hi
my speedometer is reading about 10 off, it will say 80 kph and im actually going 90
the faster I drive the more inaccurate it is. If im only going 50 its bang on.

Question is would that VSS on the diff have anything to do with my cable driven speedometer?

I tried unplugging the vss with no change to the problem.
My truck is a 1990 f150 5 speed with no cruse control.

Thanks

 

Your differential VSS has no effect on your mechanically driven speedometer reading. For your year & not having the cruise control option, it's (VSS) main function concerns your RABS system.

 

Thank you im having a bit of trouble getting the vss out and I don’t want to brake it.
Now im just going to put the bolt back in and leave it alone.

 

I concur. If it ain't broke...

 

Bigger tires ??

 

My thought too, but he say's at 50mph "its bang on"! Maybe a worn drive gear at the tranny that only slips/skips over 50mph? Easy to visually inspect & cheap date to fix/replace.

 

I am running 235 75 15 wheels.
but for more of the story and I will try to keep it short…..

my original speedometer started jumping back and forth at higher speeds.
So I got another one with a tach off an 89. this one didn’t jump around but
It reads too slow for the speed.
Just for the heck of it I reinstalled my original speedo,
and now it flys all over the place no where near the speed im going.

I did pull the cable off at the transfer case and spun it by hand.
It seems to spin real nice with no resistance.

At this point im not sure if the problem is in the transfer case,
or if I have too bum speedometers.

 

I'm going with ymeski on this one. The worm gear, inside the tranny on the output shaft is probably different from the loaner vehicle than yours. Check the loaner vehicle if available for the color of the gear, otherwise ask Ford about the differences between the 89 and 90 clusters and the gears involved.

 

I don't think it's the speedo/tach unit itself. I put took one out of a 91 Ranger & installed it in my 87' F-150. I'm thinking the drive on the end of cable but Inspect the socket of the speedo unit for play in union of the two.

On the other end, the worm gear inside the case that matches the gear is probably made of a material harder than the drive gear on the end of the cable. For the reason that the gear on the cables are easier to change. So check for wear on the cable gear itself.

Maybe grab a gear off a donor vehicle speedo cable. Make sure it has the same amount of teeth as yours.

 

Ok I have the 89 I will take a look at that gear.
When I unclip the cable at the t-case it just pulls out and has no gear on it.
It looks like I’ll have to take a bit more apart.

The speedo end of the cable kooks good.
One difference between the speedometers, is when I spin the back by hand
I can feel the one with the tach has more resistance than my old one.
Not sure if that even matters though.