I have a problem with my '92 E-150 similar to others discussed in FTE, but I'm still stumped.

The speedometer has started to either read correctly, read nothing, or jump between the two. Sometimes it's several minutes of correct, then minutes of nothing, then jumping. No pattern. Odometer doesn't increase when the speedo isn't reading.

The REAR ABS light comes on after about 20 seconds and, now, stays on. It doesn't go out when the speedo is reading correctly.

Trans seems to shift fine; brakes seem to work as well as ever, but I haven't tried to really lock 'em up. Master cylinder is topped up; brake lights work.

I checked all the connectors I could find, including those behind the instrument cluster, and removed and cleaned the VSS on the top of the differential. Wiggling any suspect connector or wires while the vehicle is in motion has no effect.

When I turn on the ignition with the RESET switch pressed the speedo swings to full and back and the display shows "E 03"; press again and it reads "972CAL." I conclude the PSOM computer is working.

I understand the REAR ABS light may mean a problem detected, not necessarily a failure or no ABS action. I also understand that the ABS computer uses the same VSS signal as the PSOM, but I don't know who gets it first. I can't find a wiring diagram for this system.

The only free conectors I could find inside are the single for PSOM cal (L-BLU/YEL wire) and a 6-way connector over the parking brake with 4 wires (L-BLU/BLK, RED/YEL, BLU, BLK). None near the ABS computer on passenger side. Chassis may be "commercial".

Any help would be greatly appreciated.

 

I had the same problem in my 1993 Ford F-150 XLT. I replaced the rear speed sensor above the differential for about $15 and it fixed the problem instantly. Also my cruise control worked again. It's either that or a problem in your console.. I pulled my whole console out before I thought of the speed sensor. DOH.

 

It doesn't sound like the PSOM is the problem. Sounds like either the VSS or a damaged tone ring on the rear axle (the toothed ring the sensor reads off of). Pop off the rear diff cover and inspect the teeth on the ring behind the ring gear. Look for any broken or damaged teeth. Since the problem is irratic, I'd lean more to the sensor than the tone ring, but it's free to check it first. Check for corrosion on the connectors with that sensor, and follow the wires back as far as you can. Check for any cuts or broken wires. That system is very sensitive, as it's a pulse signal and anything that interrupts that signal will give you trouble you've described. If the ABS light is on, the ABS system is not functioning (not active).

 

I replaced the vehicle speed sensor ($23 from Ford). Problem gone! Thanks for the help, and...Doh!