Help! Speedometer issue

[chrizti4]

Everyone, my 96 LT4 is my second C4, my first was a 95 Base Auto. Never an issue with it, bought it at 61,xxx in 2000 and sold it at 98,xxx in 2004. The car was dependable and solid. Now my 96, which I just bought has 51,xxx and the speedometer shows me going between 4 and 16 mph while sitting and idling, mainly 6-9mph but does vary between the first set of numbers. Once I start to roll it will give true, GPS and Hand Held Radar (im in L&O) speed, matter of fact, right on the money with maybe 1 - 2 mph variance. The wire from the sensor at the rear of the transmission does have what looks like an aluminum wrap to the computer under the hood. All scanning, both internal and hand held show no problem and if I could afford to take it to GM, I could afford a brand new Grand Sport. I liked this car from the moment I started looking and liked it even more the more I looked at others. So I believe I am very happy with my purchase, just would like to get this little issue fixed, as I am racking up mileage at the stop lights!
Christopher

 

[boomdriver]

Everyone, my 96 LT4 is my second C4, my first was a 95 Base Auto. Never an issue with it, bought it at 61,xxx in 2000 and sold it at 98,xxx in 2004. The car was dependable and solid. Now my 96, which I just bought has 51,xxx and the speedometer shows me going between 4 and 16 mph while sitting and idling, mainly 6-9mph but does vary between the first set of numbers. Once I start to roll it will give true, GPS and Hand Held Radar (im in L&O) speed, matter of fact, right on the money with maybe 1 - 2 mph variance. The wire from the sensor at the rear of the transmission does have what looks like an aluminum wrap to the computer under the hood. All scanning, both internal and hand held show no problem and if I could afford to take it to GM, I could afford a brand new Grand Sport. I liked this car from the moment I started looking and liked it even more the more I looked at others. So I believe I am very happy with my purchase, just would like to get this little issue fixed, as I am racking up mileage at the stop lights!
Christopher

Could be signal interference from somewhere...
I'd get under it and physically trace the harness from the speed sensor to the harness and beyond. These wire harnesses are very bad about the insulation cracking and degrading with age and exposure to cleaning chemicals. Cross-signals start, things stop acting right. Since the car is managed by delicate resistance values, poor connections or dirty contacts can send all kinds of bad signals. In this case you're speedo is picking up some voltage from something else. When tracking the harness pull it apart gently and see if that makes any difference. Next I'd look at the bulkhead seal and the connector near the battery where the harness enters the cabin.
You can experiment and d/c the speedo signal generator and see if you still get some kind of signal to the speedo. If NOT, a bad speedo generator/sensor is possible. If you DO get a signal then that proves interference.