Losing 4th gear

[BoxMasters]

Hey Guys,

My 1991 Suburban 2500 7.4 tbi Transmission went out in 1996 at 60K miles and the factory replaced it with a new one for free due to a factory defect recall.
Now my Suburban has 137K miles on the truck and around 60K+ miles on the trransmission it's failing again.

The transmission is a 4 speed automatic and I'm losing 4th gear. It's starting to slip in fourth. The error code 68 means Overdrive Ratio Error, or 4th gear is slipping in and out.

I'm told I can put the gear shift lever in 3rd gear for now and keep the speed below 50 and I can still drive it around town.

Any ideas on the condition of the internal workings of the transmission.

Am I looking at a new transmission or can this one be rebuilt or is there any hope for a simple repair?

Thanks to all for answers!

[2000CAYukon]

Are you losing 4th or are you having problems with the TCC lockup?

Take a look at http://www.transmissionspecialty.com.../34994-01K.htm as this can cause your code 68 and the TCC not locking up.

This page may also be interesting to you http://www.transmissionspecialty.com.../DM/4L80-E.htm

//2000CAYukon

[BoxMasters]

Great site to see and maybe there's hope for my automatic transmission.

First I started getting a "Service Engine Soon" lite, a local shop checked it and got a code 68. Overdrive Ratio Error.

Two days ago I noticed the transmission seemed to shift odd between 3rd and 4th (overdrive)
I've really been listening and paying attention to the feel and sound of the transmission and engine. I noticed the engine runs a higher rpm when running between 50 and 60 mph. The shift would always happen at about 45 into 4th but now I don't feel it shift everytime.

This afternoon everything shifted normally, tonight it never shifted out of 3rd gear. I keep the speed under 50 now until I can it fixed.

My main problem is we only have the suburban, and can't have it down for a few days for repair.

The problem is shifting into 4th gear does not always happen even at 50+mph.

[2000CAYukon]

If downtime is an issue, you could always have the valve body replaced. This is more expensive but should fix your code 68 (and the problem with shifting into 4th) in the fastest amount of time. Otherwise, someone needs to pull the valve body off and see if the problem is the TCC Regulator or Actuator Feed limit valve.

A rebuilt valve body will already have addressed these issues.

Good luck and let us know how it goes.

//2000CAYukon