Grand Prix GXP erratic speedometer at idle & slow speeds

jrharris

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I have a 2008 Grand Prix GXP with a little over 160K miles. It has recently developed an odd electrical issue where the speedometer will jump erratically while I am at slow speeds or stopped at a red light. When the speedometer becomes really erratic the car will even die and has to be cranked back up. Since the speedometer is jumping around erratically, it makes accelerating very difficult because the transmission keeps shifting gears based on the speedometer reading. Once you get up to around 20 or 30 mph the problem goes away and the car drives just fine until you begin slowing down and stop at the next red light. I'm guessing that this is a bad sensor or more likely a bad wiring harness that is getting shorted to ground causing the erratic behavior. I have looked all under the hood and especially around the PCM looking for any bad wiring but can't see anything abnormal. I have also disconnected the battery for 10 mins to reset the PCM but as I expected the problem still persists.

I have posted a video at this drop box link showing what the speedometer does while sitting at a red light.

https://www.dropbox.com/s/qrk2gp5l0ogpydh/IMG_5738.mov?dl=0

While troubleshooting the problem I have noticed the following details:
- The problem does not happen while the car is in park, but it does happen in reverse, neutral, and drive.
- It seems like the car has to warm up a bit before it begins to occur.

Any help would be appreciated. I usually do all my own mechanic work but with this one I really don't know where to start.

Thanks,
Jeremy
 

jrharris

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the sensor causing your problem is the vss, vehicle speed sensor. The wire harness connected to the sensor should be looked at and if nothing found try replacing the sensor.
https://www.youtube.com/watch?v=d1Oj3GGIGNE
I am now getting a check engine light suggesting that my throttle/accelerator positioning sensor is bad. Is there any possibility that this could be causing the odd electrical behavior?

Also there is a vehicle speed sensor for both the input and output shafts of the transmission. Any way to know which one could be faulty?
 
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