how can i fix my rev counter on my toyota townace?

Rev Counter on my 1990 Toyota Townace Diesel 2ct engine 1970cc works when first started up then stops working after a couple of minutes

When engine stopped and restarted everything is back to normal, then same thing happens again.

Any ideas what can be wrong with it ?

Would I be right in assuming that the Rev Counter is impulse driven from the voltage from the alternator ?