How can I hook a tachometer for RPM's on my 1977 75HP Johnson?

Question

I have a 20' pontoon that I have a 75hp johnson on and I was wandering how i could hook up a tach to my motor to see the rpms. I am trying to find out the right size prop for my motor to push my pontoon.

Answer 1

Yes. the gray wire in the control box is the tack lead.  hook up the tack to ign and ground and the gray to the sender.  Good luck.

Answer 2

There's a little more to do the Gray wire is tack> black ground> Purple Hot> Blue light> And on the back of the tack is a plug this is to adj the RPM to the engine so yours is 3 cylinder so set to 3> 17 pitch is the prop for the 20 toon>

Answer 3

and a tach will help how

Answer 4

if you have the standard control box with push-to-choke keyswitch, there's a socket, might be covered by a rubber boot, located just underneath the throttle tension control k**b (which is just under the keyswitch), that accepts a tach wiring harness.  I got my harness at basspro.

Don't have the harness, tap into the black wire (keyswitch terminal "M") for the tach GND post, the purple wire (terminal "A") for IGN, and the gray wire in the control box connects to SND.

The LT post, you can connect to your helm lighting, or your anchor light, or if you connect it to the purple wire, the light will come on whenever the key is "ON"