2015 Suburban-Periodic rear end rumble strip sensation at highways speeds, between 1k-2k rpm

[montcrw15]

Same feeling when you drive over a "rumble strip" on the highway occurring at highway speeds. The sensation feels like it is coming from the rear of the vehicle, will last for a second or two along with a rpm drop of about 100-250. This will repeat rhythmically sometimes, and randomly at times. Occurs while maintaining a speed, accelerating modestly and no throttle. If the rpms are above 2k, this event does not occur and vehicle drives normal. Also, abnormal shifting at lower speeds sometimes happens.

Vehicle was purchased used with approx 120k miles, "new" trans at 98k per seller, (shown paperwork, but I do not remember if it was rebuilt or replaced).
Did a trans service before a road trip from Ca-Az and back! About 1200 miles total. Checked trans fluid and appeared to be dirty if not burned, dropped pan and replaced filter and pickup, gasket was in good shape. decent amount of particulate on pan magnet but no chunks visible. Biggest concern was a long fastener in the pan, just hanging out in the fluid and several other fasteners finger loose on the valve body. In fact, the fasteners were loose enough that the valve body hung down so much it was nearly impossible to remove the pan. While tightening the fasteners, you could actually see the valve body being pulled up. When done, the pan slipped back on without getting hung up on anything. About 5 quarts of fluid were added and the dip stick indicated full. Drove to AZ and back to Ca with no issues until about a week later. No engine check light. No warnings on a scanner that can read transmissions either.

While trying to diagnose, I have replaced:
driver side motor mount- Absolutely shot
spark plugs and wires- overdue and dirty
throttle body cleaned- a bit dusty
exhaust manifold bolts were found to be loose on both side in the process of replacing motor mount

Vehicle still has same issue. I still would like to drop the pan again, check the fluid and look for any loose fasteners, and it was suggested to maybe try a trans relearn, check the rear suspension(replaced soon after I bought the vehicle), driveshaft, transfer case, and rear axle before taking it to a trans specialist to diagnose. I do not mind doing the labor if it saves me money, I just want to avoid spending a huge chunk of money I do not have and having to replace the whole transmission when it was "replaced/rebuilt" less than 100k miles ago. I tried to add all the details for this problem and I am sure some info irrelevant but if anyone has any ideas or advice, I would appreciate it, I love the thing, its our family hauler and my wife's daily.

Thanks!

 

[Tahoe14]

Take a picture of the fastener left in the pan and see if @NickTransmissions can help to identify it or he could give suggestions on what to check for the issue you are describing.

 

[NickTransmissions]

Vehicle still has same issue. I still would like to drop the pan again, check the fluid and look for any loose fasteners, and it was suggested to maybe try a trans relearn, check the rear suspension(replaced soon after I bought the vehicle), driveshaft, transfer case, and rear axle before taking it to a trans specialist to diagnose. I do not mind doing the labor if it saves me money, I just want to avoid spending a huge chunk of money I do not have and having to replace the whole transmission when it was "replaced/rebuilt" less than 100k miles ago. I tried to add all the details for this problem and I am sure some info irrelevant but if anyone has any ideas or advice, I would appreciate it, I love the thing, its our family hauler and my wife's daily.

Thanks!

The fasteners in question are the six inverted Torx Plus 50 (TP50) valve body assembly to case bolts. They must be torqued to 71 inch lbs on reinstallation (start at the center then work your way out). That said, the sealing integrity between the valve body and both front pump as well as the center support was compromised when those bolts came loose as apply fluid intended for the 1-2-3-4, 3-5-R, 4-5-6 clutch (front of the unit) along with the 2-6 and low-reverse clutches (rear of the unit) was bleeding into the sump instead of going to those clutch packs to keep them clamped in place when shifting through the gears.

You can try a fast adapt relearn however I would plan on having to pull the unit and rebuild it sooner rather than later as most if not all clutch packs have been severely stressed once those bolts came loose. I'd also be wary of gear train damage (front and rear planetary carrier assemblies) and the rear Ravigneaux style planetary assemblies are relatively expensive to replace.

If you do drop the pan, I would actually remove the valve body assembly all together and check to see if any of the non-inverted TP50 bolts are loose. There's 18 on the TEHCM-facing side (including the two 8mm bolts going into the side of the TECHM/upper valve body half) and 12 on the speed sensor side (the side that actually faces the transmission and has the seals. I'd replace all of the valve body-case seals as well while you were at it.

If the rumble is coming from the transmission, it's more than likely coming from one of the two planetary carrier assemblies due to going in and out of perfecet mesh w/their respective sun gears and ring gear though if the planets were beginning to break up, you'd see pieces of gear teeth in the pan. You'd also get delayed engagements that felt like something was 'locking up' the vehicle temporarily then would release when you shifted between drive and reverse. This happens when pieces of broken gear teeth are getting wedged between the mesh and preventing it from moving then clearing when you put it into reverse (or drive if it initially happens in reverse).

Check all of your U-joints as well as the pinion yoke-to-driveshaft bolts to ensure they are tight. Also check the fluid in the T-case if you have a 4WD.

With the sort of failure you experienced, you wont throw a DTC until the intput and output speed sensors detect a gear ratio error (ie non-conforming relationship between rotating elements the speed sensors are monitoring based on what the ECM should see when the trans is at a given speed in a given gear). When you see those, trans definitely has to come out for an overhaul as that means one or more applied elements are shot.

You can refer to my 6L80 tear down and inspection and 'assembly' videos on my Youtube channel to get an idea of what's going on inside the case as well as what's involved in tearing one down and reassembling it.

LMK if you have any questions and I'll respond as soon as I can.

 

[rick1024]

i just bought a tahoe and it was doing this. needed a whole new transmission