LSx '85 C10

Hello all, I had my trans rebuilt 8700 miles ago(1 year of driving) and last week I lost both 3rd and 4th gear, when the trans was rebuilt I had the upgraded sonnax shell installed, what would cause the trans to lose 3rd and 4th so soon after a rebuild? I'm not pushing a lot of power through the trans although I have beat on it a few times, I noticed before the gears went out that at WOT it wouldn't want to shift from 3rd to 4th very quickly or almost at all, I had a similar problem 4 or 5 months ago where 2nd and 3rd went out for 2 miles or so then it fixed itself and everything was back to normal. any ideas?

mrvedit

When the 3/4 clutch goes out so quickly it is often due to a leak in the 3/4 apply circuit. A bad 3rd accumulator check ball is one leakage area, but there are quite a few more. I have documented them all here, but don't know which thread right now.

LSx '85 C10

my trans builder thinks it abuse because I have a "high hp" motor he thinks that 325 to the wheels is 550 at the crank. he should see that its a build problem shouldn't he?

mrvedit

Don't send me jokes while I am drinking hot coffee.
Seriously, you need a better builder.
I just read a recent post here from someone running a twin-turbo C5 with 800 rwhp for several years through a 4L65E from RPM! Both RPM and FLT have a reputation for building exceptional 4L65E like that.

LSx '85 C10

I just figure if it goes again I'll do a 4l80e or a t56 I don't care which, I plan on doing some road course events in the future what would you go with? P.S. I'll have a sleeved 6.0 427 when i do roadcourse

mrvedit

For road course racing you will want a T56; a manual trans is much more precise, predictable and enjoyable.
To road course race an automatic you would not want anything more than stock stall speed; a higher stall converter will slip too much and build up too much heat. And a stock stall is no fun on the street and drag strip.