A lot of these buses use electronic sensors which are no more than magnetic switches or proximity switches mounted in or on the transmission. They pick up pulses, usually from the final drive shaft or output gear, count those pulses or convert them to a variable analog output that drives your speedometer readout. Moisture, looseness, or misadjustment of the sensors can cause loss of signal to the speedometer circuit. So the first thing I always check is, is the sensor tight, are the wires attached, and does it appear that the insulation on the wires is intact. There can also be a "black box" which is the amplifier board, either behind the dash, or in the area of the power distribution box (above and behind the left headlight assembly on my bus). Check to make sure this is firmly mounted (the mounting screws are normally the circuit ground), and that the power and sensor plugins are clean and firmly inserted. Last, but not least, check to be sure that the breaker or fuse feeding this circuit is connected well and is not corroded or overheating. I hope this helps and you find and fix your problem.
Here's a link that also might help. http://schoolbusmechanic.blogspot.co...on-wiring.html