1999 Pontiac Grand Am Q&A
1999 Pontiac Grand Am Question: If my speedometer is off by 30 mph, could it be the sensor?
Pofessional scanner will tell if it is or not!! Scanner live data will show what the computer sees and if it differs from odometer it's the dash cluster, if it's the same ,,possible sensor! - pushrod
The ECM uses the sensor output for the speedometer, but also for the transmission and mixture controls. If it was a sensor, the trans would shift into O/D at 20 mph. It sounds more like the decoder chip or the circuit board at the back of the cluster that changes the pulse signals to a voltage signal that runs the speedo. Plug in a code reader that has live data capability and take it for a drive. Verify that the live data reads the proper mph's compared to the dash. If they are different, it's the C/B or decoder. - techpaddle