Sounds like the internal amplifier for the speedo head has failed. Experiment: Inside the trunk, drivers side trace the wires coming up from the axle speed sensor and disconnect the connector. If the needle bangs the stop when you turn on the key the speedo head amplifier is shorted. If it does not move look for a short in the wiring down to the pickup coil.
The amp is a rather sensitive Op Amp device and it doesnt take much to blow it out, just the act of jump starting another car may spike the op amp.

The only way to fix this is to replace the speedo head or the PC board that is mounted to the rear of the speedo meter assembly. A 700 series from about 87 to 91 should use the same driver board and all that is needed is a philips screwdriver. I like to replace the board rather than the whole speedo head because you will also have to replace the odometer and any dignified brick owner with 200,000+ miles would rather die before he replaced his odometer.


Good luck

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