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Speedometer Jumping

6558 Views 12 Replies 4 Participants Last post by  cocobolo95
I recently purchased a 99 Trans Am WS6.. I will post pictures & info on it in another thread..

Some Specs for diagnoising my problem...
LS1 Stock Engine
6 Speed Tranny
3.42 Gears
Original Speedometer

When I am driving from 40+ mph my speedometer starts to jump UP & DOWN within a 5-10 mph range... (Tach has no problems - Just FYI)

I read online the ALTERNATOR was going out (which it was) so I just replaced it. I installed a Remy New Alternator - 102 Amps Part No 91524.

And my speedometer is still doing the same thing... What could it be? Any diagnostics or tips would be very helpful, thanks! :thumbsup:
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Need to put car on a scanner that does real time sensor data. One of the readings you can view will be what speed the pcm is seeing from the vss. If the reading on the scanner fluctuates like your speedo, then theres a problem with the vss, it's wiring to the pcm, or the pcm. If the reading the scanner gives is correct and the speedo is off then the problem lies in the speedo itself, or the wiring from pcm to speedo.

Common problem with electronic speedometers is the circuit board inside going bad. One of the first symptoms, many times is a needle that jumps around.
thanks coco, i'll see if i can borrow one from a friend locally and i'll update you & everybody else here in-case someone else bumps into this issue... I have searched online for this issue for a while now and nobody ever seems to put the answer to the fix...

I'll make sure I do.:thumbsup:
If you can't find a scanner locally and you have a pc that can be put in or very near the car, this one will work: ScanTool 423001 ElmScan 5 Compact OBD-II Scan Tool and OBDwiz Diagnostic Software : : Automotive

There are cheaper Chinese-made ELM327-USB ones, but you would have to get good software to go with it, since the supplied software will not be adequate. I can tell you how to do that if you need it.

Another possibility is to use an Android-based phone and get the Torque-BHP app and a Bluetooth adapter:

It will be slower than the USB versions but should do what you need if the fluctuations are not too brief. That method will require some learning of the app and some possible assistance. Again, I can help if needed.
Garydoug I will order that today cause I don't know anybody who has a scanner, and I might as we'll have it for the future as we'll since I will be keeping this car. I will update the thread as soon as I get it and scan the car. Thanks again!
If you are buying that exact one and need to fill out the $25 for free shipping, I would recommend the ($4) 15 foot USB extension cord, as the one on the tool is only 5-6 feet long and it would sometimes be handy to put the pc outside the cabin.
Coco & Garydoug, I went ahead and purchased that OBD2 scanner from amazon and ran the program w/ the dashboard view while driving last night. The refresh rate was a little slow but enough to tell that the PCM is increasing on the MPH normal. So the issue lies between the speedo itself, or the wiring from PCM to speedo correct?

Are their any other tests to further diagnosis the speedo itself or the wiring from the PCM to speedo?
if the vss is sending the correct reading to pcm then your most likely suspect is the speedometer itself. The circuit board inside is probably going bad. Time to find a replacement instrument cluster.

I see them fairly regularly on craigslist, here in Florida, don't know how your area will be. Try this salvage yard search engine. All the yards ship parts so if you don't find one nearby, branch out to your whole state, region, or entire country. Auto Parts Market
had the exact same problem a few months ago on my 99 camaro. v6 auto by the way. the speedo would work one second then start jumping all over the place the next . by any chance does it also drop to zero likes its not working then goes back to jumping?mine would work ,not work start jumping around etc,etc. Well anyways i went under the vehicle on the passenger side and saw a wire going up the side of the transmission and couldnt see the connector but i believe that to be the vss. i was going to replace but couldnt even put my hand to reach it or see it for that matter. got pissed just wiggled the wire for the hell of it and speedo been working perfectly for four months
coco, i would really like to keep my Original Mileage from my speedometer.. I think I read online that people can re-program a speedometer to whatever # you want... Maybe I can use that to re-program the new one to my original mileage?

jrsgalaxy, my speedo never drops to 0. It just moves up & down after 40 mph and i've noticed that when im on a smooth road it doesn't do it as much, and when im hitting a bumpy road it moves up & down much more. (night & day difference)

Maybe a part inside the speedometer thats broken?
I believe cars with digital odometers need special equipment and a program to change the reading. But do a google search to make sure I haven't forgotten the correct info.
To coco, Gary, and all... I have figured out the resolution to my problem so if anybody else has this happen to them they may fix it as we'll.

My resolution to the problem was doing a tranny fluid change. I used 4 qrts of Dex 3 ATF Valvoline. It's been a week since the fluid change and it hasn't happened again, btw it's my daily driver so i really kept an eye on it!! :)

The tranny fluid change corrected the speedo jump and butter like shifting now!! :D

I thought I'd share cause I've read lots of threads here that have helped me in the past and i would like to give some help back and to others... Thanks again!!!
That just goes to show us how something seemingly unrelated can cause a problem.
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