First I had a f250. 96? Gas 4x4. Speedo didn't work. Wouldn't shift out of 1st gear. Replaced speedo and did a Trans drain and fill with new filter. Shifted good but seemed like it would downshift at highway speed then shift up then back down..now my 94 f350 4x4 is doing the same thing. I just bought this truck. Drove over 500 miles with this issue. Last week my oD light flashes off and on. I went and replaced the speed sensor on the rear diff. It seem to shift fine and od light went off. Later in the day it started shifting down and up and down and up at highway speed. But I turned off od and the rpm shot up to about 2800rpm. I hit the od to on and rpm went back down to 2100 which is normal. But when this downshift upshift issue happens the rpm only changes maybe 100 rpm. Example I'm doing 60 the rpm is at 2g...the issue happens and the rpm goes to 2100 for a few seconds then the rpm go back down to 2g as if the truck is downshifting and then up shifting but doesn't seem to be a big change in rpm like it's actually changing gears..any idea? Did that make any sense haha. Of by the way previous owner said transmission was rebuilt about 6k miles ago

 

Sounds like the torque converter locking and unlocking.

 

How do I correct this? It drives fine. It's just annoying. Overdrive doesn't flash on/off anymore since I changed the speed sensor

 

Check for codes. Without codes it's just guessing.

 

I thought the od light flashing was a warning? But it hasn't came in since I changed the speed sensor. No check engine light or anything

 

So I did some more research and I found it possibly could be the tps? Not sure tho. But also as stated in the above reply that it seems like the torque converter is locking and unlocking at highway speeds. How do I fix this? Will it hurt the transmission? As stated the previous owner said the transmission was rebuilt about 6k miles ago. It drives fine its just annoying to have it jump up and down on rpm and sound of the engine.

 

Check all brake lights, including the center light in the cab roof. Does it happen when you're changing lanes and using the turn signal, by any chance?

And +1 on the codes, in case any are remaining after the VSS replacement.

 

I'll pull codes if there are any. But no not changing lanes or driving fast etc etc. Yea all lights work and dome lights and interior lights. Would it be the solenoid pack inside the transmission? I'm not sure. Like I said my old f250 had the same problem but I sold that truck

 

Dome lights aren't relevant. The center STOP light in the cab roof is. That works?

 

The light inside my cab comes on when my drivers side door is open. It turns off when it's closed. The little lights beside it works when I turn them on as well. So yes it's completely working

 

Those are the interior lights. What about the STOP light?

 

Forgive me. The light on the outside rear of cab. When the door is open it lights up yellow. I haven't tested to see if it's a brake light. I honestly didn't know it was a brake light. I'll pull over and check. I'm driving now and it's doing fine but when I first left my house it did the upshift downshift issue it only happens sometimes but I'll check the brake light

 

Okay so all brake lights are working including the 3rd high mount brake light on back of the cab.

 

Any other ideas? I'll get codes pulled asap but I honestly don't think there are any.

 

It is a warning. It warns that one or more transmission codes have been stored in the computer's memory. The codes will stay there for about a month after the last time the light flashed.

That's possible. It's one of about 50 different things that can make the OD OFF light flash.

You start by reading the codes. If you don't want to read codes I can list all the parts in the electronics and hardware and you can start replacing them until you find the bad one. I don't know how to find out what's the problem without reading codes or using very expen$ive diagnostic tools.
Yes it will.

That's another of the 50 things that can make the OD light flash. It would be on my list of guesses to change and see if that's the problem. It's only $250-$400 for one, do you want to try that to see if it is the problem? Or you could read the codes and then we'd know where to start looking. Your call.

I know that codes have been stored. It is possible that they have been cleared, and it is also possible that you won't find the right scan tool and won't be able to read transmission codes. But they are there....