Ok, I know the speedo gear is in the transfer case, I know the 'vehicle speed sensor' is really in the t-case, HOWEVER;

The problem is, half the owners of these trucks thinks the speed sensor is in the rear diff, including some of the parts stores around here. I'm getting RABS code 6 occasionally. I have a bouncing speedo, replaced the sensor but I really think the speedo gear needs replaced.

Well, because everyone has a different opinion on where the speed sensor is for my 1990 Bronco, I'm not sure which speed sensor is throwing an erratic signal.
For this code, is it for a messed up signal at my RABS "speed" sensor or at my differential at the "vehicle speed sensor"?

Thanks!

 

For that vintage truck the speedo and RABS systems are seperate, the speedo is cable driven off the T-case and the rear axle sensor feeds the RABS system only. Somewhere around 1992 Ford integrated the two systems and did away with the cable driven speedos, you can tell what system a truck has by looking at the odometer, if it's mechanical then the dash is cable driven but if it's LCD it is electronically driven from the combo RABS/VSS sensor in the rear axle.

 

Paul's method of determining which speedometer/odometer system is in he truck is spot on. With that in mind, 1992 was indeed the first year for the PSOM (Programmable Speedometer/Odometer Module) that allowed the ABS and the speedometer/odometer to use a common sensor to determine both vehicle speed and wheel slip. The location changed from the tailhousing in the T-case to the top of the differential housing in the 8.8 rear axle assembly in 1992 with the advent of the PSOM.

The term used to describe the sensor may have changed when the location changed but no matter where it is located, the VSS (Vehicle Speed Sensor) does the same thing... it counts the teeth in the trigger ring and the computer calculates RPM of the shaft the ring is attached to using preprogrammed parameters. That is how the VSS is used by the PSOM to calculate vehicle speed. The ABS computer "knows" how many RPM the shaft SHOULD be turning at a given speed AND engine RPM. (This is why you have to reprogram the PSOM if you swap tire sizes or axle gears if you want the PSOM to work properly). The computer compares the "known" information with the "actual" information coming from the VSS, PIP and preprogrammed MPH information. If the "count" coming from the VSS is too LOW based on the engine RPM the computer alters the brake pressure to the rear brakes IF they are applied when this condition exists. A low count from the VSS indicates that the wheels are locking up under braking so the system reduces applied pressure to the brakes.