Engaging 4wd While Moving Causes It To "slam" Into 4wd

[Fortune50]

Whenever I engage 4wd on my '01 HD (which has only 4 hi, 2hi, N, and 4lo) into 4hi (it's a pushbutton or electronic manual transfer case) it slams into the mode. What does that mean? I'm now too scared to engage it while moving very fast. How is it suppose to engage so it doesn't "slam"? BTW I changed the fluid just last Nov w/ the GM specified fluid (originally Dexron III, now is called "manual transfer case" fluid). Also at that time I changed the front diff oil to the GM specified oil...

[Fortune50]

no its called auto trax,seems as though the synchromesh blocker ring MAY have failed and of course changing the oil does not fix anything ,needs to be diag'd by a technician,maybe taken out and apart for a look see.

Thanks for that reply. It doesn't have the AUTO button, and it's in an HD, so it's an NVG 263-NP1 transfer case with buttons only for 2hi, 4hi, & 4 lo. How does the system work when switching to 4wd? It would seem to make sense that it would want the front driveshaft turning BEFORE it would engage the front diff, right? That's the only thing I can think of that's not right as to why it would be so harsh in engagement because it's working fine once engaged otherwise....

[gmtech4]

t case engages frt shaft with shift fork and frt diff actuator engages frt diff,now read what I wrote 2 posts up

[Fortune50]

t case engages frt shaft with shift fork and frt diff actuator engages frt diff,now read what I wrote 2 posts up

Just to clarify, where do you think the problem is at, in the front diff. or in the transfer case?

[gmtech4]

I can't tell from this side of the internet.I said it MAY be the syncromech in the t case ,it should not thunk into 4wd,get it diag'd,possible tear down ,definite road test by a tech,get ur done,

[Genuinefine409]

My 2002 HD does the same thing. My old truck did too and the syncro ring was shot in it so I figured something like that was wrong with this one but didn't get around to pulling it apart yet. The slam you are hearing the the front drive shaft going from sitting still to whatever speed your going when you push the button. For now I just dont engage it while moving.

[gmtech4]

no its called auto trax,seems as though the synchromesh blocker ring MAY have failed and of course changing the oil does not fix anything ,needs to be diag'd by a technician,maybe taken out and apart for a look see.