2018 duall 4WD duramax Allison. Slack in drive train

[HoSeR4785]

My 2018 3500 dually 4x4 has an issue with slack in drive train. sitting still and you take off normal there is a short “slack“ period before everything feels tight and normal. No clunking, no noise, but you feel the “bump” when this happens. When driving and you let off the pedal and go back on it you feel the slack again. It has done this since I bought it a year ago. Funny thing is yesterday in traffic I flipped it in to 4WD and the drive train is tight like a new one no slack at all. Everything under the truck is tight and can find nothing visible wrong, no leaks, all bolted tight, drive shaft solid in park. Thanks in advance.

[DmaxMaverick]

Welcome aboard!

This sounds like something we haven't visited in quite a while, but I hear about it from time to time. It can happen on any later year GM and others, and more so on longer wheel bases. Rear leaf spring wrap, combined with slip-yoke friction. It goes away with 4x4 engaged because the spring wrap is greatly reduced (the rear axle alone isn't reacting to the entire mass of the truck), as is the slip-yoke movement. The solution is inexpensive, and very easy to do. Simply grease the slip-yoke spline. I like high pressure marine grease, although GM suggests lithium grease (common chassis grease). It's also a good time to refresh the U-joints (and center carrier, if it has a 2-piece shaft), the rear transfer case shaft seal on older vehicles (the seal hardens over time, and can weep ATF), and the transfer case ATF. Drain the transfer case (Dex-VI ATF, less than 2 qts), disconnect the U-joint at the rear diff (mark the pinion yoke and rear shaft, so the U-joint can be reinstalled where you found it), and slide the slip-yoke out of the transfer case (ATF will run out if you didn't drain it). Apply grease to the entire circumference of the internal spline of the yoke. It only needs a coating that will distribute as the spline is reinserted. I use a good size dab on a fingertip, and wipe it into the spline grooves an inch or two deep. Work the spline in/out a few times as it's inserted into the transfer case to distribute the grease (apply a little ATF or grease to the outer yoke surface to lubricate the seal). Return the remainder of parts where you found them, fill the transfer case, and be done with it, at least for a long while. The transfer case service interval is about 50K miles, so if this is done at that time, you should not experience the problem again.

[HoSeR4785]

Outstanding. Thing about to drive me nuts. I will take your corrective actions and let you know. Thx again.

[DIESEL/VETTE]

Sure would like to know if you tried this, and it fixed your porblem.

[HoSeR4785]

Have not been home yet to start repairs.

[DIESEL/VETTE]

Thank you

[More Power]

Welcome aboard!

This sounds like something we haven't visited in quite a while, but I hear about it from time to time. It can happen on any later year GM and others, and more so on longer wheel bases. Rear leaf spring wrap, combined with slip-yoke friction. It goes away with 4x4 engaged because the spring wrap is greatly reduced (the rear axle alone isn't reacting to the entire mass of the truck), as is the slip-yoke movement. The solution is inexpensive, and very easy to do. Simply grease the slip-yoke spline. I like high pressure marine grease, although GM suggests lithium grease (common chassis grease). It's also a good time to refresh the U-joints (and center carrier, if it has a 2-piece shaft), the rear transfer case shaft seal on older vehicles (the seal hardens over time, and can weep ATF), and the transfer case ATF. Drain the transfer case (Dex-VI ATF, less than 2 qts), disconnect the U-joint at the rear diff (mark the pinion yoke and rear shaft, so the U-joint can be reinstalled where you found it), and slide the slip-yoke out of the transfer case (ATF will run out if you didn't drain it). Apply grease to the entire circumference of the internal spline of the yoke. It only needs a coating that will distribute as the spline is reinserted. I use a good size dab on a fingertip, and wipe it into the spline grooves an inch or two deep. Work the spline in/out a few times as it's inserted into the transfer case to distribute the grease (apply a little ATF or grease to the outer yoke surface to lubricate the seal). Return the remainder of parts where you found them, fill the transfer case, and be done with it, at least for a long while. The transfer case service interval is about 50K miles, so if this is done at that time, you should not experience the problem again.

The above has been the recommended fix for the "bump" since way back when. Thanks Greg. The spline is lubed by the ATF used in the transfer case (or transmission in a 2WD truck), but ATF isn't slick enough on its own to keep the bump away. Most truck owners never lube the driveline splines. I haven't heard of a wear problem in those trucks that don't see regular lubing of the driveline splines.

[DIESEL/VETTE]

I HAD THE SAME PROBLEM AS HoSeR4785 HAD. SO FOLLOWING DMAXMAVERICK'S FIX. HERE IS WHAT I DID, MARKED THE FRONT AND REAR OF THE DRIVE SHAFT SO WHEN IT'S TIME TO REASSEMBLE EVERY THING GOES BACK AS WAS. WHEN YOU PULL THE DRIVE SHAFT OUT OF THE TRANNY THERE IS NO TRANNY OR TRANSFER CASE OIL TO RUN OUT, THE TRANSFER CASE IS SEPARATE FROM THE TRANNY. THATS WHY IT IS IMPORTANT TO DO WHAT I AM ABOUT TO DO EVERY SO OFTEN. I COMPLETELY CLEANED THE SPLINE AND THE SLIP-YOKE OF THE OLD LUBE. I USED LUCAS RED "N" TACKY GREASE FORTIFIED WITH ANTI-SEIZE. ON THE SLINES ID AND OD. AND PUT A LIGHT COAT ON THE YOKE WHERE IT MATES WITH THE SEAL. I CHANGED THE OIL IN THE TRANSFER CASE AND USED RED LINE D4- ATF OIL 2 QUARTS DOES IT. WHEN I WAS DONE I TOOK THE TRUCK FOR A DRIVE AND IT WAS LIKE WHEN THE TRUCK WAS NEW.
WHAT A DIFERRENCE IT WAS LIKE A NEW TRUCK.