Noticed not every time but sometimes when i use my turn signals my speedometor will fall to zero then back to speed. What would be causing this. I took my dash apart and thought mine ran off a cable but all i saw was eletricaly wires. Any ideas on where i can start looking for the problem

 

I've looked for loose wires or a bad ground on my 92 because it was doing the exact same thing. Every now and then when I first fire up the truck it wont work and then driving for a few minutes it jumps to speed. Good luck with it and if you find the problem please share!

 

mine does that sometimes too, it seems like a bad earth on the instrument cluster

 

I replaced the broken wire connecting the battery ground to a bolt on the inner wing, speedo hasnt done the drop to zero thing since.
Guess that it was just poor body to battery ground.