speedometer works half ass

i have a 1990 chevy 2500 and the speedometer works half the time. it seems to work fine untill i am driving for about 10 minutes at highway speeds, then it drops to zero. sometimes it is not working for a few minutes and will start working again. other times it stays at the zero mark till i shut the truck off and let it sit for a while. then it goes thru the same routine after starting. seems to work find around town and at lower speeds.

Mine is doing something simular. I'm finding at lower speeds its about 7-9mph slower. Then at High speed 65-70 its about 12mph slower does anyone know where to sensor is located so we can read the voltage?

if i smack the top of the dash really hard above the heater controls, then it pops back up to normal speed and is accurate.....is this common in this year truck? i have heard the "smack", can cure alot of problems!

i have a 1990 chevy 2500 and the speedometer works half the time. it seems to work fine untill i am driving for about 10 minutes at highway speeds, then it drops to zero. sometimes it is not working for a few minutes and will start working again. other times it stays at the zero mark till i shut the truck off and let it sit for a while. then it goes thru the same routine after starting. seems to work find around town and at lower speeds.
Is it a four wheel drive or two?

Take a close look at the I/P cluster contact fingers:

http://72.19.213.157/files/InstrumentClusterBack.jpg

And the mating contacts in the dash.

its a 4wd!