Transmission slip in/out of 4th

[NGAneer]

Have an issue related to 4th gear in my 4L65. Truck has ~250k miles so it may mean full rebuild but this doesn't seem like the band is completely gone. Only trans mods are Corvette 2nd gear servo and a Superior Billet 4th gear servo.

Leaving from a dead stop truck drives completely normal. First gear through 3rd is fine on the upshifts. When the trans tries to shift to 4th the RPMs run away. If I get out of it the RPMs will drop down and 4th will engage. Once engaged 4th seems to work fine at speed (75mph cruise on the interstate). If the trans tries to kick down to 3rd, the RPMs run away again but not if I drop the shifter into 3rd manually, just if I'm in "D".

Ideas on what may be failing here? Short of a full rebuild, my thoughts are...

1) 4th gear servo seal leaking
2) Shift solenoid

EDIT- Added a video, see the link below. First is a trip through the gears up to about 50mph. You'll see the RPMs shoot up when the truck shifts to 4th. Next you'll see a ~50mph cruise when I get to a hill and the trans kicks down to 3rd. You can see how the RPMs again shoot up.

0 new items by Matt O'Brien

photos.app.goo.gl

 

[NGAneer]

Bump

 

[swathdiver]

Have an issue related to 4th gear in my 4L65. Truck has ~250k miles so it may mean full rebuild but this doesn't seem like the band is completely gone. Only trans mods are Corvette 2nd gear servo and a Superior Billet 4th gear servo.

Leaving from a dead stop truck drives completely normal. First gear through 3rd is fine on the upshifts. When the trans tries to shift to 4th the RPMs run away. If I get out of it the RPMs will drop down and 4th will engage. Once engaged 4th seems to work fine at speed (75mph cruise on the interstate). If the trans tries to kick down to 3rd, the RPMs run away again but not if I drop the shifter into 3rd manually, just if I'm in "D".

Ideas on what may be failing here? Short of a full rebuild, my thoughts are...

1) 4th gear servo seal leaking
2) Shift solenoid

EDIT- Added a video, see the link below. First is a trip through the gears up to about 50mph. You'll see the RPMs shoot up when the truck shifts to 4th. Next you'll see a ~50mph cruise when I get to a hill and the trans kicks down to 3rd. You can see how the RPMs again shoot up.

0 new items by Matt O'Brien

photos.app.goo.gl

Where've you been? It's been awhile. Can't answer your questions, might want to look through Precision Transmission videos on youtube and see if they mention it. Oh wait, I think I have a troubleshooting guide for yours...

Well, I found this in my GMT900 4L60 manual, not sure what to make of it:

 

[NGAneer]

Where've you been? It's been awhile. Can't answer your questions, might want to look through Precision Transmission videos on youtube and see if they mention it. Oh wait, I think I have a troubleshooting guide for yours...

Well, I found this in my GMT900 4L60 manual, not sure what to make of it:

View attachment 362032
View attachment 362033

Been busy with alot without much time to spend on here. Work, coaching sports year round, plus I just haven't done much with the truck except keep it going as long as possible. That was going pretty well until this issue popped up.

I've got servo seals on the way for the 4th gear servo. I think I'll try that first and then maybe the shift solenoids. If neither of those do the trick I'll probably limp it along while saving for a rebuild.

Appreciate the info by the way. That's the kind of direction I'd been looking for but couldn't find any documentation.

 

[swathdiver]

Been busy with alot without much time to spend on here. Work, coaching sports year round, plus I just haven't done much with the truck except keep it going as long as possible. That was going pretty well until this issue popped up.

I've got servo seals on the way for the 4th gear servo. I think I'll try that first and then maybe the shift solenoids. If neither of those do the trick I'll probably limp it along while saving for a rebuild.

Appreciate the info by the way. That's the kind of direction I'd been looking for but couldn't find any documentation.

Glad things are going well. As our rigs get older, it's good to have a little nest egg for these kinds of things.

 

[rockola1971]

Id first pull the pan and filter. Check the pan for a substantial amount of what looks like dark brake dust. This is 2-4 band lining material. With your 250k mileage and no mention of a previous rebuild its likely that your 2-4 band is failing. This could have been caused by a stuck check ball in your valve body separator plate or just normal wear and tear. It wont get any better. Eventually you will loose 2nd and 4th since those gears require the 2-4 band to hold the input drum stationary.

 

[Geotrash]

Any codes from the TCM?

 

[NGAneer]

Well, today it got worse. Going up a hill in 3rd the RPMs ran away. So now I've got no 3rd or 4th, revs to the moon after 2nd. What's your manual say for that @swathdiver?

 

[NGAneer]

Id first pull the pan and filter. Check the pan for a substantial amount of what looks like dark brake dust. This is 2-4 band lining material. With your 250k mileage and no mention of a previous rebuild its likely that your 2-4 band is failing. This could have been caused by a stuck check ball in your valve body separator plate or just normal wear and tear. It wont get any better. Eventually you will loose 2nd and 4th since those gears require the 2-4 band to hold the input drum stationary.

Nope, no rebuilds. Just regular transmission services from yours truly. With it being just 4th gear I thought I had a decent plant of attack with changing the servo seals and the solenoids to eliminate that but now I've lost 3rd and I'm thinking it may be the 3-4 clutch. We'll see.

 

[swathdiver]

Well, today it got worse. Going up a hill in 3rd the RPMs ran away. So now I've got no 3rd or 4th, revs to the moon after 2nd. What's your manual say for that @swathdiver?