4L60E Thunk (Harsh Shifting)


Posted by dan rollins on April 19, 19100 at 19:25:32:

Vehicle: 96 Tahoe w/4wd, 5.7L, 4L60E, 3.72, 31K easy miles with about 700 tow miles

Problem: Transmission has harsh engagement from park/neutral to Drive, 1-2 upshift is very harsh, 2-3upshift is harsh, 3-4 upshift is better (probably due to gear ratios), and TCC does not lockup. The MIL "service engine soon" is on. The tranny shifts so hard that I think it will self-destruct soon!

Circumstances: The problem occured after changing motor oil and filter plus tranny oil and filter. Used Mobil 1 ATF and converted to the GM deep pan with the extended filter that I've seen on this board. Tahoe was on jack stands with the rear wheels off the ground and front wheels on ramps (tires touching). When checking new ATF level, put tranny in all gears to flow fluid throughout tranny. (don't remember if MIL was on or not at this point). MILdefinitely was on slightly longer during initial start up after engine oil pressure built up. MIL is on steady at this time.

Attempts to Solve: After a test drive around the block and the harsh (thunk) shift from 1-2, immediately checked wiring harness to tranny--OK. Then dropped the pan and checked the wiring on the valve body--seems OK nice and tight connections. Consulted local GM dealer's tranny guys and showed them both stock and deep pan setup. We all agreed that the deep pan should not set a code and/or create this type of shifting problem. They suggested checking the wiring again. Consulted a tranny shop to scan the code. He used a Snap-On scanner with the tranny module. Guess what? No Codes on the tranny side. How reliable is this scanner? It is suppose to be an OBDII. Tech said the code must be on the motor side and his chip or module will not pick it up.

Theories: Default was set by some event (slipping) and now have full pressure through all shifts. If true, how could the VCM detect slip if the wheels turned with the tranny in gear when it was in gear on the jack stands? Will this default setting clear after 40 starts if there is truly no problem? Ran across a Service Bulletin #99-07-30-005 which may be related to this problem, but I have no I idea what this bulletin says.

What do you guys think? I know Traveler thinks anyone with a 96 or 97 4L60E needs an extended warranty, but I didn't discover this board until 6 months ago. Soooo, I need the sharpest tools in the shed to help me out. Is there something simple I'm overlooking?
Thanks in advance

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