Thud when shifting from 4 Hi to 2 Hi.

jaguarjoe

Original poster
Member
Nov 22, 2012
73
We have an '02TB with the flaky 4wd/2wd selector system.

When the TB is at higher speeds (40 mph or above) and it magically jumps from 2wd to 4wd, we just switch it back to 2wd and all is well again, no noise, no thumps no nothing, it just shifts. When it jumps into 4wd, the TB doesn't handle any different, steer any harder, or sound any different.

Now, when we are going very slow, especially when pulling into a parking spot or driveway, many times it will jump from 2 to 4wd. Everytime we switch it back to 2wd, the TB makes a loud thud or thump as it goes back to 2wd.

The wealthy PO had a GM installed new TCCM and a new encoder/motor.

Any ideas on where to start, or what the thump is?

Thank you,

Joe

(I'm not a newbie to cars/trucks at all, just to the TB)
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
The thud is entirely normal, and caused by driveline wind-up (torque energy stored in the driveline due to different turning radii of the front and rear diffs.) Driving fast and straight, you don't feel it, and it builds up very slowly if at all due to infinitesimally different tire diameters.

Allowed to continue, it will wear the clutches in the transfer case or break a u-joint or something.

When you say it switches into 4HI on its own, is this without the switch being turned? That's entirely an issue with the switch (dirty contacts usually). How do you get it back to 2HI? Turn the switch to 4HI and then back to 2HI?

The switch was redesigned around 2005 for better reliability, and is surprisingly cheap, even at the dealer. I think under $30 list price. Until then, just turn it end-to-end 100 times or so to clean up the contacts and see if it gets better.

Here's a drawing I did years ago to explain why we get driveline binding during turns in parking lots.

envoyturning.jpg
 

jaguarjoe

Original poster
Member
Nov 22, 2012
73
the roadie said:
When you say it switches into 4HI on its own, is this without the switch being turned? That's entirely an issue with the switch (dirty contacts usually). How do you get it back to 2HI? Turn the switch to 4HI and then back to 2HI?



You are right, it will be in 2wd then all on it's own, it will switch to 4Hi. We rotate the switch from 2wd to 4Hi and back to 2 again and it's ok.
 

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