Erratic Speedometer, O/D Light Flashing, But No Shifting Issues

[plut]

I have a 96 Explorer 5.0 AWD with 225,000 miles. My Speedometer has started acting up. It works fine when the vehicle is cold. After about 20 miles usually at highway speeds (65) it will start to wiggle between 60 and 65.

After a few more miles the O/D light will start flashing. The speedometer will gradually drop (still wiggling) into the 40-45 range even though I'm still traveling at 65. If I slow down below 50 it will not register at all.

I have read up on this and changed out my ABS wheel speed sensor in the rear differential. That did not help.

Most of the search results tend to have issues with shifting, ABS light, and Cuise control. Mine does not.

So after a trying a new abs wheel sensor here is a list of my symptoms

Cold first 20 or so miles, Speedo works, no OD light. Everything perfect.

Warmed up (after about 20 miles)
Erratic Speedo
Odometer turns at the rate of the speedometer
O/D light constant flashing.

No other issues
Overdrive button still works when pushed.
No ABS Light
No Check Engine light
Shifts perfect
Tachometer Works perfect
Cruise Control works but tries to constantly adjust for erratic Speedometer

 

[plut]

It ended up being the Speed sensor in the Transfer Case. In 96 the Sensor in the rear differential was only for the ABS system. In 98 they started using it for both ABS and Speedometer.

A good sensor is supposed to ohm out between 190-250. When I tested mine cold it ohmed out at 215. Sure enough I took it for a ride and the speedo was working fine. After driving for about 10 miles it started acting up like clockwork. I ohmed it out again and it measured 238.

I changed it out even though it was in range. The new one I put in now reads 215 ohms warm or cold.