91 Suburban Speedometer Not Working

[Rich84]

I installed a '91 cluster in my '86 LQ4/4l80. All the gauges work fine except the Speedo.
As soon as I plugged the harness into the cluster the needle started moving all the way until it is pegged against the Trip knob. When I start the truck the needle moves back down to about 68 mph and stays there.
Do you think I have a bad connection in the cluster somewhere or is the speedo broken.
The temp gauge functions properly except it pegs while starting the truck, then it immediately settles to the proper reading.