speedo problem

my speedo doesn't work properly, some times it gives you the speed, then it goes back to zero then a few minutes later it will work again for a little then back to zero.....
yesterday I went with my kids to the desert to have fun on a dry lake, cuz it rain and it's a little muddy. no big pool or anything. I pressure washed completly the truck. could some mud causing this?
how does the speedo work ?is it a sensor or is it a cable ?
what I should look for ?

thanks again for your help !!!





 

Need to know the year and model to know what drives the speedo???
Mike

it's a 2001 disco II.

That speedo is an electric drive system from the tranny to the speedometer
Could be the sender unit on the tranny, that would be my first bet.
Mike

thanks, so probably the only way to fix this is to replace the sending unit...
thanks again !

well .... it start working properly after a few days.....  so I guess the truck doesn't like to much being in the mud / water.....