tH400 neutrals at full throttle??


Ain’t no feeling like Oldsmobling!
Feb 10, 2008
I went to Dick Millers Olds/Buick/Cadillac race in Norwalk zohio this morning. First pass was ok, next pass I staged the car, lightly torqued up the engine to about 1300( (just enough to take all the slack out of the driveline, just like I always do on the track) last yellow I swap feet. The car launches normally for a split second, front end comes back down and the engine screams. I thought for sure I broke a driveshaft or U-Joint. The is no carnage, I start the car and it goes right back into gear. Hmm?! I thought maybe I bumped the shifter into nuetral?? Next pass, same thing!! I looked closely at the driveline, I drive the car hard on the street, everything looked at felt normal. I attempted on more pass with the same results. At that point I decided to put it back on the trailer. This is a 400 trans with a modified factory valve body, dual feed, deep pan, fluid level is correct and clean, I’m scratching my head. Is there any possibility the low roller clutch could be slipping under full throttle during high traction conditions? Other ideas or things I should look for during the autopsy??
are you running the column lockout rod setup , linkage could be moving the trans selector lever with body twist
No, factory floor shift with the park rod lockout rod left off. Wouldn’t fit around the headers.
Is this a full manual valve body ? Reverse pattern ? Is this happening in manual 1st gear ?
There are some possibilities that come to mind. If its a fixed line pressure valve body and the pump has a heavy spring it is possible the PR valve is pegged and it will block converter feed at that point in the PR valve position. This is just a theory based on the description of your problem. Just to avoid situations like this I normally run the mandatory 7/64 converter feed restriction but also drill a very small hole in the PR circuit to short circuit the converter supply oil with the pressurized oil to guarantee converter oil supply regardless of PR valve scheduling. Some will disagree with this mod but with the converter feed restriction further downstream anyway its a non-issue and will not hurt anything.
Another possibility is if the trans was built with iron or worse yet, chrome sealing rings, it is possible that they wore out the pump sealing ring groove for the forward clutch circuit. I have seen this happen really bad on fixed line pressures at and above 200 psi. This will cause a bad leak in all forward ranges.
Reference picture for the modification I mentioned before. Again, can't stress enough the importance of running the 7/64 converter feed restriction if you opt to perform this modification.
This is a auto shift valve body, blue regulator spring with 2 horseshoe shim (about 165ish line pressure if I remember correctly) I built the Trans about 8 years ago with Teflon rings, internal dual feed, I have modified the governor to auto shift at 5500 (right after the 5400 shift light comes on) I normally leave the trans in drive at the track, I have tried leaving in manual first with the same issue. Keep the ideas coming, I’m scratching my head with this. I’m seriously thinking of pulling another core trans out of my stash and building another. It’s summertime, I prefer to use the winter for maintenance and upgrades, but I want to enjoy the car while the weather agrees
Maybe the forward clutch is just wore out and cannot handle boost?
Put a pressure gauge on it and be sure it still has good pressure
Ok, finally got around to pulling the trans and tearing into it. All the frictions look like new, the pump is perfect, the only issue I found that I don’t like is the rollers on the low/reverse roller clutch fall out of the retainer, zero spring tension. I also used a early center support without the thin snap ring to protect the case, I can’t believe I did that!!! Other than these 2 concerns, the trans looks perfect. I did try a different valve body, I didn’t think it would matter, and no change. I’m scratching my head on this.
I would assume 165 should be enough? Factory blue pressure regulator spring with 2 horseshoe clips.
I have installed a new roller clutch, and a later model center support that uses the thin snap ring to protect the case. I’m putting it back together as is, other than the aforementioned parts everything looks new. I’m going to roll the dice and hope for the best.
I have installed a new roller clutch, and a later model center support that uses the thin snap ring to protect the case. I’m putting it back together as is, other than the aforementioned parts everything looks new. I’m going to roll the dice and hope for the best.

So you haven’t tested it since the new roller clutch was installed?
I’m a little intrigued over the condition of the original roller clutch. What do the bushings look like in the back on the low/reverse drum?

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I am with turbo bit on this one I think you answered your question the first post the only things holding in first on a turbo 400 is forward clutches low roller clutch and low band and that would only be in manual low after installing the new roller clutch I would put it back in and try it if it was a pressure issue you would have seen the low band or forward clutches starting to blacken from slipping. The low roller is starting to slip cuz it can't handle the torque at that level especially if the rollers are just falling out with no tension on them. I would put it back in the car and try it
I would have to believe that the low/roller clutch was the issue however not normally a problem area on a TH400 and is why I was asking about the conditions of the bushings. Also possibly was overheated really badly at some point in its life to kill the accordion springs.
The bushings/beatings/thrust washer wear appears normal. Unfortunately, I can’t duplicate the issue on the street. It will be early next spring before I know if the clutch was the issue. Thanks.