4th Gear Slips
 

Hello Guys,I'm a Master Auto Tech My self and have worked on all brands vehicles for over 20 yrs.I own a 96 DeVille with a 4.6 Northstar Engine and I belive an 4T80-E hidramatic Transmission.Unfortunately,Transmissions are NOT my Best area.When I purchased this vehicle,I have to replace the "Transmission Speed Sensor" This one is Deep inside the tranny (NOT the Vehicle Speed sensor) Without Removing the trans out of the car.So I'm familiar with this Transmission.
Now I have a very Bad Problem on My car,when takes off,Shift Perfecly From 1-2,from 2-3 but when Shifts from 3-4.......OOPPS engine revolutions go up,Transmission slips with no traction at All.(Like if goes on Neutral).
From my little knowledge of transmissions,I would say,there is a problem with the trans 4th gear Clutches,Band or servo piston.
my question is :Anyone with transmission Experience,Could repace the 4th gear chutches,band or piston servo without removing the transmission?
PS. This vehicle and Transmission have 89K miles,Ran perfect,fluid it's very clean and NEVER have slipped,or gave any sings of malfunction.This problem happened all of the sudden!!!From day to night.

Any help will be apprecciated.

RE: 4th Gear Slips
 

There was a TSB for the lower channel plate not sealing prperly and alowing 4th to slip.
Not sure which years it covered but it was fixing them.
I've only seen 1 with a servo problem and never just a worn out band without something causing it.
If you catch it early enough the band and drum are still ok.

4th band is the closest to the driver side and the servo right behind the steering rack is the one that controls it.
By dropping the pan and lower V/B you can get a real good look at the band and drum to see if its scored or not.

If you can put a ISS in the car than a 4th band and revese drum are fairly close behind it.
I perfer to R&R the unit but it certainly can be done either way.

NOW, you didn't mention anything about checking the shift solenoids.
That is the #1 problem the early 4t80 has. It's easy to check and you're right at the common mileage for failure.
They are in the lower valve body and can be inspect by just dropping the pan.
GM has an updated set that fixes it for good.

Hope this helps.

RE: 4th Gear Slips
 

Thank You FLcaddy tech for your help,now I don't understand some terms you used.
1-"Lower Channel Plate" Is that The Lower valve body Gasket?and if is this Would I be able to see from where leaks?
2-If you can Put an "ISS" what is an ISS?? Sory but I have no Clue!
3-"Shift Solenoids" Wolud a solenoid would do this condition?Would do this without turning Service Engine Light on?
I have no SES Light on or Trouble codes...NONE

RE: 4th Gear Slips
 

This is the channel plate that the lower V/B bolts to, a sandwich metal/paper gasket goes between them.
No, you won't be able to see the problem.
Input Speed Sensor. Thats the one you said you replaced with the trans still in the car.
Yes, sometimes you get a code, sometimes not. This is where i would start. The lower channel plate has to be removed from the V/B to do the solenoids so you may want to change it while its out. Also replace that large metal/paper gasket between them.

RE: 4th Gear Slips
 

Hey ,FL Caddytech,Im tinking in start an inspection on this tranny,but i have no intention on droping the frame and trans.My plan is to remove lower oil pan and vlave body,inspect 4th band and servo,if it looks ok then I'll drop the chasis (only form driver's side)lower the trans,remove side cover,inspect and test solenoids,and if nothing wrong found at this point,I may even go deeper to 4th drum and clutches.
I would like to have your advice of what to do.

Already went to the trans shop and the guy said this car don't have a band for 4th gear.Definetly NOT going back to him since He doesn't know what is saying.(I saw the band and servo in AllData)

And just keep this in mind,this trans shift perfetly and runs great,untill shifting to 4th gear.When Shift to 4th,trans DON"T SLIP,It just goes to neutral,no traction at all! No noises,no dirty oil no dtc'sAnd one more thing,It happens all of the sudden,from day to night!No warnings,no sings of malfunctioning and did nothing wrong to provoke damage to trans,happens while driving straight on flat street.

RE: 4th Gear Slips
 

Shift solenoids are in the bottom.
Very easy to inspect to see if one broke. Just drop the lower pan.
If you have to relpace them, the V/B has to come out and seperate from the lower channel plate.
We charge 3.5 hrs labor for this repair.
I still think this is the most likely cause for your cars symptoms.

Most tranny shops stay away from 4t80's.

RE: 4th Gear Slips
 

Hey FLCaddyTech,Good News!I removed the oil pan to inspect the Shift solenoids and Found 2-3 Solenoid plastic part Broken.Droped the valve body,separated from Channel plate and replaced both solenoids with the updated p/n kit.The rrans is working perfect!!!.
I wanted to thank you for answering all my questions and for your help,you hit the nail in the head.The problem was exactly what you said it was.Again, thankyou very much for your Help.I would end up replacing the tranmission,but thanks to You I saved a LOT of Money,A LOT!!.

RE: 4th Gear Slips
 

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