Monzsta

Oh yeah.

When I had bought this car it had a broken engine piston in #7. The fellow had said something about a "shift kit". I commenced to tearing the engine and tranny out the bottom and began scrounging from my co-worker's huge pile of LS1 parts. The transmission looked new(ish) so I didn't bother giving it a going through while I was putting an engine together.

Fast forward about a year. I finally get the car running and the first drive the transmission was slamming into second, nice and firm into 3rd and 4th. Part throttle 1-2 was a tire chirping torque arm slamming mess. Almost like you had hit a manhole cover with the seat pan. WOT 1-2 would send you into a spin about 50% of the time. Full hydraulic shifts. Whee.

I told myself one day I'd get in there and see what the hell was going on. That day came a few months ago.

Rolling down the highway about 70ish with the cruise control on all locked up I needed to move for merging traffic. So I begin feeding in a little gas and was rewarded with disproportionate climbing rpms. ****. SLIPPING! I pull it into 3rd and it goes in firmly. I accelerate away and begin feeling out the transmission. It now begins to **** with me. It cleverly decides to not slip in 4th or 3rd, and is now acting normal. I beat the **** out of it. Works great. So I settle back down with the cruise on again and after about 5 minutes I catch the tack slowly climbing again on a slight hill. WTF???

I pull the transmission and found the input drum was cracked all to hell. (Probably from all the bang shifts.) I also find the 4th accumulator is blocked, and two pistons and a few washers in the bore. No spring. In the second accumulator I find two more pistons and washers in there. No spring there either. So I go shopping for the good stuff. I throw in the following parts for a nice sturdy transmission.

* Good used input drum
* .500 Sonnax boost valve.
* Overhaul kit with a Z-Pack 3-4 clutch kit
* All new bushings
* Sonnax seperator plate (I didn't touch it.)
* Sonnax TCC replacement valve (bore the valve body and installed new sleeve/valve)
* Beast sunshell
* Molded steel piston kit
* Sonnax input shell reinforcement sleeve kit (with the aluminum overrun piston and spring)
* Trailblazer converter.

Everything else looked cherry so I put it together with an old spring I had from another transmission replacing one of the pistons in the 2nd accumulator.

The trans worked wonderful, and almost made me forget everything else wrong with the car until the other day. (Had been together about 2 months) Leaving work, the car began shuddering when coasting, and almost made me think I had broken a diff. Then it had a bind and a god awful gear stripping noise in 4th. I pull it into 3rd and the trans acts normal 1,2,3, and reverse all there.

So I limp it on home and pull it out again, expecting to find stripped gears, needle bearings everywhere, sprag blown up, SOMETHING to account for the god awful noises it was making. Nothing but a smoked overrun clutch pack and the legs spread on the forward clutch piston. The reaction plate for the overrun/forward clutch had spread the stands on the piston and had pancaked down on the overrun clutch pack, pinning it solidly when they should be free spinning in 4th.

I've never seen anything like this. Sonnax makes a billet piston for about a $100, but I feel like there's something else going on here.

A few points to ponder. I have not put a gauge on it yet. There is a mystery tune in the pcm. I don't have the equipment to read what's in there. And I'm not ruling out someone screwing with the EPC solenoid, either. I probably have stupid high line pressures from the new boost valve working in conjunction with the unknowns elsewhere in the car.

I'm going to repair what'd damaged and install a vacuum mod kit this time. That seems the best way to null out both a bad tune and a tampered EPC.

I just wanted everyone's thoughts on this. I know it's a bit of a read. Thanks.

Gilbert@Ace Racing

I've built hundreds of these and have seen my share of fractured aluminum pistons. But I have never seen one spread the legs like you are describing with either steel or aluminum pistons. The only thing I can think of is maybe you had clocked the piston incorrectly when it went in? I dunno....

g

Monzsta

Got some pics up. It was installed correctly. All my witness marks prove this out, too. I wonder why the plate doesn't have hooks on the edges like the 3/4 reaction plate does. There's plenty of room between it and the 3/4 piston shell.

Vince B

iTrader: (1)

I've personally probably built more of these units than most who visit this site and have never seen nor had this happen to me. I would question two things here. Is that an aftermarket piston or were there neutral drops involved?

Monzsta

No neutral drops. I had an instance of wheel hop I quickly got out of. The piston kit was aftermarket from Whatever It Takes. The forward accumulator is stock/unmodified.

Vince B

iTrader: (1)

Do me a favor and look at the bottom side of the piston. Look for the name national molded into the bonded rubber seal. If you could take a picture of this for me. Thank you. Vince

Monzsta

I went out and snapped a few pics. I also dug out my take outs from the cracked drum episode.

This first picture shows the back side of the bent forward piston. It has no identification marks, letters, or numbers on it anywhere.



I grabbed my old piston I replaced when I built it last. It has a "National" stamp in the rubber.



The unmarked forward piston was packaged with the overrun piston which I didn't use and the 3-4 piston, which I did use. Both of these parts bear the "National" stamp on one and a discreet "NS" for National Seal on the other.



Here's the bent piston at the top of the picture with the good used "National" one on the bottom of both pics.

Monzsta

Id' thought i'd update my thread. It's been almost 5 months and 5,000 miles of hard driving, with no reoccurance. I put the old coast clutch in with the old forward piston and replaced the EPC solenoid. I also softened up the second gear a bit more (It was a touch too hard for my taste.) by removing a washer I had left in to limit the travel of the accumulator.

I did not install a vacuum mod kit. The transmission always went into gear hard before, and now it goes in to gear nice and soft.

The transmission operates much smoother with the new EPC solenoid. Looking at them side by side I couldn't tell any difference in the height of the screw, but the old one clearly had a problem. My guess is that the higher pressure was bottoming out the forward accumulator and slamming the clutch pack every time "D" was selected, sort of like a neutral drop, causing the legs to spread.

Transmission fixed.

Vince B

iTrader: (1)

Thanks for the update. What you have said makes sense as to why you had the issue you did. I have personally built to many of these units to count and have never had this issue. I think you nailed it!

joecar

+1 thanks for the update.

JUNK

The pressure could have been max in the tune, then the boost valve, and then someone could have cranked the **** out of the epc.....would put u north of 300psi for sure. It's the kind of thing that will break snap-ring lugs out of a drum, but like Vince I have never seen that. Broken alum. apply pistons, but never seen a bent steel one like that. U can bet your *** I'll be looking now