Morning, gents. I'm starting to have some issues with my 8L90e transmission and I'm looking for some advice. It's starting to shift hard between first and 2nd gears (up and down) and I'd like to solve the problem. It seems like there are a few modifications that can solve the issue, mainly in the valve body (zip kit, aftermarket valves, thermostat mod, etc). Unfortunately, calling a bunch of shops around me, while they will replace the transmission, no one wants to work on them because they don't want to be held liable for any broken parts they might find that they can't get replacements for.

While I can understand their position, this makes it rather difficult to solve any problems I may be having. Given that I don't have a space available where I could safely do the work myself, it's seeming like my only real option would be to order a rebuilt/modified/aftermarket replacement transmission and get it swapped. If I'm going to spend that much money, I'd like to buy once, cry once. What would your recommendations be for a replacement, preferably of the variety that I wouldn't run into this refusal to work on again?

 

Given the harsh 1–2 / 2–1 shifts you’re seeing on the 8L90E and the fact that most shops won’t touch valve body work due to liability, the smartest move is not piecemeal mods but first confirming exactly which transmission variant you have, since build date, revisions, and known problem ranges make a big difference running your best free vin check from https://carfast.express/en/cars/check helps clarify that quickly; if it turns out to be an early or problematic version, the true “buy once, cry once” option is either a GM factory reman with the latest updates or a reputable aftermarket rebuilt unit from a company that openly supports and warranties the 8L90E, as those typically include revised valve body components, updated internals, and thermal fixes, and are far less likely to put you back in a position where shops refuse future service.

 

I had the same problem with that transmission. Hard shifts from first to second can come from the valve body or fluid.

If shops will not open it, a full replacement is the simple path.

You can look at a rebuilt unit from a known builder. Many sell units with updates to fix common faults. These units also come with a warranty.

Ask for:

updated valve body parts
new clutches and seals
tested solenoids
warranty

You can also check if a dealer can do a full unit swap.

Before you replace it, try:

fluid and filter service
check for software update

If the problem stays, a rebuilt unit is the next step.