why is my overdrive coming on

Your overdrive button/switch needs to be replaced.

If you got it fixed please share, I'm having the same damn problem

The Oval Drive stays on blinking but it don't it don't cut off on a Grand Marquis 02

4-12-2020 - I have a 1997 Cougar that has the same issue, I had the transmission redone and the flashing begun few weeks after, the harness of the car connecting all the sensors and solenoids under the car, has individual wires that had dried off and oiled through the years, some of them were touching, that caused the computer to store codes and ordering all kinds of mix open and close solenoids and sensors. My transmission doctor replaced an output speed sensor (part number DR6179) that made a big difference. Don't buy NON Ford products, after market makes a bad outcome. on Monday we're checking the computer yes tomorrow. Older cars and specially ford will have this issue because the harness once again is wrapped around places of high heat and the wiring harness will cause many issues. I experimented driving at steady 60-65 Mph and suddenly the transmission just went NEURAL on me while Drive was on. those will be the input and output speed sensors. Carlos my Trans Doc has been using a Snap-On Diagnostic plug in and we took the car for a spin on the highway real time, O/D OFF started flashing again after 9 miles and the graph on the Snap -On was reading all kinds of errors coming from the Car Computer. That's why he said, its your computer, suddenly the graph went to Zero while driving 55-60 mph, the Snap-On could not read the output speed sensor. I'm in Guadalajara, Mx wanting to get back to Houston Tx, safely. without any transm electrical problems. hope this helps .

Adding to, Yes I turn it off , let it cool, turn on, light is out for a few minutes and no problems, until it starts again, I think that the temperature is causing it, mainly I think that the harness and some sensors leaking a bit of oil to the connectors are contributing, more to discover.