Stephen Carrigan

Hi all,
I have an 08 Silverado ltz with the 6.0 and the 4l70 tranny. I'm having an issue with hard shifting first to second. It started off really intermittent, like one trip every 6 months, but now is pretty constant. Every once in a while it shifts fine, but the rest of the time is a hard shift. I've replaced the 1-2 accumulator and the TPS. I'm out of ideas. Help!

oilcanhenry

Drop some Dexron VI in it nd see if that helps out.

Stephen Carrigan

I was short a quart, so I added one after work. Should it help instantly? It didnt seem to change anything on the way home.

oilcanhenry

One quart won't do much. Depending on how many miles are on your transmission, Seafoam and Lucas are said to make some real good additives for hard shifts. I don't know from experience, just what people say. Lucas makes a milder additive for lower mileage transmissions that are shifting hard and another for high mile tyrannys that are slipping and really needs a rebuild so I would not use the second one if you go that route. In order for Dexron VI to have any effect, you'll likely need to drop the fluid and refill.

Kyle Money

My 08 shifts hard on the first shift of the day. Then it's fine. I was told not to change my transmission fluid. That the new fluid would break down my seals and ruin my tranny. Idk just what they told me at the lube center

KChevy75

Hard to say without pulling the pan and valve body. The TCC valve bore can get worn due to the constant pulsing of the TCC PWM solenoid. Sonnax makes a kit and reamer that fixes the problem. Even though it's the TCC valve they say it also caused a harsh 1-2 shift somehow.

How many miles and has it ever been fully flushed? Mine was having harsh 1-2 shifts occasionally on fluid that had 90,000 miles on it. Did a full flush and the 1-2 hard shift was gone.

I had shifting issues on mine even after I rebuilt it. I reused the original pressure switch manifold and turns out that was the problem. Replaced it and it shifted perfectly. It's bolted to the valve body and is easily accessible after dropping the pan.

Irish_alley

this is BS to a point. yes changing the fluids can make them crap out but only because it was on its way out anyway. when i buy a vehicle with miles over 100k i change the fluid regardless of what may have been done because its a cheap insurance and you dont really know the history of the fluid. heat kills the fluid and dont never flush the system. even GM put a a SB about the only time to use the flush machine is when youve replaced the trans and need to flush the cooler/lines