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1990 Deville not shifting into overdrive.........Does anybody read these "help" topics anymore, and offer advice?

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My 1990 Sedan Deville is not shifting from 4th gear into overdrive anymore, thereby dramatically reducing my fuel mileage at higher speeds. No codes on computer. But, I can manually make it shift into OD by utilizing the self diagnostics mode on the cooling/ heating control unit. I wasn't aware of how many different diagnostics and readings can be gleaned from this procedure (engine temp, RPM, etc.) Any advice would be appreciated, it seems not too much advice is offered on this site anymore, and a lot of advertising instead....HELP!   Thanks

Bill in VT  

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