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Answered 2015-07-15 19:39:48

thats ur transmisson, my friends Jeep does it too. try some of the stuff they carry at the auto store. if that doesnt work, its expensive to fix

My '97 Bravada had a new tranny put in after it did this... (Fortunately mine was still under warranty)

This is a known issue with GM vehicles that have the 4L60E transmission. The issue normally presents itself after the transmission has had time to warm up and has been driven approximately 50 - 60 MPH. At highway speeds the torque convertor locks up for increased efficiency and so that it generates less heat. The lockup is accomplished by a solenoid in the tranmission's valve body which applies the torque convertor clutch (TCC). The solenoid and valve body itself are made of different metals which cause them to expand and contract at different rates. This constant variance in rates causes the valve body to wear away. The wore valve body allows Transmission Fluid to slip past the solenoid, causing a weak TCC apply. This weak apply is detected by the tranmssion as a slip. When a slip is detected the computer increases the line pressure to the shift solenoids to make the shifts firmer to minimize damage. This increased line pressure is what causes the hard shift that you feel, it is most noticable from 1st to 2nd gear. If the transmission detects the slip on two consecutive trips it will illuminate the "Check Engine" or "Service Engine Soon" light and store the code P1870.

If you ignore the problem, you will end up damaging your transmission and it will require a rebuild or replacement. However, if you catch it in time you can generally save your transmission. There are two ways to remedy the problem. 1) Replace the valve body. (GM has a new updated one) 2) Repair your current valve body, this can be done with special repair kit available through Sonnax.

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