When on the highway my 1997 ford tarus GL wont shift into 5th gear

Are you saying that the clutch is not working so that you cannot change gears while the car is running? With the engine stopped, and the car not moving, can you shift the transmission into the other gears? If this is so, then your problem would be a hydraulic or cable malfunction with the operation of the clutch system. If you have a hydraulic system, make sure that you have fluid in the clutch master cylinder and that it is working properly. If it is a cable system, make sure that it is working. c

Your car has went into a fail safe mode, either one because the speedo is not the proper gear ratio for that particular vehicle, not all of them were the same, it depends on the exact model. For instance, a Tarus SE is different from a Tarus LE / SE / on and on, and if the gear ration is not correct, then the TCM will not ssend a signal to the shift solenoids to do their job thus the car wont shift. It sounds to me like your original issue may have been that the speedometer didn't function or work at the cluster, ad you've made repairs to the wrong part. Change your Spped Sensor, and it will be ok, IF the cluster is for that particular vehicle. If it is the proper part, your only issue is the speed sensor.

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check the linkage on the end of the gear shift,

its could be the handbreak thats jamed

it could be the valve body sticking, or worn clutch plates in the 3 & 4th gear planetary drum, or 3rd &4th gear solenoid is bad

its the solonoids on the solonoid block, 3 of them are sticking, there is alot of videos on what to do,on you tube, very common problem, mine runs and shifts great now

You're 5th gear syncro has just gone out. It keeps your shift forks and slider gear from popping off. It also makes it possible to shift the gear in. It's going to take a rebuild of the transmission to fix this problem.

indicates a TCM problem
run the fault codes to determine the exact problem area

have the system tested for a trans related trouble code, u may have a defective OD speed sensor.

Either you have a worn shift fork and fifth is not fully engaging or the synchro for that gear is worn Both are internal and if fork is worn I suggest you have synchro replaced as well..