Quote:
Originally posted by f5fstop:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by ChevyHighPerformance:
There are speedo recalibrators out there that change the frequency output from your VSS (vehicle speed sensor) to what you need. Some also have a switch to accomodate two differnt size tires (i.e., winter or summer). The speedo box connects or splices in the wires going to the VSS (use to be on the tail of the tranny).
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Please explain further. The I/P cluster on the H3 is a dumb cluster. It receives all readouts, except five, from the BCM/PCM via class 2 serial data circuits. There is no input to the cluster from the speed sensor. (Those five readouts are left and right turn signal, seat belt, security, and MIL control.)
There may be aftermarket calibrations out there that change tire sizes, but nothing at GM. If you have a 32 inch tire, and upgrade to a 33 inch tire, the dealer can request, at customer expense, a callibration to change the tire size.
The VSS is located in the tail of the transfer case. </div></BLOCKQUOTE>
The VSS is a reluctor based sensor that produces XX number of pulses per revolution. The output of the VSS looks like a square wave. The speedo recalabrators splice or connect into the VSS wiring (and get power and ground too), measure the VSS frequency, and feed the PCM another frequency scaled from the original. Here is a PCM screen shot of a silverado's speedometer calibration. You can see the the PCM needs tire diamter, final drive gear ratio, VSS pulses per revolution, etc for speedometer calibration.