I'm having this "Tach Data Error" issue on my 95 Continental as well and I had a mechanic hook up a scanner to try and find the related error code(s).
However the scanner couldn't read anything at all from the computer.
He told me that this most likely means that the computer itself is dead and needs to be replaced.
Before I go down that road and spend the money to replace the computer, could there be any other reason for the scanner to not be able to get codes back?