My 1979 Data Marine Wind Vane stop indicating the correct wind speed.  In fact it wasn't reading at all.  I called DMI Marine and they said to isolate the cockpit meter from the mast head sensor by doing the following:

     While watching the wind speed meter, and using a shoring wire, temporary short out terminals 1 and 4 (white & black) a few times (see diagram below for Rear Connector for Meter).  The meter indicated 10 mph, and then 12 mph and 15 mph depending how fast I shorted the terminals out (make and break).  This showed that the meter was reading ok but the mast-head sensor was not providing the correct voltage.

So the problem was above. 

     Once I had the masthead wind unit down I carefully remove the wind cups (they will bend but also break), [Picture 4] and the top direction vane (Note: mark on the shaft the position of the set screw).  Carefully remove the bottom set screw and gain access to the magnet shaft.  [Picture 5]  Remove the center shaft and magnet pick-up and clean it up [Picture 6].  It was a bit gummed up.  Originally installed in 1979 and going through 4 hurricanes (at least while I had Mikhaya),  it needed attention.  I didn't really think I could fix it.  It's not captive, and will slide out easily.  After cleaning, replace and close up the assembly.  You can't really do anything else with the mast head unit.  Replace the cups with the set screw [Picture 7].  Replace the top wind vane at the same position it was removed.  Remember I told you to mark it!

      I thought about using the mast head connector and jumpering the pins to the back of the meter to check it out but it seemed like a lot of work.  So the picture of the connector pins is correct but I never jumpering it the cockpit meter.  I just climbed back up and installed it.  I included a couple of pictures of my mast ladder.  Got that at a marine flee market.  Works ok.

While at the top of the mast it will be necessary to hold the wind vane, top part toward the direction of the bow of the boat.  If you put the set screw back in at the exact place it came out of then it's already referenced back correctly.  If you didn't then someone below must reference the vane to the meter heading by turning the only center screw adjust on the rear of the meter.  A real two person job.  You could just wait till the wind is  blow directly from the stern to do this also I guess.

There is another link that explains the "wire" on the wind vane arm brace.

Datamarine Corinthian LX-360 Wind Machine 

(Thanks to Sea Dragon for the manual)