Question about 1994 Pontiac Grand Am

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Transmission speedometer problem

I have a 1994 pontiac gram am. i'll be driving on the expressway doing about 70mph steadily and after about 20 mins or so my engine makes a sound almost like its switching gears, my car will continue to go the same speed but my speedometer will rapidly drop close to zero and then jump back up to 70mph. from that point on anytime im driving and let off the gas the speedometer will drop 5-10mph,when gas reapplied it jumps back to my current speed.but never while the speedometer is fluxiating does the car lose speed.my speedometer almost acts as an rpm meter which my car does not have.then after when i make a complete stop my car accelarates very poorly till it reaches above 20mph.when this happens and i am stopped i place the car in park and turn it off and when i restart it the problem is solved temporarily.3 months ago this happened and i brought it to my mechanic and they thought it was the cruise control. they replaced the computer and ran diagnostic checks and ultimately they replaced the vehicle speed sensor and powertrain control module which states was causing intermittent inaccurate speedometer readings.i got the car back and it ran fine for about 2 months and now im having the same problem again.i am hestitent to bring it back if the problem is going to reoccur or is just being temporarily fixed.any suggestions?

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  • Anonymous Mar 28, 2014

    Speedometer cable started jumping put graphite in cable temp fix stopped working now replaced cable not fixed . Unhooked cable to see if turning not turnig

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  • Pontiac Master
  • 64,034 Answers

This all can be caused by a defective Vehicle Speed sensor, also called a VSS, also run a test on the computer to see if any codes are in memory that will help to sort this problem out.

Posted on Nov 08, 2008

  • mwpahl Aug 27, 2011

    I am having the same problem just with the speedo on my 94 grand am. I replaced the VSS prox all to no avail. Should I just have a diagnostic run on the ECM?

  • yadayada
    yadayada Aug 27, 2011

    Hello
    There is a small DC electric motor that operates the needle of the speedometer, these do fail and the needle either stops working or it reads erratic or reads the wrong speed. This problem has become known to me only in the last year or so, 3 years ago when I answered this post I hadn't found this information out yet. Any speedometer repair shop like "Speedotach.com" can repair the unit.

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Posted on Jan 02, 2017

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