Tach only works with key

[duhn]

"I have an 86 Merc. 150 hp bla

"I have an 86 Merc. 150 hp blackmax. The tachometer works when the key is turning the engine. As soon as you let go of the key when the motor is running, the tach doesn't work.

Any suggestions?"

 

[galamb]

Recently rewired? or bad wire

Recently rewired? or bad wire - sounds like the tach is getting it's power from the keyswitch in the "start" position - so either wired wrong or bad connection.

Normally feed the tach with a hot lead from the key switch in the "on" position.

Are you having the same problem with any other gauges? if not look to see whether the tach is piggy backed from a single hot lead to the other gauges - check for bad connections etc...