rev counter only works with alternator off

[redgti]

Does this make sense to anybody i have never been able to get my rev counter to work however, my friend borrowed my alternator belt and i noticed the rev counter working. so been messing around today.
 
The plug has to be plugged in to the alternator and the big live wire has to be off in order for the rev counter to work however the speedo is doing weird things on idle is reading about 80km and as you rev the engine the speedo goes down.
 
can\'t work it out think gonna change it tomorrow and see what hapens then. i have 2 sets of dials and they are both the same.