Hi, I'm new here but did alot of searching and found similar issues but still need help.

I have a 92 suburban with the 4l80e trans. Not diesel engine. 5.7tbi. Sorry

My problem is my trans will go into Overdrive and right back out within seconds. If I slow down and gain speed again it will do the same thing so turning on and off the truck makes no difference.

I checked the fluid and it didn't seem bad but I changed fluid and filter anyway.

I am getting codes 68, 85, and 87. (I've found different info on what codes mean???)

My transmission doesn't seam to slip whatsoever.

Engine temp doesn't seem to effect problem.

When I go down a hill it's like the trans was put in neutral. RPM drop to idle. This is at highway speeds.

I've checked connectors on outside trans and in pan and none seemed loose or damaged.

Trans was rebuilt less than 30k ago with new TCC (I will find receipt with breakdown and post.)

Any help would be great. I can get a decent deal on a used trans but hate to replace if it's not an internal problem.

Thanks guys, from searching around this site seems to have some of the more knowledgeable people on it.

1. Fluid level okay while running, warm, level surface. yes
1a. Fluid color and smell. good
2. Speedometer reading correctly. seems so
3. Shifting normal/hard. little hard, but I found info in glove box for a shift kit?
4. Shifting into OD okay? No
5. Any service engine light: yes/no. No, but has codes 68, 85, 87
6. If SES light on, what codes do you have? 68, 85, 87 no light
7. Model year: 1992
8. Describe symptoms: Goes in overdrive and right out
9. Does the engine come up to full temperature? Yes
10. Any service work recently done that may contribute to your issues. No
11. Does the TCC lock up (or how many "shifts" do you hear?) lockup? shifts4 then back to 3rd
12. Are you experiencing any kind of slipping? No
13. What type of rear do you have? (Check RPO codes) 4.10/ 3.73/ 3.42 or 3.08. Not sure
14. If you have an accurate tachometer, what is the RPM at 55 or 65? Factory Tach 2600/2700 @ 55mph I don't go to 65mph due to high rev.

Follow my previous posts and you will see me tracking this very same issue with more than a few with the same problem. Research not only my postings but any you find.
This is an education process for you to be able to ask the right questions if you should so choose to take your transmission to a reputable transmission repair shop.
I can almost vouch for the fact you will not need a rebuild to your transmission.
Key words are: Torque converter solenoid.
Torque converter solenoid valve.
Torque converter solenoid valve bore wearing out and fluid as it thins will leak past.

I've read alot of your stuff. Seems like a big starting point is the TPS adjustment. Problem is on my 5.7 it doesn't appear adjustable??? I'm going to go ahead and order the shift solenoids and TCC solenoids and see if that makes a difference. They are only about 15 bucks each so it's a cheap place to start. I also have info on a tsb that talks about a similar issue and says to drill the feed orifice for the B solenoid to .37 to .40 I believe. I looked for this orifice and couldn't see it. The diagram on Aldata doesn't match what I see inside the pan. Anybody have any other info on this or tried this?

Do not replace the solenoids.
Transmisison component slipping: DTC68.
Undefined ratio error: DTC 85.
High Ratio error: DTC 87.

The computer expects to see input and output speeds. VSS sensor if you have one Should be checked into. If it is OK then an out of range pressure switch could falsly indicate code DTC 85 and 87. If you want to purchase anything and replace it would be the pressure manifold switch AKA transmission range pressure switch. However Do check Pins N R P at the transmission connection. There should be a sticky for it. The switch id designed to let the computer know what gear you selected. It expects to be in that output ratio.
An unserviceable or dirty output sensor AKA VSS sensor sure won't help anything.
An unserviceable Transmisison range pressure switch sure won't help anything.
This post takes precedence.

Thanks Gen... I'll look into that.

Stupid questions, can I just throw a non electric turbo 400 on it and call it a day. I also have a4l60e in my back yard but think that would cause too many issues.

If I do the TCC lock mod will this make the tranny go into OD?

I definitely can't advise you there. PM the ferminator so look for some ov his older postings.
Gassers are not in my repertrois. MikeL is another who might be able to help you out.

Thanks for all the help.

I just wanted to give an update. I found the problem. It was a stupid problem and I cant believe I missed it before.

The front VSS wasn't plugged in all the way. Previously I check the main plugs and all the plugs in the pan but never the VSS plug. I didn't really see it at first.

I plugged it in and now I have perfect shifting. Overdrive works perfect and now have lockup.

Things like this happen to the best of us. I was searching for a connector myself.