Join Date: Dec 2008
Location: Jersey Shore (Not Seaside!)
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Going 12 feet / 2 mph is not really a test I would sign off on that everything is good. It's been some time but I remember the speed signal going to the ABS first, then distributed to the other electronics of the truck. If there is any inconsistency off the VSS you can have issues.
While it was replaced, is the VSS a Motorcraft or other vendor part? Some aftermarket are actually the same part as Motorcraft, some are not and do not have the same resolution. It also would not hurt to pull the VSS and make sure it was not quickly contaminated by excessive iron particles in the rear axle lubricant. It will be fuzzy if it is. Anytime I have replaced a VSS I also have replaced the lubricant.
I would also closely check the wire harness between the frame and the VSS as some have experienced a broken wire from movement fatigue.
The best method for checking the signal is to look all the sensors from stopped to 50 mph. An oscilloscope graph view is best but depending on the type of test instrumentation a numeric value that has a high variability at moderate to higher speeds diagnosises a problem.
Formerly FMTRVT, Manager of Vehicle Testing - Brakes
2003 F-350SC Lariat 4x4 6.0L Auto
Truck Modifications as below in Pictorials on Facebook. Google - Facebook TooManyToys
Step Lights;Painted Flanges; Bypass Oil Filter; Heated Mirror Switch; Reverse Lights; 7.3L Fuel Reg Shim; 6 Disc Radio Speed Volume Mod; Coolant Filter, etc.