Is my speedo going out?

This has been going on for months. Speedo will jump up to the right speed and then rest at zero for a while. Over the past 3 days I've only recorded 9 miles. I'd really like it to work again. I'm almost sure its the instrument cluster, which I have a spare for. But where in hell does it get it's reading from? Considering I just did a 5spd swap and there is no speed sensor in the trans that is hooked up it would have to be the ABS system right? Not the ABS computer, already swapped that out. Also in the same system but may be unrelated, my rear diff lock fails to engage. All of this has been in non sequential order so... ABS light on with no code (Jan 2011) Speedo going on the fritz (March 2011), diff lock blinking and never engaging (July 2011) and just did the trans swap.

Any ideas? Hate to rip the dash apart to throw in the cluster if there is a simple fix.

 

Speedo issues: You have a broken arb sensor wire (it may not be visible). What wheel were you messing with last before the speedo started acting up? That is the corner that is upset. Go to the yard and buy one for $20. RR then enjoy.

Rear locker: Have you tried engaging rear diff while in neutral? Or going slower and steering back and forth? What happens is its waiting for the r/p teeth to line up just right so it can engage. Not saying that is forsure your problem but it doesn't sound electrical to me. Sounds like the electrical signal is being made but mechanically its not engaging. Before you tear apart your dash put the truck on jack stands and try to engage it by turning your tires by hand after you pushed the button (ignition on but not running).

Hope this helps, kind of a old post now that I look at it so you probably have it all sorted by now.