My analog speedometer fails, it's jumping all the time, like if for a few millisecond there's no signal or something, but it only happens when it's a hot day, as the temperature starts to fall down then the speedo works just fine. The other instrument gauges in the dashboard are working just fine.
SPEEDOMETER FAILS on 1998 Nissan 240SX
check the connection of the wire front to back, make sure no shorts or anything,check your speed sensor if not check your cluster. for more indepth answers, check other posting of my answers for Speedo.
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It could be that the Vehicle Speed Sensor is going out. I would check for any codes. Better yet, have someone with a scanner drive the car and watch the speed sensor output to see if its signal gets erratic.