Technical rpm sensor doesn't work

[alex fiat marea99 1581]

hi everyone
i have a fiat marea 1581 year1999 elx
the problem is that the rev counter(rpm)doesn't show me anything.after driving some minutes ive notice that he started to work,and after that once again he stop.there is no problem with the engine,works fine,all the time.
i want to know where is the location(maybe a picture)and if i can change it myself.
thanks:worship:

 

[Venters]

Try banging the dash when it's not working first. The connections at the back of the clocks are notoriously bad.

 

[alex fiat marea99 1581]

i had tried this,nothing.also i open the dashboard,try all the cabels,no luck