Pontiac Repair: Speedometer (2) issues with 2005 Bonneville, 2005 pontiac bonneville, pontiac bonneville

Question
We have a 2005 Pontiac Bonneville that we truly thought very highly of UNTIL... the speedometer started going nuts. It would stop and stick and then release at different speeds while we drove it. It would say we were running 10 mph when we were on the Interstate and obviously (with traffic) in the 70 mph range. I took it to the Pontiac Dealer (now Buick and GMC) and they said it would be about $500. to replace the cluster. I could not afford this at the time and opted for a used one from a reputable supplier. We had the GM dealer install it... and it too now is doing the same things. A friend ( a mechanic in another town ) said it sounded like possibly a speed sensor going bad... on or in the tranny. Your thoughts are very appreciated.

Answer
Hi brian:

In a nut shell the way the system works is there is an output spped sensor in the trnamission. thatsenor sends a sigan to the engine control module inturn that module sends the siganl to the speedo head. In stead of guessing what need to be done os the car needs to be driven with a scan tool and compair the tranmsission output sepeed senor to say the ABS sensors and see if they read the same. If they indeed read the same then chances are the output from the engine control module to the speedo could be the issue. If the output sensor reads what the speed is reading and it's way off what the ABS wheel senors read then it could be an issue with the speed senor in the tranmission another thing that can effect this is EMI spark plug (cheap ones) wires that are routed too close to the transspeed sensor can induce a false reading for the speedo. Doubt ful but it's it possable usually when that happens the speedo will be erratic and or will jump/spike when that happens. Hope that helps you out. Good luck :-}