90 Sierra Speedometer

[ram1220]

1990 GMC Sierra 5.7l. Last night when starting from a red light my speedometer wasn't registering anything. My truck has the moon gauges and the truck wasn't mooning me. Nothing until I gave a light tap on the plastic face of the gauges. Then the speedometer came to life again. Worked fine the rest of the night. Does this truck use a sensor to send info to the speedometer? If so where is this sensor located? Or is the truck old enough to still use the old fashioned speedo cable? Thanks.