Hey all, I've got an extremely rough 1-2 shift on my 4L60E and it's making the truck nearly undriveable.
The truck is the GMT400 in my sig, it is a 1994 GMC Sierra K1500 with the original TBI 350 and a reman 4L60E.
The only code I have is for the EGR valve. The EGR valve hose is disconnected as the valve isn't holding vacuum and needs to be replaced. I've had it like that long before I've had these transmission issues. I know someone is going to ask, and yes I have replaced the TPS with an ACDelco. That was one of the first things I did lol.
So, what's it doing? When driving, the truck has a violent 1-2 shift. And by violent, I mean that it has shifted so hard that it has cracked my dash in several spots (not that that's exactly hard to do in these trucks lol). No other gear does it, 2-3 is smooth, 3-4 is very smooth, and all downshifts feel fine. It doesn't clunk going into drive or reverse, the transmission fluid level is right where it needs to be, and I measure it exactly how the owners manual tells me to. It does it regardless of temperature; if it's below freezing outside, or if I'm in 110 degree Oklahoma heat it still shifts rough. RPM's don't usually make a difference either, if I'm shifting at 2200 rpm going through a parking lot, shifting at 3000 rpm when driving normally, or 4700 rpm when going WOT, it still shifts uncomfortably rough. Sometimes if it's an especially rough shift at high RPM's, the engine kinda stutters and bogs down before it goes back to normal. My assumption is that it's doing this because the line pressure for the shift was too high and maybe the torque converter didn't have enough fluid. But that also could be because of my worn out injectors. Also, there is no delay in shifting, it doesn't seem like the trans is slipping at all, honestly it feels great other than that hard and immediate 1-2 shift.
Rarely, it will shift smoother than usual. I've noticed it most when driving around a corner at an intersection or something, sometimes it shifts when I'm partially though the corner and it almost feels normal. It's not smooth every time, but I'd say it's a 35/65 split between smooth and rough when going around corners. Also, after I dropped the pan and replaced the 1-2 accumulator piston and springs, it will occasionally shift much smoother if I keep the RPM's under 3k. Unfortunately it's still not consistent.
Alright, now here's what's been done to the truck. At 270k miles, the transmission cooler got clogged and the transmission cooked itself. I cleaned the lines, cleaned the transmission coolers, and made sure every last bit of junk was out of the system. I replaced the transmission with a remanufactured one, and from day one it's shifted hard 1-2. After 1k miles, I replaced the transmission fluid and filter with Dex 6 and a ACDelco filter, I did the same procedure again 5k miles later. Nothing I did changed how the truck shifted. At 282k miles, I took a lot of things apart to fix the many engine oil leaks, and while I was there I replaced all the wires going to the transmission connector. I had issues with the TCC wire not grounding correctly and the rest of the harness was in rough shape, so it got a new connector and new wires. That didn't change anything, so I decided to tackle the infamous 1-2 shift accumulator piston. I took the pan off, took the accumulator housing off, and the piston was still the pinned type, and it had a single spring that fit right on the inside edge of the 1-2 piston. Nothing seemed damaged, nothing was scorched or had any marks on it. I took that system apart, and replaced the piston with the Sonnax pin-less piston that replaced the pin with a small ball bearing, I also changed the single spring to the big outside and small inside spring type that a 94 should have stock (from my research). I put it back together and took it for a drive. The first 1-2 shift was very smooth, the second 1-2 shift was also smooth. After that, it started to get harsher and harsher, and after about 6 first to second shifts it was right back to where it was before. I've had to drive the truck about 500 miles since then, and I'd say the shifting has only gotten worse.
I may take the pan back off and swap the double springs for the single spring like how it was before. I've read that that's how the newer 4L60E's are set up, so maybe it would help. I'm not confident that will fix the issue though. There was nothing visibly wrong with the old piston.
Anyways, I'm no transmission expert and now I'm stumped and don't know what my next step should be. Any help diagnosing or any ideas on what I should tackle next would be very appreciated!
