tach help needed

[chevythunderz28]

I am working on a 1971 mercury 500 sn#3176669. My tach was working okay but now it works intermittently. It seems to increase from 650 to 1100 rpms, On some occassions I see the rpms increase as I throttle and it seems to work fine, then it goes back to 1100 even at wot. According to the book the brown wire goes to the switchbox, black to ground and white to lighting. Bad tach? Bad switchbox? Or something else I need to check out?
Thanks
JR

[chevythunderz28]

Guess I'll try a new tach. Not much help here. Thanks

[oldman570]

Most tach wires go to the rectifier or voltage regulater and not the switchbox.
Oldman570

[westexasrepublic]

most times its a bad rectifier. replace that $25 part and be done with it.

[chevythunderz28]

Thanks for the replys. I looked at the rectifier and it has 3 wires, 2 from the stator under the flywheel and a red wire to battery and starter solenoid. The switchbox has 3 wires on the blue terminal, orange to keyswitch, blue to ignition driver, brown to mercury switch. In the manual it looks like the brown tach wire from the remote control connects with the orange wire at the engine harness plug.
Make any sense?

[westexasrepublic]

Yep makes sense. And?
Did you replace part and fix the problem?

[chevythunderz28]

I ordered a rectifier so I'll see if it fixes the problem. I don't know how it ties in to the tach but I'll give the ole 6th grade try.

[chevythunderz28]

Well the new rectifier didn't solve the problem.