VetteRedLS6Z28

Hello all,

My overdrive keeps slipping back to 3rd while I drive highway speeds. It doesnt want to stay in 4th at all, the transmission will shift up the gears fine all the way to 4th but after driving in 4th for a a quarter mile or so it shifts back down to 3rd randomly. When I let off the gas it will go back into 4th however once I give it gas again and drive for a little while it will go right back into 3rd. Other gears are fine, I can drive around the city with no issues.

Any insight on what this might be? Ive read a lot of threads but have not seen the answer to my problem..

mrvedit

First, I would use a scanner that displays the commanded gear to make sure the PCM is not making the gear change.
If the PCM is not forcing the gear change, your problem is likely due to a worn band and excessive servo end play. Therefore, next (or even first) I would determine the servo end play by pressing on the servo cover with a crow bar and measuring/estimating the end play. Spec is .075 or about 1/16" inch. Much over .125 (1/8") indicates excessive end play which might cause slippage. Here are some related threads and my suggestions for a temporary fix which helped some people:

https://ls1tech.com/forums/automatic...t-trouble.html
https://ls1tech.com/forums/automatic...-gear-all.html
https://ls1tech.com/forums/automatic...ckup-help.html

PONTIAC MUSCLE

I have a similar problem: "I just lost 3rd and 4th gear in my transmission, it will shift into but not hold when I give throttle. I have a 1998 Pontiac Trans Am with the 4 speed automatic code #GU5. Does anyone know the best solution for this or know of anyone parting out their A4 LS1 Firebird/Trans Am/Camaro with the GU5 Transmission? Any help is greatly appreciated, thank you."

mrvedit

Nope different problem; your 3/4 clutch is worn out, the most common problem reported here.

mrvedit

Pontiac Muscle started his own thread with the same question as in #3.
https://ls1tech.com/forums/automatic...g-3rd-4th.html

So lets only continue with VetteRedLS6Z28's original question.

VetteRedLS6Z28

Thank you for the advice mrvedit.
Im going to have a friend hook his scanner up to it highway speeds and see if we can get some info on this through the scanner then go from there.
I know in one of those posts you said you had a friend where they replaced the servo, 3-4 accumulator and finally the valve body to fix their issue. Could this possibly be the same issue I am having? If so should I go through the same process?

mrvedit

I do recall the thread where the OP finally replaced the valve body to fix his 4th gear problem. Unfortunately he made no attempt to determine exactly what was wrong with the valve body; obviously he was just happy to have his car on the road again. Based on the assistance I have given here and the feedback from those threads, starting with the most likely problems:
1. Worn band - lengthening the servo pin may give you a bit more life
2. Problem in servo area - cracked piston, worn rings, etc.
3. Problem in 4th accumulator - cracked piston, worn bore
4. Valve body - (unknown problem)

Determining the end play of your servo will be important in nailing this down.

lsxstudebaker

who needs over drive.

VetteRedLS6Z28

Okay so Im not entirely 100% sure where the servo band is located (please excuse me as I am a novice with transmissions). I did lift the car up and try to take a look underneath. What I did find just by looking is when I moved the drive shaft a bit it has quite a bit of play where it connects to the tranny (the pics show where I moved the drive shaft). Is this mostly normal?
Also, could you direct me to where the servo would be?
Help much appreciated

mrvedit

Its the cover on the passenger side. Here is a pic of hitting it with a malet to loosen it for removal or measuring end play:



The servo is hard to remove in most cars as the driveshaft and rear trans mount need to be removed to lower the trans enough to access it. Obviously the trans needs to be supported when the mount is removed.

(Sorry I don't really know much much play you have in your driveshaft and how much is normal.)

mrvedit

In the following thread I detail how to open and remove the servo starting at post #8:
https://ls1tech.com/forums/automatic...ry-smooth.html

Another thread on the servo:
https://ls1tech.com/forums/automatic...ervo-swap.html