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Resolved! I just want to close this thread out on a positive note. The 3-4 shift problem has been FIXED. As some of you suspected, myself included, the source of the trouble was in the valve body, specifically, the 3-4 and Low Servo Capacity Modulator Valve circuits. I inadvertently swapped the springs between these two valves (thanks JK080 for the spring arrangement info) during the initial rebuild process. Having the lighter tension "white" spring behind the 3-4 modulator valve apparently created an imbalance in the 3-4 shift circuit. It should have had the "yellow" heavier tension spring installed with it and the white spring installed with the low servo valve. Now, although I did replace the tcc solenoid and the harness assembly with new ones this time around, I did so purely to extend the operational life of the transmission. There were no problems detected with either of these original components. The trans shifts beautifully and is silky smooth at the top end with OD, and perfect closed throttle coasting conditions without going into that rapid 4-3 shift cycle. Thanks to all those who offered their experience to get this one resolved!

Lance Bowling
Technician/Owner
Lance Auto Diagnotics
Gallatin, Tennessee, USA