I think I have a 2 speed rear issue on my 49 f-6. I checked vacuum...it was good all the way back to the diaphram. When I drove, the truck could only get 35 mph tops and the speedometer was up and down...zero to speed, zero to speed. Question is...is this the top speed? Or is the high speed not working? I just think that it should be able to go faster and I wonder if the speedometer being jumpy have something to do with it? Help appreciated!

 

The two speed is much faster than that. When you shift be sure you have your foot on the gas pull out the **** let up on the gas and you will feel it shift. The speedo is jumping because there is a valve that shifts it as well.

 

You'd know it if it shifted.

35 in low would be about right, with maybe 55-60 in high.

Do you still have the "how to shift" instructions on the dash? (Dad's '48 had a metal plate and the button was on the dash, iirc, vs. later ones on the stick)

 

yes, i follow the instructions on the dash.push in accelarator, shift, let it out.

 

The speedometer jumping is most likely from a bad speedometer cable.
Generally the 2 speed Eaton axle has about a 28% difference from low to high, so if you were maxed out at 35 mph in low, you would max out at just under 50 in high.

I do not know too much about the vacuum mechanism on the axle, but it should work the same as the electric mechanism - all it does is move a spring loaded shift collar. You can check out the movement of the mechanism on the axle by connecting the diaphram actuator to a hand operated vacuum pump and seeing if everything moves freely (with the wheels blocked of course, as you can get the axle in an intermediate position where it is in neither in high or low, but in sorta neutral).