Power outlet configuration on 20hp Johnson

[koff1]

Hi!
I’m helping a friend restore an old wooden boat. The outboard have a three pin power connector which I measure AC voltage across several combinations of the pins (measured using oscilloscope). I wonder if anyone knows the electrical configuration of the three pins. Which pins can we draw power from? The outboard is a Johnson 20hp motor from around 1990.
Thanks

 

[jtexas]

Re: Power outlet configuration on 20hp Johnson

What you see there is the tachometer wiring harness. One pin should be ground, one is 12VDC (should be very nearly the battery voltage) only when the key is in the run or start position, and the third is an AC signal from the rectifier (for the tachometer) only when the engine is running. You can use the 12V and ground pins without a problem, that circuit has a 20 amp fuse located on the powerhead.

If you can trace the wires to the connector, ground is black, 12V is purple, and the tach send wire is gray.

 

[koff1]

Re: Power outlet configuration on 20hp Johnson

Thanks for the reply.
You are probably right about the tachometer, but the outboard does not have electric start, a battery or rectifier so I’m guessing that one wire is common, one is ac power and one is tach output. Can anyone confirm this?

Or could it be three phase?

 

[CATransplant]

Re: Power outlet configuration on 20hp Johnson

Give the model number. It's possible that it's a Euro model with an AC lighting circuit. The model number will tell all.

 

[koff1]

Re: Power outlet configuration on 20hp Johnson

Give the model number. It's possible that it's a Euro model with an AC lighting circuit. The model number will tell all.

The model number is BJ20BFCOS

 

[jtexas]

Re: Power outlet configuration on 20hp Johnson

ok then, it is a European motor...sorry I jumped the gun.

Still, if you have one pin powered only with the engine running it'd be the tach sender, and if the other is powered when the key is on it's your power source. Probing the engine block or other good ground should get you zero volts on one pin, to ID the ground. sorry, I meant "common".

Of course that assumes I'm right about it being the tach harness plug.

cheers!