Noticed a strange thing this week. The transmission was locking and unlocking the torque converter and acting funny while driving around town, and I looked at the gauges and noticed that my speedometer needle was fluttering. It would display the correct speed, then the needle would drop a few mph and bounce a little, then it would correct itself after a couple seconds. When accelerating from a dead stop, the needle would sometimes "stick" to the bottom peg (0 mph) and then suddenly jump up to the correct speed (usually 30 mph or so) and proceed to climb the gauge like normal. It did this at almost every stop. The truck behaves fine when it's cold, but drive it for 3 minutes and the needle goes bananas. Seems the longer you drive it, the worse it gets (or maybe it's temperature related?).

By the time I got home, I had a check engine light: Error P0721 for "Output Shaft Speed sensor signal (signal out of range)." According to the archives, diagnosis of error code P0721 involves checking for:

- Wiring misrouted
- After market add-on
- Wiring damaged
- Wiring insulation wear
- Check routing of harness.
- Check wiring and connector for damage.

I took the truck to dealer (the "Overdrive" light on the column shifter was flashing by this point) and they had the truck for several hours and then told me "is there any way you can leave it with us for an extended period of time? We need to wait for Ford to reply back to the question we posted about your truck on their message board." Huh?

- They replaced the output shaft speed sensor (OSS) on the back of the transmission.
- They replaced the vehicle speed sensor (VSS) on top of the rear differential
- They inspected the wiring for any worn insulation, chafing, or clearance problems
- All connectors and harnesses looked fine
- They said they looked at some ring or gear inside the rear end and the teeth looked fine

And they still have no idea what is causing the speedometer to go crazy. They did say that when they pulled the old VSS out, they noticed some metal buildup (more than usual for my mileage) which they assume is from wear inside the diff. They checked the new VSS again later in the day and noted it had already accumulated some metal too ("large particles"), and suggested the tiny shavings could be interfering with the magnetic sensor.

Any other ideas???

 

Additional clue:

I went to run some errands this morning, and it was chilly out, about 50 degrees. I was able to drive a good 30 minutes around town before I noticed the speedometer needle wiggle. Toward the end of my trip, after a solid 15 minutes at hwy speeds, I noticed the speedometer needle starting to misbehave again, but no where near as bad as it acted yesterday. It was 60 degrees by the time I got home (still relatively cool, compared to the 80 degrees we had yesterday afternoon).

I guess this confirms it may be (operating) temperature related?