97 Suburban 4WD driveline issues

[Keith Seymore]

Sounds like I’ve killed something in the driveline of the Surburban.

I’m getting a “brrrrrrrrrrrrr” on hard launches, sounds like a fan hitting a shroud except more metallic. May be coming from under my feet.

Does not do it on soft launches. Does not do it in reverse. Does not do it in 4wd.

Also had a couple occasions where, in reverse, it’s like it “pops” out of gear with a single metallic bang and then the driveline “freewheels” (ie, you can hear the trans/tcase spinning even though the vehicle is stopped. Only way to stop the driveline is to put it in Park and let the pawl catch it).

No obvious outside damage: rear shaft, front shaft, everything on the outside of the trans looks ok visually.

Trying to decide if it’s converter, transmission or tcase. T case controls? Any thoughts?

K

[83GMCK2500]

Sounds like transfer case. Is yours manual shift(241) or electric shift(243)? I was dinking around one time and missed the shift from 4L through to 4H and I ended up with intermediate parts rotating with no way to stop them other than throw it in Park, Dddddddddddddddt.

Two weeks ago I rebuilt an electric shift case from an '07 Classic that had experienced pump rub and both of the shift forks had worn pads, one down to the metal. If the mode fork is not moving or holding the mode sleeve in the correct place I think it could cause the condition that you are experiencing. There is a hub and syncro ring in there that it slides along. Also, if a snap ring or bearing has failed (in the tailshaft) it will cause the mainshaft to slide forward, changing where the mode sleeve rides.

Have you rolled under it to check and see if all the heat shields are still in place, nothing rubbing driveline, exhaust still hung correctly?

[Keith Seymore]

Thanks for the response and ideas -

Yes - I've crawled around under there and externally and visually everything seems to be in order.

I did get the T Case snap ring failure diagnosis on another forum, so I may start with that.

It is the pushbutton T case (RPO NP1), so I assume that's a NP243.

Thanks again -

K