Transmission slipping 1st -2nd

[carkhz316]

Wow. Keep on rocking 'er till she goes then I guess. Surprised it's lasted this long.

[Randy Lahey]

Old thread, but I *think* I am having this issue right now on my 1999 Sierra 2500, 4x4, 6.0, extended cab, long box.  I'm no expert on trannys, but I get the odd clunk/pull when it shifts from 1st to 2nd , very randomly about every 20th time I drive it (only in town).  It has 297,000 kms on it, and I have no idea if the last owner ever changed it.  He did swap the engine.  I only paid $2,500 for the truck in 2012, and I barely drive it, but maybe I should throw a can of Seafoam in it?

 

EDIT:  I don't know that I know what 'slipping' actually feels like.  I assume it's either lurching forward or hesitating when it slips, or is it just a hesitation?  Mine is 'lurching' forward, not hesitating...

Edited April 14, 2022 by Randy Lahey