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4T80-E won't stay in 4th gear


dickstrauss

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96 Eldo. Transmission will shift into 4th, but at the slightest demand for more throttle will downshift into 3rd and stay there. No codes are set. When this has happened I manually shift into 3rd gear, then shift into 'D', keeping a steady throttle. Trans will momentarily shift into 4th, then drop right back to 3rd. Anyone know what is causing this? I replaced the VSS with a new one; same results. Resistance on the old VSS was 1453 ohms; resistance on the new VSS was 1415 ohms. I believe both are within tolerance. Also replaced the TCC solenoid about 1200 miles ago. Using Dexron III.

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I would suspect the shift solenoids, but you do not have any codes. Shift solenoid problems will set codes and the SES light. Isn't 4th gear an overdrive gear only used when cruising? Maybe there is nothing wrong with it.

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A damaged piston seal on the 4th gear servo will also prevent the trans. from shifting into 4th gear. I would keep the gearshift lever in D3 until you find the cause of the problem or you run the risk of cooking the 4th gear band.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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Thanks to all who replied. Today I checked resistances (ohms) on all solenoids and sensors at the 14 pin harness connection. All were within tolerances. Several test drives result in the same symptoms; will shift into 4th, then drop back to 3rd. Still without a code being set. I guess my next step is to take it to the local transmission shop for their diagnosis.

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