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-   -   4L60E Screeching in Manual 3/2/1 (https://www.hotrodders.com/forum/4l60e-screeching-manual-3-2-1-a-519507.html)

spfautsch 01-29-2019 04:53 PM

4L60E Screeching in Manual 3/2/1
 
I've been searching with zero luck for days on this one. It seems there are some experienced 4L60 mechanics here so I thought I'd ask and see what turns up.


Vehicle is a 2000 GMC Jimmy 4x4 with the 4 button "soccer mom" auto 4wd. I'm currently sorting out issues with that, but believe my transmission issue is unrelated.


We've owned this vehicle since new, and I'm hoping to get it back in repair after 5 years of use as my son's short-trip transportation - back and forth to school and work, less than 6 miles a day about 6 days a week. Before that I did tow our boat with it and on one occasion overheated the trans pretty badly. The fluid has been changed at 75k and now at 139k is again in need of a change.


With normal use everything seems to be fine - no slipping and all gears seem to work and shifts feel normal. But if manually shifted below overdrive (d3/d2/d1) the engine bogs down, the lights all dim and an incredible screeching sound is heard until the selector is shifted back to overdrive.


I've searched extensively but haven't found anyone complaining of this particular symptom. I'm wondering if possibly there's a stuck check ball or some gasket material missing allowing a leak between the D3 and the reverse input apply circuits.


Back in my teens I rebuilt a half-dozen TH350s with 100% success, so I'm not necessarily intimidated by rebuilding this much more complex beast. Its the removal and replacement task that at age 47 I'm not quite so enthusiastic about.


I think I'm going to drop the valve body, clean it and see what I can see. Was wondering if anyone has any ideas as to what to look for and where. Any technical ideas greatly appreciated.

454 Rattler 01-30-2019 01:12 PM

I would guess that if this is near the top of the page you might get some action.


454 RATTLER

Crosley 01-30-2019 08:08 PM

In the shifter position of 3 , 2 , 1 ... Not OD .. the over -run friction pack is applied. The clutch hub for the over-run frictions is weak. Often the center breaks out. This may be the cause of the noise you hear.

spfautsch 01-30-2019 08:27 PM

Overrun clutch was the first thing I considered, but would a broken hub cause the engine to bog down when sitting stationary? Seems like overrun clutch should only be absorbing torque when moving faster than the selected gear dictates.



Also, to describe this generously, is a blood curdling squealing / screeching sound. To put it mildly, I'm afraid to test because it's so loud and obnoxious I feel like if I leave the selector in a manual gear for more than a few blinks of an eye hard parts are going to start flying.

spfautsch 01-30-2019 08:39 PM

Also (i wanted to edit the post but couldn't figure out how) according to the range reference chart in the GM powertrain manual the overrun clutch shouldn't be applied if the selector is in D3 but the trans is in low. I'll test tomorrow to be sure but nothing seems amiss (like the trans starting out in 3rd / direct drive) from a stop. But the squealing most definitely happens when stationary and selecting manual D3.

BogiesAnnex1 01-31-2019 12:49 PM

First place I suspect is also as others is the overrun clutch, it is common to all forward manual gear selections of 1, 2, and D3 but not with D4 or reverse selection. It is a common failure of transmissions that have been overworked and overheated.

Bogie

spfautsch 01-31-2019 05:55 PM

1 Attachment(s)
I appreciate the input, but again that's where I went initially and have since ruled it out because the overrun shouldn't have apply pressure when shifting into D3 while stationary, only when the trans has shifted into 3rd.

spfautsch 03-01-2019 09:22 AM

Thought I'd come back and update the thread in case someone else finds it searching for this symptom. Teardown of the valve body found nothing amiss. I was expecting to find metal in the pan if something broke internally but the amount of steel on the magnet seemed consistent with the mileage.



I'm still at a loss as to what could be getting apply pressure when manually selecting d3 while not in motion and the trans is in 1st, but I didn't find anything at all out of place. Oh well...


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