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I understand that your transmission is slipping with overdrive engaged, but it is not slipping when when overdrive is turned off, as I'm understanding this the transmission is slipping in 4th gear but not any of the lower gears, correct? If this is the case I would like you to try shifting into the manual 2 position, if the transmission also slips in manual 2 then I suspect that the overdrive band is slipping and needs to be adjusted. If it does not slip in the manual 2 position please let me know and I will continue to work with you to help diagnose your problem.