What is VSS Vehicle speed sensor?

[larshell]

Hi I'm new here. I'm from Manila, Philippines. Very few owner's of Expedition here and I'm having trouble with my 2001 Ford expedition XLT 4X2.
Problem is on shifting if I step on the gas it does not down shift, it runs very slow. especially when incline position. RPM will drop like the engine will stall then it will slowly go up.
I change the fuel pump, got new plugs, new fuel filters and clean IAC etc. Did not do anything. Went to local Ford dealer to have it check
scanner said VSS malfunction vehicles speed sensor but as per the technician it has nothing to do with my complains. According to them VSS is for speedometer only.
It will be a waste of money if I change VSS according to them.
This problem occurs intermittently. Sometimes it doesn't have the problem.
Has anyone here got the same symptoms or problem?
What does VSS do?
What is the reason for it to malfunctioned?
Please help. Thanks!

 

[jayhawk2014]

What speeds is this happening at? If you are going top-gear fast, just need to push the pedal down farther until it shifts.

 

[larshell]

Around 40 to 50 kph. Problem is 3rd gear going 2nd gear or 4th going down to 3rd gear.
I have floor it down for the transmission to response. I got no engine response if step on the gas just enough. I have to floor it to have a down shift even on low speed. THanks!