'08 Sierra 1500 5.3l Service 4WD

Matthew208 · February 3, 2020

Recently lost my 4WD when shifting it from 2WD to 4WD. I have electronic shift dial on the dash. I've replaced front diff Encoder motor, T-case Shift motor. Last week after I swapped the Tcase motor the 4WD worked again for about 2 days. Now its out again. Ran codes on it today at Oreilly's and its got CO 387 and CO 374. Looks like the 374 is a 4WD speed Sensor? and that failing is throwing the other code 387? Any ideas on what the root cause would be?

swathdiver · February 4, 2020

C0374 is called General System Performance Temperature High. It could be caused by a wheel speed sensor, not that the problem is a wheel speed sensor.

C0387 is called Unable to Complete Shift Circuit/Component Protection Timeout.

Now this one sets if Motor A Control and Motor B Control are shorted together (wiring problem) or the shift detent is binding. Could also be the control module. Either way, an ATC motor learn has to be done.

Matthew208 · February 12, 2020

Update. 4we went out again. Tonight I crawled under and noticed some gear oil dripping from the passenger side of the front diff. The axle and plate to which the CV axle bolts has about 1 1/2" of play in and out of the front diff. I'm mildly unfamiliar with the front differential. Is this going to be a major repair or a simple unbolt and replace?

Edited February 12, 2020 by Matthew208

davester · February 12, 2020

Your diff should be similar to mine (even though they are different generation trucks). There's a c-clip that goes on that stub axle around where the diff splits where the actuator screws into the diff. Most likely, it either wasn't installed or not installed correctly and came off, but it's possible some bits have broken inside as well.

You can unbolt the diff where it splits and examine the parts for damage, but I would just hit up junkyards/people parting out trucks to find a used diff of the same size & gear ratio and swap it, for minimal downtime.