Rear gear swap now trans slipping.

[chevydog66]

72 GMC 1500 truck with a junkyard stock 01 6.0ls and purchased new 4L65 trans 4 years ago only 10k miles at the most. Just swapped rearend gears from a 3.07 to a 3.73. First test drive after it has sat all winter and now trans is slipping like crazy in 1st and 2nd only above 2800 rpm. It's not in limp mode. No check engine light. Speartech did the computer and harness specific for the truck. Speartech says should be no problems with computer. And trans fluid is reading way too full on dipstick. Trans is slipping bad, almost like throwing it in neutral above 2800 rpm. Should I just pull trans??

[BR3W CITY]

Had you previously driven on this engine combo successfully (with the old gears?) You didn't have any problems with the trans at all?

The other question to be asked is....how did you get too much fluid in it? Too much fluid screws with line pressures and can cause a trans to slip much like running too low can.

What do you mean by "Speartech says nothing can be wrong with the computer"? If you changed the rear gears, and didn't adjust the gear ratio in the tune, then the computer is going to be a bit confused. The ecm will see that the engine Rpm doesn't match the vehicle speed for that gearing, and think that your slipping or something is broken. This will lead to actual slipping, issues with lockup etc.

I'd suggest getting the fluid level right and making sure you've got the right stuff, resetting the ecm by pulling the fuse or battery, and seeing how it feels. If it still feels off, I'd suggest making sure that you're tune matches the correct rear end gear and tire size.

Did you recently do any trans maintenance? New fluid filter etc....making sure you've got the right Dex VI in that trans (not DexIII like for the older gm auto's).

[Bigdav160]

3 and 4 alright? Reverse?

What type of cooling fan do you have?

[chevydog66]

I put 8-9 thousand miles on this combo even drag raced once last year. Towed my Chevelle with it also. I have no idea how fluid level got so full. It won't even shift up to 3 or 4. Haven't done any maintenance on it. Pretty sure I put dex 6 in it originally.

[chevydog66]

Speartech says the gear swap will only raise shift points I agree. No check engine light.

[Bigdav160]

Sounds like the forward clutches let go

[chevydog66]

Reverse still works.

[BR3W CITY]

And its not throwing a Limp code or anything?

[chevydog66]

No code. Red light isn't on next to obd port.

[Special-Ed]

problem #1 you raced a 4l60e
you look at those funny and they stop working...

id say pull it and get a performance rebuild and then baby it.

[JNJchev77]

Check your fuel pressure... I had similar symptoms a couple months ago and my post pump filter was clogged, wasn't getting enough fuel... If the truck sat all winter, you could have something in the fuel line.

When I would give it 3/4 to WOT the engine would rev but truck stopped pulling. Thought it was a trans problem... turned out to be crap at that filter causing some cavitation. It kept getting worse and eventually got to a point if I gave it any gas it would bog down... I hope it's that simple for you.

[Wasted Income]

Originally Posted by Special-Ed

problem #1 you raced a 4l60e
you look at those funny and they stop working...

id say pull it and get a performance rebuild and then baby it.

Or slap a 4L80E in like a 6.0 was meant to have, and go have fun and not worry about it.

[BR3W CITY]

Originally Posted by Wasted Income

Or slap a 4L80E in like a 6.0 was meant to have

I just assigned this post to a Macro Key on my keyboard. Now when I press MacroS5, it says "Or slap a 4L80E in like a 6.0 was meant to have"

[Wasted Income]

Haha.

Do you ever wonder why GM put an 80E behind even the 4.3 and 4.8s in the Express vans?

I don't. lol.

[clinebarger]

Fluid doesn't appear out of nowhere, Water or antifreeze could be mixed in...And quickly kill the trans. You are idling on level ground when checking?
If no contamination is found & your checking the fluid correctly, Correct the fluid level & try again.

Pull it down into manual 2nd, If the slipping is now gone....The problem is in the forward clutch or forward sprag. In manual 2nd the Overrun clutches apply & bypass the forward clutch & sprag, Don't dog on it, There are only 2 overrun frictions.

[chevydog66]

Originally Posted by Wasted Income

Or slap a 4L80E in like a 6.0 was meant to have, and go have fun and not worry about it.

No $$ for a 4L80. Although I do have a 4L80 with transbrake in my 66 Chevelle with a 406 sbc 573hp.

[chevydog66]

Originally Posted by clinebarger

Fluid doesn't appear out of nowhere, Water or antifreeze could be mixed in...And quickly kill the trans. You are idling on level ground when checking?
If no contamination is found & your checking the fluid correctly, Correct the fluid level & try again.

Pull it down into manual 2nd, If the slipping is now gone....The problem is in the forward clutch or forward sprag. In manual 2nd the Overrun clutches apply & bypass the forward clutch & sprag, Don't dog on it, There are only 2 overrun frictions.

I'll try that thanks Clinebarger and thanks to everyone for the help.

[chevydog66]

Corrected fluid level , it doesn't look or smell too bad. Drove it in manual 2nd, slipping is gone. Still slips in D won't make it out of 1st gear. Guess I'm pulling it.

[clinebarger]

Originally Posted by chevydog66

Corrected fluid level , it doesn't look or smell too bad. Drove it in manual 2nd, slipping is gone. Still slips in D won't make it out of 1st gear. Guess I'm pulling it.

Fixing it yourself? This failure isn't hard to fix. If it really is a low mileage trans...It may not need a full rebuild. Just a full tear down & inspection of the Input drum, (The Input drum houses the Forward clutch, Overrun clutch, Input sprag assembly & 3-4 Clutch.)

[chevydog66]

Not fixing myself. Taking it to a local experienced builder with all the tools. Thanks for the info.