97 Taurus Transmission / Speedometer relationship

Greetings,
Our Taurus has 78K miles on it. Tranny was replaced in 99 under warranty, so we are about 7 years and 50K with this one. Lately weird things started to happen: when we start driving the speedometer is stuck in zero speed, the RPM goes up to around 2500-3000 without changing gear. Normally, at around 2500 when you accelerate hard the gear would change from 1st to 2nd.
In this situation, we need to either keep pushing over 3000 RPM, or let the accelerator go, which is when we feel some kind of a kick, the gear is changed from 1st to 2nd, and the speedometer needle is either remain stuck OR jump to whatever the speed is (25-30 mph).
I believe that the speedometer is indeed connected to the tranny, but what I would like to know is - what is the relationship between the two? Assuming there is a problem with the tranny that prevent the smooth change of gears, why does it effect the speedometer? Aside of that, any idea of what may cause this problem? Also, it doesn't happen all the time...once every now and then. Other two pieces of info: there are no leaks from the transmission and fluid level is good (full). And also, nothing happens during cruise, it is just during start from Stop to movement of the car.
Thank you.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John wrote:

You need one of these:
http://www.partsamerica.com/ProductDetail.aspx?mfrcode=GPS&mfrpartnumberw99723
Your car thinks it's not moving, so stays in 1st gear. Check the wiring & harness 1st, & all connectors for a tight fit.
Rob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks. Can you give me more info...what is this part (other than what it is called), where does it go and also - is it a big deal to replace it myself? I mean - do you need to remove the transmission for that or something? Also, where can I find info about which wires/connectors I need to check and where they are?
Thank you!!

http://www.partsamerica.com/ProductDetail.aspx?mfrcode=GPS&mfrpartnumberw99723
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I'm working on a 99 Ford Windstar Van. Don't think you have the same transmission as it has but someone else might respond to this msg and let you know if this might help. The tranny in mine has a VSS on the transmission. That's Vehicle Speed Sensor. Imagine a little AC generator shoved into the most inaccesable place on the tranny. (On top buried behind engine and above exhaust piples, etc.) The Power Control Module gets its car speed from this unit to make all its decisions. It also drives the speedometer. The end of it is a gear that could wear and loose a tooth or teeth. Hence improper signals. Just a thought.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John wrote:

It's the VSS - Vehicle Speed Sensor - and it is installed in the transmission from the exterior. It replaces the speedometer gear and housing, as the car speed is all done electronically instead of with a cable and mechanical head. It is on top of the transmission, I think driver's side. You need to do a visual inspection & a "wiggle test" of the harness servicing the sensor. It is a small pulse generator to relate to the ECM the vehicle speed.
I advise you to go to eBay & get a $10.00 service manual on CD so you can diagnose & effect repairs such as this. It will pay for itself quickly.
Rob

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.