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You can purchase aftermarket overdrive units from suppliers such as Gear Vendors, however another option would be to install an overdrive automatic such as a 700r4, 4l60e, or 4l80e.
Automatics would include th350, th400, or the 700r4, if you want a overdrive.
TH350, unless you want an overdrive, then, I would go with the 700R4.
if you have overdrive in that car it means that your about to go into overdrive
If its a 3 speed auto, I would say its a Turbo 350. If its a overdrive auto (4 speed) I would say its a 700R4.
what would transmission not to shift in to overdrive
the overdrive sencer
Improperly adjusted TV cable, which will also lead to early Transmission Failure. These adjustments are critical and can distroy a 700r4 and 200r4 in less than 200 miles.
If your 1997 Chevy diesel will not shift into overdrive, you might have an issue with a transmission sensor. Some of these vehicles have several sensors that must engage in order to shift.
If the overdrive switch is turned off then the car will not go into overdrive. As it is not in overdrive, then it will not downshift out of overdrive as it normally would. It also will not shift to 3rd gear above a certain factory preset speed. Turn th overdrive on and see if it operates normally.
Linkage adjust
dirty trans fluid might be the cause. have it changed, but not flushd