96 B2300 SE odometer erratic

[Ximeno]

Hi everyone.
This has been going on for some time and the The "Check Engine" light is now on and I don't really know what is left to do to fix it. Also when driving the truck it has a tendency to buck (like a horse) every now or then.

The odometer does not work or it may work till 20 +mph or it may work if you let off of the gas.

Things that I replaced and done.
I replaced the VSS on the trans (with a Ford part) I check the Ohms on the VSS and it checks good.
I replaced the sensor on top of the differential (the Mazda Dealer mech suggested it).
I replaces the gage in the dash (from a wrecked Ford Ranger 96, same part no#)
I cleaned and looked at the wire harness and undo the wire connection to see if their was water or corrosion. Their was none and the wires looked good.

I don't know what is left to do? Except replace the wire harness.

My guess is because the VSS works off of pulses and some how a capacitance developed somewhere to the computer.

Oh, the truck is ULTRA reliable with very very little maintenance cost (less than 4k) and has over 240k miles from new.

 

[Ximeno]

Update

UPDATE!
I disconnected the Computer and found some wet stuff in their. Cleaned and dried it out and now the "Check Engine" light is out and the needle moves. But the old problem is still their but way less erratic and able to pass emissions inspection.

Old problem is that after going 20mph the needle will drop until you let off of the gas, then it jumps to what ever speed you are going.

My next step is taking some electrical contact cleaner and clean all of the other VSS connection and clean the throttle sensor.