This is a primary cause for TCC-related slip code P1870 and overheated converters. Exhausting apply pressure also diminishes line pressure, which can result in burnt clutches. Excess wear at the isolator valve bore reduces TCC apply pressure.
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Depth Guard to prevent over boring.
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I can almost promise you this is your issue, I spent months chasing this issue down and no one ever talks about this unless you look specifically at the transmission. The valve in this wears the edge of the valve body down until fluid can get past it and then it can't lock up at cruising speeds. You have to slowly ream it out with a special bit and a rachet, but if you put this in you will be able to lock up again. Make sure you clean the metal shavings out with some transmission fluid before installing the sleeve.
I also did these upgrades to the valve body when I was in there:
TRANSGO SHIFT KIT. TRANSGO VALVE BODY SEPARATOR PLATE. With the 4L60E 4L65E 4L70E 4L75E Transmissions. light & Hard 1-2 shift. Has new design Isolator Valve that works perfect in worn. _______ A Must to remove code 1870 _______.
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TransGo--Heavy Duty--Aluminum--Accumulator Piston & Seal--With Legs. TransGo 2nd gear accumulator piston. 4L60E & 4L70E series parts in. Fits accumulator housing with casting. Made of high grade aluminum.
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Fits all 4L60E, 4L65E, 4L70E, 4L75E 93-on. Forward Accumulator Piston. Replaces both plastic and aluminum that prematurely break and wear out.
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I also replaced all the solenoids while I was down there.
I had never touched a transmission/valve body before and this was pretty easy and I have had zero issues, just be meticulous and don't forget the O-ring on the accumulator valve or you get to take it all back apart
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P.S. My transmission had been rebuilt about 50k before this too so don't let that sway you.