Help pinpoint Low speed vibration in turn!!! Out of ideas

FS87LT

Veteran Member
Apr 3, 2010
981
DFW, TX
Age does not really affect the G80 unit, nor does miles. My '77LT has over 750K on it and no issues with the G80 unit. Fluid and GM additive only a few times in that time, too. It has never had the "lh turn jerk" issue as our '69 C-10 has had since new (even with fluid changes and GM additive in it, when newer).

To me, if the clutches are flaky, it should have similar "feels" in either direction, not just one. Clutches have to slip both times.

When the fluid is changed, with the rear cover removed, there should have been "friction material" in the drained fluid. IF the clutch plates were dying, I suspect.

In another forum, a poster discovered that the aftermarket limited-slip additive did not work as well as the genuine GM PTrac additive did, in relation to the "chattering" on corners.

How long has this situation been going on? When did you first notice it? Have you enlisted the aid of a friend to look and listen to the car, on a quiet and empty large parking lot, to see if the origination of the issue might be determined? Does the "feel" start immediately or after the rh frt wheel makes a revolution or two?

For grins, jack up the rh frt wheel, turn the steering wheel to simulate a rh turn, apply the brakes, then release them, and see if the wheel turns easily or is dragging due to the rh frt fluid line starting to de-laminate internally, keeping pressure on the caliper causing that front brake to drag when the wheel is turned for the rh turn. Jack the wheel under the lower ball joint so the ride height of the body is not affected. Jack the lh frt wheel, similarly, to compare the effort to turn the front wheel.

Just some thoughts,
FS87LT
 

8zero

New Member
Aug 13, 2023
8
Appreciate the thoughts. Been going on for a year. Starts about 1/2 way through a tire rotation. I did use the AC Delco additive to ensure proper fluid, maybe GM P trac is different? Yes it is both directions LH or RH turn. I pretty much ruled out the brakes but did not really inspect the lines so that is a good point. I just swapped everything including bearings, re-greased caliper slide pins, made sure everything was free etc. This is how I ended up at the rear end, it is the only thing left that had the ability to affect everything regardless of turning direction. It literally feels like 1 rusty ball in a ball bearing. 9/10ths of the tire rotation is fine, then you get to that one rough spot and it like pushes through and you can feel it through the whole car.
 

FS87LT

Veteran Member
Apr 3, 2010
981
DFW, TX
Appreciate the thoughts. Been going on for a year. Starts about 1/2 way through a tire rotation. I did use the AC Delco additive to ensure proper fluid, maybe GM P trac is different? Yes it is both directions LH or RH turn. I pretty much ruled out the brakes but did not really inspect the lines so that is a good point. I just swapped everything including bearings, re-greased caliper slide pins, made sure everything was free etc. This is how I ended up at the rear end, it is the only thing left that had the ability to affect everything regardless of turning direction. It literally feels like 1 rusty ball in a ball bearing. 9/10ths of the tire rotation is fine, then you get to that one rough spot and it like pushes through and you can feel it through the whole car.
Thanks for the additional information and clarifications. If you used ACDelco PTrac additive, that should be fine.

One other consideration might be the rear wheel axle bearings and/or related wear on the axles themselves. When that happened on my '77LT, it was a grinding noise I could mimimize with a light foot brake application, as the brake shoes against the drum would re-center the shaft in the bearing, thereby stopping the noise. Except this was all the time, including no turns.

Keep us posted, please.
FS87LT
 

8zero

New Member
Aug 13, 2023
8
Will do. I thought about pulling the axles and throwing new bearings in when I drained it but decided against it because I figured there would be other signs at higher speeds if they were worn. Plus if the clutches are gone, I’ll end up doing bearings when I replace the posi unit. Which it sounds like might be the next step after I check a few more things that were suggested here.
 

8zero

New Member
Aug 13, 2023
8
Well after further investigation and much deliberation, I made an appointment for this Monday the 18th to have the rear end rebuilt. This is not my expertise, so I’m having a reputable drive line shop do it. All new bearings, seals and a new posi unit. I’m really starting to think it may be a combo of possible worn carrier bearings and toasted clutches. It is a 43 year old unit so freshening up isn’t going to hurt anything either. Hoping they find a “smoking gun” on tear down! Will update with results.
 

8zero

New Member
Aug 13, 2023
8
UPDATE: Well, the posi clutches were shot, pinion and carrier bearings worn, axles pitted. Full on rebuild coming my way. Going with a Yukon HD Duragrip posi unit and upgrading to 30 spline axles. All new bearings, seals etc. Should drive like new. I’ll let you all know when I get it back in about a week!
 




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