OMC Tachometer Testing

[marleywars]

I have refurbished a 1973 fiberglass forester tri-hull and it has a 50hp Johnson on it. One of the last things to do was rewire the original tachometer. I found the grey wire in the OMC Power Pilot control box and wired that to the grey wire (non-regulated output) on the tachometer. I believe the other wire (black) off the tachometer was the ground and wired that to the negative terminal of the battery for testing purposes. I took a volt meter to the grey wire at the control box and started the engine. The volts were all over the place and the tachometer did not move. Does anyone have any ideas on whether my wiring is wrong, on how to test a tachometer a different way, or if that voltage that I saw is the problem?

https://photos.google.com/photo/AF1QipNGbp7hLqNI-TIDDvuW9is0AtcWsRIrabwD7fWU

https://photos.google.com/photo/AF1QipMCKuuYVGBAamx_jpgqVIjxBc7nbxT_hGJYXe7c

 

[wrench 3]

You made no mention of a 12V supply to the tachometer. You would normally tap into the purple wire at the control box.

 

[marleywars]

I thought the 12V supply came from the grey wire. This is where my electrician skills need help.

 

[wrench 3]

The gray wire is the speed signal from the engine. That's why it's voltage was jumping around.
It also needs 12V and ground to run the gauge. The purple wire at the control box turns on with the key.

 

[oldboat1]

That should work. If it doesn't, you may need to connect the grey wire at the motor to the yellow/grey at the rectifier.

The needle should go to zero with the key in run position, with the engine off.