Every time I take a highway trip that's over 1/2 hour, and then leave the highway, the transmission doesn't want to go from 1st gear into 2nd. The engine races in First gear for at least 5 to 10 seconds before it finally jumps into 2nd. The car has been doing this since 60,000 miles, and it now has over 100,000 miles on it. I'm running Mobil 1 Synthetic Automatic Transmission fluid in it, and it is less than 20,000 miles since the trans fluid and filter were changed, including the torque converter.
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1999 Ford Taurus Question: Shifting problem when automatic transmission gets warm
Answer #1CFAR from Central Florida Auto Repair Inc, January 24, 2011, 20:05Technician
Would need to see vehicle and do a diagnostic scan on the trans data to find problem-This is because the trans is shifted electronically and a sensor could be out of range when hot
Answer #2DaveJHM January 24, 2011, 21:58Master
Honestly - I'm not sure about the properties and compatability of the Mobil 1 transmission fluid. I can tell you that the Mercon V that belongs in the transmission is just fine and needs no modification - just regular servicing. It does concern me that it is a variable in the system compared to what is normally run.
It may be just fine on that front...
...however solenoids and the valve body are very sensitive to use of the correct fluids.
I lean against a soleonid issue and towards a hydraulic issue. Only a diagnosis will tell you for sure.