Re: 1987 Johnson 40 hp -tach
A tach for your motor would have a "6p" setting to fit your 12-pole alternator. That one might work, or it might not. 1st identify the posts, you need one for 12V power, one for ground and one for the sender. In your wiring harness, purple is 12V switched, black is ground and gray is tach sender. With purple and black connected and key on, the needle should go to "0". Try the different settings with gray hooked up and engine running, it might be obvious which one (if any) are correct.
