My 93 with 210K miles threw a new one at me the other day. Not 1 mile from work with AC running, it started stumbling a bit at around 40MPH. I just assumed it was the AC cycling and the Van was in OD. Dropped into D and drove her on home.

Well this morning was round 2.... got about 2 miles from home and it started stumbling again (no AC this time). Just happened to look down at my digital dash and the speed was jumping all over, finally settling on 0 MPH. I got about a mile down the interstate and it suddenly reactivated and displayed 70mph.... smooth as silk.

OK, so I know that the rear diff. has a sensor for ABS, and obviously a pulse wheel on the diff. Is this sensor the input for speed? From other makes, I understand the pulse train input is "conditioned" in another device ( hardware) which then outputs to the dash display.

Based on these assumptions, I'm going to assume the crazy speed variation was playing hell with the ECU. When it settled on 0 mph it was happy again. Can't just be a display issue as there would have been no stumble. Getting ready to drive her about 3.5 hours this weekend to go trout fishing.... obviously the cruise won't work but otherwise it appears to function WITHOUT the speed input.

Any others have this experience? Root cause(s)?

 

3.0? 4.0? AWD? CE light? Flashing CE light?


And when you say digital, Do you mean you have an Electronic cluster or an analog cluster with digital odometer?

 

4.0L, occasional flashing ABS light (for the last 10+ years), no AWD.

Today drove the old Van to work, no issues. As I believe, this CANNOT be a digital display issue as the machine stumbled concurrently with when it had the flaky speed input/output.

I've got the crappy HAYNES service manual, and doubt seriously it has any details at all about the vehicle speed input/conditioning of signal.

Of the top of your head(s), does the A4LD have a physical connection for speed signal to the dash/ECU, or is it from the rear diff. ouput?

 

The speed sensor for 1992 and newer vans is in the rear differential. The A4LD will have connections for controlling its shifting, clutch engagement, backup light, and park/neutral starter enabling.

Your symptoms sound more like a more general electrical problem than the speed sensor. It's like some critical wire is having a bad connection, causing the engine computer to stumble, as well as messing up the digital display.

Just as a reference, my digital dash went out once due to bad contacts on its modules. This lead to the display going weird, but did not affect how the engine ran. I took apart the dash and cleaned out all the contacts and re-installed, and everything showed up normally again.

The ABS light coming on/off is due to a problem in your braking system that you should fix. It will affect things like the transmission shifting and locking, cruise control, as well as the ABS itself.

 

The only continuously ON idiot light on my 93 digital dash is the REAR ANTI-LOCK.... which of course is the same sensor.

I don't doubt your conclusion about connections of the digital dash module to the wiring harness. The weird thing is only the speed display jumped around.... all other display values were rock solid.

Drove about 500 miles this past weekend and it only blipped once. I'm certain the problem will reveal itself soon enough.

 

Well the situation is clearly disintegrating.... drove the Van today at lunch and it started into a speed jumping fit for almost 3 minutes.

All other digital dash display values are rock steady.... temp, RPM, fuel.

After the seizure the speed settled in at the true value and the bucking/stumbling is gone. Guess I'll turn Google loose on this one.... there must be either a dying signal conditioner in front of the ECU or the ECU itself is going south in just this area.

Once it gives up the ghost, I'll update this for Aero posterity.......

 

My money is on: the diff sensor is coated in metallic particles. I've seen it many times on RWD Fords.

Clean the area, remove the one bolt, pull the sensor out and wipe it off with a clean rag. You'll be amazed at how much metallic particles will be stuck to it.

Here's a pic of my spare 3.73 Trak-Loc diff (click on image for larger):