I have a '99 F250 SuperDuty Diesel with an Automatic Transmision. There is a button on the end of the gear shift which allows you to turn off overdrive -- in other words the transmission won't shift into overdrive once you press the button.
Here's my problem. I start off driving normally. I've not done nothing to the overdrive button and the truck shifts normally into overdrive. 10-15 minutes down the road, the overdrive light (which indicates the state of the overdrive button) is FLASHING. Pressing the button doesn't stop the flashing. Once this flashing starts, the only thing I notice is a much more abrupt shift from 1 -> 2 gear. Odd.
This happens to be an old thread, however stick around and someone will be by to talk with you.
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my old 99 done the same thing and the sensor on top of the rear end had gone bad. it's easy to find it's right on top of the rear chunk, you have to take it out to add rear end dope, it has a magnet on the end to catch metal fragments. i know this sounds confusing but get under your truck and you'll see what i'm talking about. i think the sensor is only about 20 bucks from powerstrokeshop.com. PM me if you still can't find the sensor.
The only way to know what's going on is have the code read. When the OD light flashes an error code is stored in the computer's memory. A Ford specific scan tool is needed. The auto parts stores that say they will read codes free usually can't read a transmission code.
you need to find some one with a shop type scaner to read the trans codes. thats the starting point. not running and getting a trans rebuild . the other guys have said as it should be started every time. read the codes first.
did you find the issue to your overdrive problem? I am having the exact same issue with my '98 ranger. drives fine until I hit about 50-60 mph then o/d shuts off and shifts very hard when I slow down. The shift button does nothing at this point. I have changed atf, oil, filters, plugs and wires, etc to no avail....I am leaning towards the speed sensor but wondered if you got your issue resolved.