I have a 2001 Suburban with 140K miles. Just recently it stopped shifting into 4wd. When I press any 4wd mode, the switch just blinks. I can hear the motor on the middle transfer case engage. Is there a way to test the actuator? It seems like the switches on the dash are doing their job, and the transfer case motor is also. Would like to confirm actuator is bad. There are no check engine lights on, or service 4wd lights on. Thanks!
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!
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