I have a 1990 Ford taurus and it has recently stopped going into gear. on 1990 Ford Taurus

It goes into gear but the speedometer will not register till the car warms up usually about 15 to 20 minutes, seems like you are in nuetral. Before it was just doing it once, now it slips back and forth when it comes to a stop. I have talked to a couple of mechanics and they say it may be a moderlator, or the transmission needs to be replace.

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This is not what you want to hear. The two weakest points with the Ford Taurus is head gaskets and Transmission.
The car is 21+ years old. It is time to consider buying another vehicle.
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did you change the fluid and the filter?? they also have failures on the front pumps and converters. no matter what, it most likely will have to come out and be overhauled.