When I bought my 1990 Club Cab, I noticed that the speedometer does not work. It sometimes "works" but behaves all wacky, starts to move inconsistently more and more as if it were working, even reaching the top speed, and when the transmission shifts into another gear it drops to zero and does the same thing. When it reaches the final gear, it stops at zero.

I wornder if this is a speed sensor issue or it's the computer. The computer was working until 2 weeks ago, but the speedo still does the same thing.

 

sounds like the speed sensor on the transmission.

 

It might just be corrosion in the speed sensor electrical connector causing erratic operation.

 

let me tell you guys that sometimes when I crank the engine, the speedo moves.

This speedometer behavior is every now and then, it usually never moves, or sometimes it moves a very little aroun zero, but when it acts like I described, it does random, big sweeps as if it were a cable-operated speedo. The sweep angle increases on each gear and when it shifts to the next gear, it drops to zero and repeats the procedure. It hardly moves when it is on the last gear.

 

Although the speed sensor and speedometer are electronic the system still uses an adaptor and a pinion (gear) inside the transmission. The gear may be stripped or loose on its shaft. This would not account for movement when you crank the engine but could explain the rest.