NP243 t-case tries to engage 4wd randomly

[neo]

Hi all, Hoping one of the drivetrain guru's out there has an idea about this issue. 96 2500 burb with 6.5 TD, 4L80e tranny and servo actuated 243 t-case. When I am in low speed, starting from stop and asking the engine for power, the tcase is randomly trying (I believe) to go into 4wd hi. The control switch panel does not indicate any activity, no flashing, just the normal 2Hi indicator illuminated. The racket from under the belly of the truck is horrible. I changed all the fluids, t-case, trans, and front axle. If I place it in 4wd, this issue never happens. if I am super gentle pulling out, it may happen, just not as severe. does not happen at cruise speed.
Disconnected the front driveshaft and put a zip-tie around un-coupled yoke at t-case and front axle. Test drove. THe zip-tie on the t-case broke when I accelerated like normal. I could here it switching, but no racket and shuttering horrible sound through the truck like when the driveshaft was connected.
My question is, does this soudn like the servo going out (again?) last time, it would just flash the lights on the dash control unit. I don't want to kill the t-case, so I am not driving it. Was thinking pick-n-pull to get a cheap servo and try it out. any thoughts anyone? THanks!!

Neo

[DmaxMaverick]

Check your engine and tranny/TC mounts first. If they are worn (out), engine torque will allow things to move around enough to make some racket. It's usually an exhaust pipe, down pipe, or fan/shroud, but could be other things attached. Could also be U-Joints. It will do it less when engaged in 4x4 because drivetrain torque is more equally distributed.

I've never heard of a servo/stepper failing by trying to engage without some help (power). Usually, they either don't (completely) engage or don't disengage. If it's getting power, it should be indicated on the panel.

[neo]

Thansk, I'll check that out. I think the part that makes me relaly suspect the t-case is becuase, and please correct me if I am wrong, since the front driveshaft is out, since I wire tied the two yokes to the frame, neither yoke should spin when driving in 2wd. But the one on the front of the t-case (supplier of torque to the front axle) broke the wire ties, indicating it is engaging randomly. Wierd! I would think if everything was good under there, the wire ties would have both been uncompromised. THanks again!
Neo

[DmaxMaverick]

The front shaft is synchro-engaged, and even idle (disengaged) gears/shafts will always have some torque through them while rolling. A zip-tie isn't going to hold much. There may be an issue with the TC, but I don't think it's the stepper. I suppose it's possible it isn't fully disengaged, but it wouldn't be reactive with engine torque, or act up intermittent. Look for something specifically connected with engine torque, considering the conditions you are seeing.

[neo]

I may have figured this out, not certain yet, as it will take some slight disassembly to know for certain. What I suspect at this point is the circlip at the tail end if the tailshaft that is right in front of the splines for the slip joint has broken, allowing the shaft to move forward causing the fork at the dog for the front output to engage. Going to take it off ASAP to determine. My fingers remain crossed.

[neo]

Pulling the tailshaft extension from the t-case found the circlip in the tailshaft snapped in three pcs. Ended up using a 1.25 id x 1/2 thk 2.25 od shaft collar and turning it open to the shaft dia of 1.375 id leaving a .090 x 1.25 ID lip on one side. Bolted this on shaft (got the idea from Internet - apparently this is a common issue with the 241 and 243 cases) the .090 lip fit into the circlip groove and now holds shaft very tight. Works like champ. This can be purchased in a "transfer case saver" kit as well for about $60. I like mine for $4.59 from fastenal! Thanks again for suggestions and ideas.