erratic MPH readings on scanmaster??

[1985GN]

i just noticed tongiht while driving the car that the scanmaster is reading the MPH all over the place boucning form like 90 to 78 to 40 and all over. the mph on the dash is accurate. before i go tearing into stuff i just want to know if anyone has had this happen? im taking a guess and saying its the VSS?? if so how hard is it to replace and can i just grab one from a parts store?? this is important as my timing is MPH based since the car has a TH350 trans in it....thanks everyone

 

[SHADOWII]

Can you ...

read your malfunction codes, it should indicate code 24 for the vss.

 

[Red Regal T]

Sometimes just shutting off the car and restarting ends readout problems.

 

[turbo nasty]

Mine did that when the speedo cable needed greasing. But, the neddle had bounce also

 

[NJ Torque]

Hey Make sure your VSS is secured to the back of the dash or somewhere.. if its not, it could show erratic readings...