplate is bending and hitting the starter?

 

[unclematty]

I remember when i did a stint in a local tranny shop, there was a while there that T400s kept going into 1st and reverse at the same time causing something similar to what I'd call a stutter, unfortunatly I can't remember what caused it!

 

[rockspider]

still stuttering...

Fluid level is ok, smell and colour are normal, no water nor metal shreds into it.
Defining stutters is like there is a clutch that applies and releases at a very fast rate, giving a sensation similar to a very bad vibration, but really you feel that the power of the engine is not flowing to the wheels continuously, but in short burst. I'm not so good with english language, it's hard to define it better.
I don't know if a not enough pressure condition (due to an internal leak) gives a constant slippage or this kind of fast shock loads.
What I know is that it gets worse with heavy throttle.
It doesn't change if I start in D, 2 or 1. This leads me to think that it's not the main 1st gear clutch, as with lever in 1st the band should apply too.
It happens however only in 1st gear, as soon as 2nd kicks in everything goes smooth.
How I can tell a bad converter from a good one? If internal oneway bearing went bad can it cause this?

The TH400 was under another M1008 of my friend, and worked ok on it. The Transgo shift kit was installed at same time as tranny swap, so can't say where the fault is. Apart from this "stuttering" when starting, the tranny shifts well, slightly harder and faster than stock, pretty liveable.
I'm gonna get the tranny out again in the next week (freetime from work permitting) but at the moment I've no idea of what do I have to look for when I open it!
Also if it's a leak from a gasket or piston in the valve body, how to trace it?

 

[rockspider]

Moreover,
trying to answer to your questions:
Slips? Maybe. But not continuously.
Noise? Yes, but not coming directly from the tranny. To me the fast "Thum-thum-thum" is noise amplified from the driveshaft and from the frame, as a result of the whole engine-tranny-transfer oscillating on their supports (supports are new, by the way)
The tone is deep like hammer on the frame, and not as fast nor metallic as a rotating flexplate touching the starter would produce.
The throttle amount required to build initial pressure and transmitting some power feels normal enough for the stock converter. I'd say, based on previous experience with manual tranny, that the engine torque would be used better with a lower stall speed converter. Next buy if I solve this annoying bug now...

 

[diesel4me]

long shot...

I read in an old chilton's nanual that some 73 Gm trucks with a TH400 had a "shudder" under acceleration in the lower gears,but it was in the trucks with a 2 peice rear driveshaft--they said making a spacer out of 1/2 inch thick steel placed between the center supprt bearing and its mount would cure it---I know this does not apply to your truck,but I think that could mean it might be driveline angles that might be part of the problem..

Another long shot is the tranny case has internal cracks between the passages that look like a maize under the valve body,and is letting pressure get to some other clutches that should not be getting any in 1st gear... or maybe a valve body gasket has a tear in it allowing the same thing...could be the valve body itself too...This is why I dont like automatics very much---I'm not able to rebuild one,and several things can cause the same problem..--and unfortunately,I havent had a standard tranny in a GM truck since 1987 or so...so I'm in the same boat!..

 

[rockspider]

Maybe what you said about the two piece driveshaft applies to my truck more than expected, infact I have a two piece driveshaft, it's a swap from a heavier commercial truck, done mainly because it has thicker tubes and 1410 U-Joints instead of the stock 1350.
And the center support bearing has a rubber mount to the frame.
Never thought that could be cause of vibrations.
Maybe I should make some experiments with the height of that mount, what do you think?

I agree that if the problem is in the maze of passages under the valvebody, then the only way is a professional rebuild, and that will hurts my wallet!

 

[diesel4me]

worth a shot...

I had no clue you had a 2 peice rear shaft!--the chiltons book said to make a spacer out of 1/2 inch thich steel flat stock and put it between the center support bearing and the crossmember--this lowers the driveshaft at its center point some and changes the angle..of course,this was meant to apply to a stock truck,not one with 6 inches of lift!(I'm guessing the lift would require even more "drop" than a half inch to have the same effect?)...probably would not hurt to try it if you have the time to experiment--be better than blaming the tranny and find out the shudder is still there after you change it!

Those center support bearings seem to be sensitive to drivline angles and the sqiushy rubber mount around the bearing lets the shaft move around a lot,even when they are in good shape...I sold a lot of them at the parts store--I prefer a one peice shaft,less chance of failure on the road...I dont see how so many delivery trucks ("straight" jobs") and school busses that use several of them last any time at all!..looks like it would not handle the torque and strain,but somehow they do!....

For what its worth,my 82 K20 with a 6.2 and a TH400 feels a bit "shakey" in first gear too---I dont think anything is wrong with it--I assumed it was due to the lower stall speed converter used with a diesel,and the firing impulses of the diesel engine itself...at least I HOPE it is...

 

[rockspider]

I see your point, definitely I have to try a different location for the center support, or also try a one piece driveshaft (even if that is going to cost me some bucks, as I don't have it!)

Having never thought that the center support could be a source of problems ("Hey"- I thought- "it has a rubber mount too, so it will NOT vibrate...") I also installed it so the first piece coming from the transfer is almost straight, and most of the angle drop happens at the u-joint just after the center mount... uh-oh... only now I realize how bad that setup is!!