I Just picked up a 1995 Z-28 with an automatic transmission. When I bought the car 3rd gear was slipping. I removed the transmission and had a highly recommended local guy build me a transmission. It has kevlar bands/clutches, beast sun shell, vette servo, etc. I picked up the transmission followed his break in procedure and on the initial test drive the 1-2 shift had a very noticeable shutter and in overdrive when you stepped on it the engine would rev up to 4000-4500rpm but the car wouldn't accelerate any faster. Once you let off the throttle it would go back into overdrive. After a few conversations, he said it was a massive fluid pressure loss and the trans would have to be pulled. I brought the transmission back to him and he found a cut seal causing a massive internal leak. I installed the transmission for a second time and the problems only got worse. The 1-2 shift shutter was alot more noticeable, along with when ever you would accelerate hard (or just enought to cause a downshift) the car would feel like it was wheel hopping extremely bad and the high revving problem with no increase in speed was still there. I just installed the third transmission and I am still having the same problems. At first the transmission had a decent 1-2 shift and a really good 2-3 shift. It didn't shift into overdrive till 55mph though? After about 20 miles of driving the 1-2 shift shutter came back, 2-3 shift was still good and going into overdrive was snapping my neck back. When I installed this transmission (in hopes the 3rd time was a charm) I also installed a new B&M 2,400 stall converter and a external to eliminate those from being potential problems of the prior transmissions. Having the same probolem on all three of these transmissions only leads me to two conclusions. One being the builder himself and a transmission of poor quality or the tune. The tune has increased line pressure along with higher shift points. The original transmission didn't go into over drive until 55 though (don't know what would cause that?) Would the increased line pressure cause the transmission to slip? To me it would just cause it to hit harder on the shifts.

Any input would be greatly appreciated.
Jason

 

High pressures won't cause it to slip. Tune can be an issue, as the 4L60E shift points are completly electronically controlled. The guy I bougth my car from had a very agressive tune, and if tachs out way high before it shifts even at low throttle, the minimum speed of the gear is set so high, it can't be diven normally.

In my experience, the 4L60E (an 4L80E) are probally the best auto trannies out there, shift up and down great, respond very well, don't suck out as much power as other autos, but the 4L60E is by far not a good performance transmission by any means. Yeah, I've heard some success stories, but everyone I know has had lots of issues with them. They are great until they are raced. If you want to stick with an auto, a TH350 would be a wise choice, if you want to keep a OD tranny, get a 700R4 with a performance valve body and the right stuff. I will say you are having lots more issues than most. Maby a better builder.

 

you need to find a new tranny guy,, i have built many of these for my own vehicles and for vehicles at work (gm/cadillac tech), i dont know how so many people screw them up so bad

 

Please post in the correct forum. LT1/LT4 Engine Tech is only for "engine" related problems. Moving to "Drivetrain".......

 

All these issues sound like internal transmission problems. S "shudder" is when a clutch/band is trying to hold, but slipping.

First off...You say this is a reputable builder? If so, he wouldn't have used a Kevlar band. We all learned many years ago that Kevlar is too hard for a 2-4 band.

You may be looking at the band being burnt, the apply pin for the 2-4 servo being too short, or an internal pressure leak. I wonder if the reverse housing was under cut. This is what the 2-4 band wraps around. If it's cut, you have to mess with apply pin length. We never use cut housings, because there's no GOOD way to lengthen the pin. We always use new GM housings, if the original is too wasted to use.

A common reason for repeated band and 3-4 failure is a leaking servo release check ball/capsule leaking.

Is your transmission built to hold pressure at high RPM's? Most builders don't even address this problem. We've spent many years perfecting the 700/4L60E for racing applications. We know things about these transmissions that most are still in the dark about...Including MANY supposed performance builders.

Tune? We let the customers play with their shift points, but we request that pressure be set to stock. We make the neccessary modifications IN the transmission to help the shift quality/transmission life. Yes, higher pressure will make the transmission shift later, but it will not cause a slip.

It sounds like your builder is missing quite a few things.

Frank
CPT