J91 Buick speedometer going crazy


v-6,auto, 65,000 miles. I can be sitting still and the speedometer needle gets pinned to the right, sometimes it works good, other times, the car will be doing 50 with my foot off the gas and sometimes it will stall when the speedometer it going bonkers. Is it the speed sensor on the tranny or the instrument cluster or computer? Mechanic said it wasn’t the sensor, but I’m not convinced because I want the cheaper fix and don’t know if I can find an instrument cluster and how hard or costly it would be to install. Thanks for any help. Larry


It’s hard to tell when you are talking about the car and when you are talking about the speedometer.
Your mechanic needs to check the wiring, with a volt/ohm meter, between the engine computer and the speedometer. He needs to use the wiring diagrams…like in the Haynes, or Chilton’s, repair manual.


Thanks for the reply. I may have to bite the bullet and take it to the Buick dealer, but the housing market is down so I don’t know if I can afford it. Larry