Erratic speedometer reading

I have 1999 Windstar and the speedometer is jumping erratically around +/- 15 MPH the actual speed. I've searched this forum and google and it would seem that it's a vss problem, but I'm not sure. Any suggestions?

Also, I am confused as to what the difference between a VSS and an OSS are? I download the Ford CD service manual and it explains how to replace the OSS, but doesn't say anything about the VSS.

I believe on earlier models there was an issue with the grease in the speedo cable. I don't know if that applies to a 99 though.

I believe on earlier models there was an issue with the grease in the speedo cable. I don't know if that applies to a 99 though.
Thanks for the reply. I guess I should have also mentioned I have a digital odometer. I was under the impression that this has an electronic sensor rather than the old-type speedometer cable. Can anybody confirm that? Also, I'd still like to know about OSS/VSS difference if anybody would be willing to address that. Thanks.

I don't know of a OSS right of the top of my head.

TSS is Turbine Speed Sensor.
This is how fast the shaft out of the Torque Converter is turning.
This is before the different speed selections of the transmission.
The PCM knows how fast the shaft into the torque converter is turning by the engine speed.
It needs to know how fast the shaft out of the Torque converter is spinning to correctly handle transmission control.
This sensor is conveniently located on the side of the transmission assembly, on the driver's side.
This sensor also would not have anything to do with your speedometer reading.

VSS is the Vehicle Speed Sensor.
This is measuring the speed of the output of the transmission.
This has a electronic sensor and a speedometer cable (mechanical speedometer).
This sensor is INCONVENIENTLY located at mid point of the rear of the transmission.....over a catalytic converter, with a heat shield (sharp edges) over it.
This sensor has everything to do with your speedometer reading.

I am not familiar with the electronic dash, and don't have access to any TSB's for years other than 1996.

Thanks for the information. I appreciate the info that you and all the other regular contributors share with all of us. Being a fellow Windstar owner, I need all the help I can get.

Just to clarify about the OSS, I uploaded a section of the manual that pertains to it. It sounds like that it is the same thing you referred to in your post as the VSS. Here's the link:

http://www.commongroundband.org/windstar/

Thanks again.