Help!! connecting 2 wire tach to 3 wire system

[Blue 17]

I have an 1974 Johnson 85 hp that I would like to connect to the original tach in my boat. I have located the gray (send),purple (ign hot)and black (ground)wires, no problem.<br />Now my problem: the tach only has 2 connections, they are the same post used to mount the gauge, 1 post is marked gnd (ground) the other has no markings the only other connection is a wire for the back light. The gauge has a funny trademark style "M" on the face but no name.<br />Any help on this would be appreciated before I fry my tach.<br /><br />Thanks<br />Tim

 

[paperdesk]

Re: Help!! connecting 2 wire tach to 3 wire system

I recently bought a boat, and the tach didn't work at all. Just last week I had a look at the wires, and found that somebody had wired the tach leads from the motor to the backlite wires! When I switched the wires it worked fine, no problems at all!<br /><br />I'm not an electricion or a mechanic, but it sounds like your replacement tach doesn't have a backlite, is that correct? If so, than I would just connect the two tach wires to the leads from the motor and you should be fine.<br /><br />Anyway, I didn't have much to loose playing around with mine, if you have $$$ tied into it you'd better wait till the experts here reply <br /><br />Ted

 

[Blue 17]

Re: Help!! connecting 2 wire tach to 3 wire system

Ted, Thanks for the reply. My tach does have a back light and the main reson I dont want to fry it is I would have to replace it with a gauge that does not match the others, Thanks<br /><br />OK, I've got to try something.<br />Will it damage the rectifier or tach if I connect the purple wire and gray wire to the same post?<br />I know this will back feed 12v to the rectfier or send 5v ac (that is what I am getting on the gray wire)into the ignition system.<br />I can not find any info on this Tach connection after many hours on google.<br />Any help would be appreciated<br />Thanks<br />Tim

 

[rodbolt]

Re: Help!! connecting 2 wire tach to 3 wire system

no dont hook the gray and purple together. black to ground and gray to tach sense terminal. some of the antique anchient tachs were two wire and did not use the purple. go to west marine plunk down 69 bucks and get a decent reliable tach. I think the last of the two wires were around 1980 or so. there was a reason for the change to 3 wires.