I have a 97 f150 4x4 with a 4R70 transmission. I was driving the other day and the O/D light started blinking. I changed the transmission fluid and it was pretty bad, and the transmission was starting to shift hard. I thought I had fixed the problem, but whenever I get onto the highway for more than five minutes the light starts blinking again.
I've plugged into the ECU and the computer threw codes p0503, p1000, and p1500. I know p1000 is just saying the tests aren't completed yet. P0503 and p1500 both pertain to the VSS.
What do you recommend as a first step to fixing this? The VSS is in the rear differential, right?

 

Have you lost or does your speedometer fluctuate?
The VSS drives it as well as being used by the PCM for engine and trans control.
You should find the VSS on the rear output housing with a connector on it.
Ways of testing is is the speedo working?, back probing for a signal check etc.
Haynes book pages 6-9/10 give some info on this.

 

I have a 97 5.4L with the e4od. The VSS on that is on the tail of the tranny, drivers side, at the back. The sensor on the rear end is the RABS (rear abs) sensor. Two different sensors, both relatively cheap ($14/$15). Not sure if your 97 is the same since it is the 4r70w tranny.

 

The speedo works perfectly as far as I can tell.

 

The siganl out of it is noisey, the harness has a rubthru or some fault the PCM is detecting.
Make a decision as to what to try first.

 

If you are never losing your speedo, then it is probably noise as mentioned above. I can check the codes tomorrow at work and find out for sure. Fords actually have a code for VSS interference. Very sensitive to it.

Chad