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.