I've had a similar issue with my 01 burb, first off let's go over
how the whole system works. The switches on the dash send a
signal to the control module that then actuates either the
encoder motor on the Tcase (the one that engages the clutches
and shifts the range gear, on the front/bottom of the Tcase), it
also sends a signal to the actuator on the front differential (on
the front/top of the diff, about the size of a pop can) to engage
the pinion gear to the axle shafts. If one of these motors fail,
the other will still work, but no power will be transmitted to the
wheels because the system is not fully engaged. A good home
diagnostic test is to see if the front is trying to lock in by turning
the vehicle to run, but don't start it. Then have helper press 4hi
while you listen under the vehicle to see which one is working,
and which one isn't, both should attempt to shift at this time.
Since there is no service 4x4 light on, I would guess that your
front actuator failed, but be sure to fully diagnose the situation
before you go throwing parts at it because a front actuator is
$150 and an encoder is $250 :( If you really want to be sure
(this is what I'd do) First change the front diff and Tcase fluid
(80w90 up front, synthetic Dex6 ATF in the Tcase, 2 quarts or
so each) then, if that doesn't help, take the front driveshaft out
(mark it's orientation on the front yoke) and shift into 4x4 with
the engine off in run mode, now grab the front yoke and try to
twist it, if it's locked into the wheels, it engages, now shift out of
4x4 to see if it disengages. Next, try to twist the driveshaft on
the Tcase, it will be very hard to turn even if it is disengaged
because the clutches always drag, see if it locks to where you
can't turn it when the 4x4 is engaged, then you'll know if the
encoder works or not. MAKE SURE THE VEHICLE IS IN PARK
WITH THE WHEELS BLOCKED! Hopefully this will help you
troubleshoot the problem. Good Luck!
8 people found this helpful.