1998 CADILLAC SEVILLE, 4T80-E

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Overhaul Trans, 2nd clutch burnt, forward band down to metal & scored drum. Forward band anchor pin moved inward, forcing band into drum. Replaced clutches, forward band & drum, Staked & locktited band anchor pin, new shift solenoids, EPC, pressure switch manifold & input speed sensor & rebuilt torque converter.

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Original Customer Complaint:
Falls from 3rd gear back to 1st on acceleration when warm, but would slip badly in 2nd gear as well.
Current Customer Complaint:
Falls out of 3rd gear back to 1st on acceleration. Will work normally for the 1st couple of upshifts, but once it starts to warm up, then the computer starts commanding the trans back to 1st. Does hold in 2nd gear when shifted manually to that range.
Work Completed This Visit:
Scanned for codes - None. Did movies with scanner and with trans in 3rd gear, can watch the computer command a shift back to 1st gear. Sometimes it will do a quick 3rd, 2nd, 1st gear command. Did movies of shift solenoid command,range sensor function, input & output speed sensor, VSS data, PSM data, EPC control, even TPS data, with no glitches observed in any movie frame for any control device or sensor to cause this downshift.
Summary:
Noticed on numerous roadtests that if you accelerate VERY lightly, it will go into 3rd and stay there, but the slightest increase in torque demand, the computer kicks the trans back to 1st gear. We checked shift solenoid circuit integrity from the ignition fuse to the ECM all OK - normal resistance. We disconnected the solenoid harness from the trans, installed a power wire and two wires to control solenoid grounds manually, (basically simulating computer control), and trans shifts perfectly, 1st, 2nd,3rd & 4th, with no slippages or any performance problems. Suggested to owner that he could have a bad ECM. Owner got used ECM, towed the car to get the computer flashed to VIN, and you guessed it!!!! - still commands trans out of 3rd back to 1st. Any ideas?????