Speedometer Problem

[84gnct]

O.K. I have a strange one:

When starting cold my speedometer doesnot work, after I have driven about 10 miles the Speedometer needle will start bouncing, after about 15 miles the speedometer appears to work just fine. It happens the same way every time.
I have checked everything but the inside of the tranny and all works fine.
Any ideas what mey cause this. Fluid is fine, tranny operates well no other problems but this funky speedo reading.

Thanks in advance.

 

[moose]

I'm no expert here but after thinking about i would check the gearing in the tail of the trans where the cable goe's in. If i remember right the end of the speedometer cable is square, if it has started to round off then this would explain it not working cold, as the gear turn's the cable is not touching because it is rounded off some, as the trans heats up the end of the cable also heats up and expands enough to make contact with the square hole inside the speedo gear and while it is heating up and things start to expand the cable makes partial connection and causes the speedometer to jump. Just my theory!!