Jim Grant's Tech Tips
`03 Chevy Silverado, High Speed Speedo
My ‘03 Chevy Silverado's speedometer is constantly on 80 mph when I am stopped and goes off the scale when I start moving. What is the cause of this, my father-in-law tells me that it's the cable, but I thought the newer vehicles have all that electronic junk that controls that. I have seen no signs of a cable and none of my fuses are shot. What's the deal?
My sympathy, I get the same thing from my mother-in-law. Your truck does not have a speed-o-meter cable it has electronic junk and it sounds as if some of your junk is broken. I think you may have an IPC problem. If it was a PCM or VSS problem there would be a MIL light on. What does all that junk mean? There is a Vehicle Speed Sensor (VSS) on the transmission or transfer case that creates an electrical pulse. The vehicle’s main computer the Power Control Module (PCM) expects to receive this signal. The software in the PCM pretty much knows when this signal is right or wrong and will turn on the Malfunction Indicator Lamp (MIL) aka check engine light if there is a problem. You never said that the check engine light was on, so we have to look else where. The Instrument Panel Cluster (IPC) has it’s own computer that converters the speed information from the PCM to an analog based display, the speedometer. The IPC also receives a class 2 message from the PCM to compare the values received. If there is an error the IPC uses the class 2 message from the PCM to indicate vehicle speed. The Instrument Panel Cluster (IPC) can talk to a scan tool. Have your technician plug in and check for IPC trouble codes. I’ll bet a coffee that there will be a code for some broken junk that the IPC computer will have to share with you.