On the first blink of the turn signal to either side the speedometer will drop to zero and back to normal speed. But will only do this once and only the first blink. I am sure with the condition the frame was in when i got the truck that the grounds are in not so good condition also.
This on my 92 pickup with electric speedometer.

So where do the blinkers ground to?

 

Electric is not my strong area but I think the ground you are asking about is located by the fuse box behind the left kick panel. I take the 10mm bolt out and scratch the paint off the A-pillar and make sure the brass connector is good and clean.

Another thing I have done that seems to help with some other issues is to pull the cluster out and re-tighten all the screws in the back. They tend to back out some times.

 

Thanks Terry, I had thought there was one in that area but wasnt sure. I am going to check and clean up and the few I know of this weekend sometime. Its not a big issue with what its doing right now but i dont want to let it linger and turn in to something big later.