Glad to hear that I'm not the only one with this problem. It started doing this about a month ago. Diff sensor OK, speedo is OK (when working) and the error code is 'speed signal missing'. I traced the circuit and found thae speed sensor is fed to the ECU from the back of the instriment circuit board, note my '89 wagon has the manual 5-speed, so the up-arrow is illuminated for the shift points based on the ecu sensing of speed and rpm. I think this is the same connection for an automatic. I now know all of my connections and grounds are good, but it still stops working once in a while. I've taken the speedo apart and checked the pins to the board and everything checks out OK.
I am tending to think that the power source that runs the speedometer is the culprit, which is a feed from the ign switch. It is also posssible that the temperature compensator board has an intermittent open in on the board somewhere.
Still nothing conclusive. Let me know what you find.
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'89 245 Sportwagon, '96 960 sedan