I have a 91 legend and the D4 light flashes all the time what does this mean?

HI my friend
I have that one too.and the same issue.
but it can run ok right?
don't worry sometime ECU has a problem

The Issue you are having is a result of a faulty VSS (Vehicle Speed Sensor) or it's circuitry. Your first step should be to unplug the electrical connector in the wiring harness at the sensor, and then use a voltmeter to check for voltage at the black/yellow wire terminal. The circuit should have battery voltage available. If there is no voltage available, check for an open circuit between the VSS and the fuse box. Using an Ohmmeter, check the black wire of the connector for continuity to body ground. If there's no continuity, check the black wire for breaks or poor connections!

Try these few stps out and then let me know what you find!