When I turn on the turn signal my speedo needle flops down from proper speed reading and back up again.....whats up with that....thanks for your input.....93 P/U 3.0 4x4

 

My truck does this as well only when I turn on the right turn signal and not the left. Interested in this as well.

 

I would guess in both cases you have electronic speedometers

When you use the turn signal of choice it draws more current then normal due to higher then normal resistance .

Hence the glitch in your speedometer .

This is just a guess without confirming by meter

 

You could have a slowly failing VSS1 - the electronic component in the transfer case that generates the electronic signal. "Drop to zero, then springs back up" is the common symptom.

But if it ONLY happens with the turn signal (not the headlights, etc.), then I would start by looking for a short in the turn signals. Heck, I'd start by looking at the turn signals while someone operates the switch; if one isn't lighting it could be shorted.