I've spent a few days searching for this issue with no real solutions for me.

Truck is 01 2500 Suburban 4x4. 117k miles.

Tranny is all stock with recent fluid flush. Up until last week the truck had the stock LQ4 and ran great. NEVER any hard shifting or other issues with the tranny.

Last week I swapped in a built LM7-based 383 stroker engine. During the swap, I removed the engine wiring harness which had some connectors on the drivers side of the transmission. I also accidentally removed the PLUG that goes into the rear speed sensor housing on the tranny (for 4x4 models). When the PLUG came out, a small amount of tranny fluid came with it (less than 1/4 quart). I put the plug back in, and also re-connected all the wiring harness plugs after the swap was done. I checked and re-checked to make sure no wires were damaged or left unconnected.

The only change to the truck (besides the engine) was the exhaust routing. The LT header install required me to remove the heat shield on the passenger side of the tranny, and also the drivers side crossover pipe comes closer to the tranny now, possibly resulting in more heat transfer to the tranny.

Ever since the engine swap, the tranny shifts hard from 1-2 and 2-3 when the engine is warmed up. It's more pronounced after stop-n-go driging. The factory tranny temp gauge does not show hot, it never goes over about 195-deg (same as before engine swap). My engine tuner (Patrick G) said he slightly adjusted the shift pressures, but not nearly enough to cause the hard shifting we were witnessing on initial startup of the new engine.

During the first drive, we did 3 or 4 WOT pulls to 6k rpms, and it started hard shifting when we got off the freeway after that. I didn't think that would be a problem for the tranny, the engine is probably only making about 400-450 hp in its current configuration.

Any help is greatly appreciated, I'm stumped on this one!

 

I checked the electrical connections on the transmission to make sure they weren't damaged during the engine swap, and they were OK.

If the tranny doesn't get above 175-deg, it drives normal, no hard shifting. It still does not go over 200, but when it gets to about 190 the hard shifting starts. 1-2, 2-3, and even 3-4 sometimes is hard.

My best guess right now is that the extra heat from the exhaust pipe next to the tranny pan is causing this. My first plan is to wrap that section of pipe in header wrap and add a larger tranny cooler. We'll see how that goes...

 

I am guessing from the fact that its a 2500 and that you said it had a stock LQ4 that the transmission is a 4L80E.

I wonder if the Manifold Pressure Switch is going bad in the transmission. I am not overly familiar with this but a friend had a similar problem and that was the fix for him. I am not entirely sure what you would look for to Diagnose that switch as going bad.

 

That and the title of the thread says 4L80E hard shifting... lol

OP, is it tuned?

 

The temp sensor voltage can be tested by back probing the L terminal at the transmission connector and using a known good ground. As the temp increases the the voltage decreases.

I am not sure if you can see this value on a scan tool.

At 176 degrees voltage should be 2.24 Volts

At 239 degrees voltage should be 0.96 Volts

The higher the temp goes the lower the voltage goes.

 

By Pat G if you read the first post...

 

Any codes? I'm pretty sure a bad PCS wont turn a light on but will have a code

 

at myself... haha.

It almost sounds like it is hitting the Hot Threshold in the tune and jacking up the line pressure. Why it didn't do it before and does now is the question, as 1Fastbrick mentioned checking the Temp voltage to see if it is reading right would be a good start.

 

thanks for the replies. before I read any of them I went and bought a 50'x2" roll of header wrap. $50 at Oreilly auto parts.

And it seems to have WORKED!

after installation, I let the truck warm up for 20 minutes, then took it on a BRUTAL 40-min test drive in stop and go traffic. The shifting was flawless and smooth.

I used about 35' of the wrap and SS ties at the ends:



I may need to seal the wrap with a silicone spray, we'll see. I'll post an update in a few days, hopefully the problem is fixed.

 

Great!!! Thanks for the Update. Keep us posted on how well it works.