Transfer Case Issue

[houstontaylor]

I am not a professional mechanic but wanted to pass on my experience with a 2003 Tahoe with 306,000 miles and what sounds like similar issues. The transmission had been repaired about 5 years ago, but not rebuilt or replaced. I had had the rear differential replaced in April of this year (2021) with a helical or Torsen type from a GM supplier (AAM) for better traction in icy conditions. In June the car started to lock up as described above and there was noise coming from the transfer case. Fluid replacement and addition in the transfer case muffled the sound but I had the transfer case replaced when I got back in town. Things seemed good for a while. Then it locked up again when I was trying to back out of a parking spot. It was like it was in Park even though it was in Reverse and it wouldn't go back more than a little bit. The tow truck driver thought it was a rear differential problem so the car was towed back to the differential shop who had replaced it in April. They said it was not the differential and they thought it was the transfer case because noise was coming from there even though the dealer had just replaced it. I drove it to the dealer because it was fairly close and it did make it, although it was shifting funny and was making scary noises. I told them the story and said I thought it might be the transmission but asked them to look it over. They checked it out and believed it was the front differential, which admittedly had not been serviced much over the years. When I got it back and was driving away, it seemed like the transmission was stuck in 3rd gear and it was difficult to get moving after stopping. That went away by the time I got home and I made an appointment to get a GM remanufactured transmission installed by a local shop. I drove it locally while waiting for the transmission replacement appointment and at one point I was going about 35 mph and it seemed like the transmission shifted suddenly into Park by itself and made a terrible racket. I quit driving it until time to take it to the shop. Replacement of the transmission did solve the problems. So, perhaps that transmission could be in Drive or Reverse and somehow shift into Park at the same time.? At least that's what it seemed like, not knowing if that is even mechanically possible. If the transmission on the above vehicle is as old and high-milage as mine was, at least consider the transmission as the source of the locking up. With my remanufactured engine and drivetrain I am ready for another 300,000 miles.

 

[Bill 1960]

I think @houstontaylor has given you a good lead.

There’s not much in a transfer case to lock up, other than a broken chain, which should be obvious under all conditions and easy to detect. It can’t be in high and low simultaneously because there’s a gap (neutral) between the gears.

The transmission OTOH, can suffer a variety of malfunctions which can engage a clutch that shouldn’t be, resulting in two gears applied simultaneously. Which will lock it up.

 

[Postmark]

Worth checking out.

 

[exp500]

Ref post 10- Were you able to manually shift "between" ? When you re-installed shift motor, was it clocked to proper position per manual? You may have luck cleaning and lubing the motor control . There are a few threads on that here.

 

[Fless]

I've done that. As soon as I start the truck, it reverts to being stuck in between.

Are you leaving the encoder motor off? Have you tried to leave it off and position the T/C into 2hi? Forget about what the switch says.

Or are you saying that the T/C shifts itself with no encoder motor attached?

 

[Postmark]

Are you leaving the encoder motor off? Have you tried to leave it off and position the T/C into 2hi? Forget about what the switch says.

Or are you saying that the T/C shifts itself with no encoder motor attached?

 

[Postmark]

With the encoder removed, I can manually shift the t-case. I can also put the transmission in neutral, and spin the wheels. With the encoder re-installed, everything is locked up again once I start the engine.

 

[Fless]

What does it do if you leave the encoder motor uninstalled (strapping it out of the way) and manually shifting the T/C into 2wd? If that seems to work, then there is something wrong with either the encoder motor or circuit, or the TCCM.

It would really help to have a Tech 2 on it; that way you could monitor what mode is commanded vs. what mode it's in, or thinks it's in. On my NBS, same T/C, the commanded mode was 4HI but the actual mode was AUTO. That ended up being a bad encoder motor.

 

[Bill 1960]

Does it unlock if you just shut the engine off and leave the trans in neutral? Can you push it / roll it in that state?

 

[V327839]

The tcase has two speed sensors on it. They are identical. One for the front driveshaft and one for the rear. You can try replacing them to see if they are bad. I have a bad front sensor and it shows no code or display until I shift into 4wd, then the bad sensor won’t let me shift back until I turn the engine off and restart. Also, the electric switch unit in the dash could be bad. I had a 01 Yukon like that.
Just a thought.