Need a tach on a 78 Merc 700

[halfor]

I?m at a loss trying to figure out a tach for my 70hp 1978 Mercury with a Thunderbolt ignition, there are tons of post here but they all are a bit different and I?m haven trouble following them. I have not purchased a tach yet because I don?t know what I?m looking for. Can I buy any new Teleflex and make it work by simply following the instructions? Do I provide my own wires? sounds like I cannot simply buy a harness to plug into the controller for some reason hence all the confusing posts about how to hook up to a Thunderbolt ignition. Any advice is welcome. Thanks

 

[Chris1956]

Re: Need a tach on a 78 Merc 700

A standard Outboard tach of any brand will work. You will want a 12P setting, which they should all have. I would think the three prong tach harness should be availible since they were so common, but since I never needed to buy one, I never looked. Try a Merc Dealer, if no listing at a marine store.

The tach will require +12VDC, Ground and the tach signal. All of these are avail in the Tach harness.

 

[halfor]

Re: Need a tach on a 78 Merc 700

Thanks, I have both the harness and tach on order. Now I just wait and see if it all comes together. I was running WOT this weekend trying to tell when I was at max speed, once plained out I raised the motor until the prop sound would change and I would speed up slightly, any higher and the prop would get a little louder and I slowed down, the GPS said 35mph I?m thinking may be the best the old 70hp can do. I?m hopefull the tach will aid in getting a feel for what the motor is doing and when.

 

[halfor]

Re: Need a tach on a 78 Merc 700

have all the parts including the tools to cut the hole, about $120 invested at this point, want to test befor the hole is cut. but as I hook it up nothing happens, the rpm gauge went to zero when I fired it up in the driveway. I can't seem to find anything that sounds right as I search the fourm, I don't have a bunch of yellow wire like most are talking about, but then again I am not sure what the alternator looks like I'm stuck till I find some info, any ideas???

 

[halfor]

Re: Need a tach on a 78 Merc 700

I was ohm-ing out some wires and 2 of them just fell apart, crumbled in my fingers, guess being a 78 has its issues, any way I'm wondering if the signal wire on the tach (brown) should be grounded, I was assumed it was a open closed type of signal from a positive type terminal but now i have to ask anyone if it is a negative signal and the fact it shorts to ground is what it should be doing???

 

[Chris1956]

Re: Need a tach on a 78 Merc 700

The brown wire should appear open. If it is grounded, something is wrong. Look for a yellow wire from under the flywheel. The brown wire should connect to it at the rectifier.

 

[halfor]

Re: Need a tach on a 78 Merc 700

figured out what the rectifier is, saw the brown wire that is on the yellow. after disconnecting the brown, the brown no longer shorts to ground, and appears to be the wire going to the control box. I assume I should move this brown wire to the other yellow wire? only other color on the rectifier is a red. should I be concerned that the first yellow that was where the brown was shorts to ground? not sure of the function of the rectifier so I have no idea if it is working. the motor was running great before, but if it's for the charging system I have a new battery so I may not know if it working? I won't be able to fire this thing up till I repair the now 3 wires that crumbled in my fingertips to even check what going on. any thoughts?

 

[halfor]

Re: Need a tach on a 78 Merc 700

I finally did it, took 2 weeks some money, time, and patience. But here it is.

 

[Tony T.]

Re: Need a tach on a 78 Merc 700

This is my next issue,seeing if the "tach" on my new to me 95 Suntracker even works, " my new project pontoon boat" I have now. I already had to do some re-wiring on the boat, so who knows if the tach even has power to it, The tach didn't move when I fired up the new/old motor that I installed on it . I don't have any tach install instructions to go by for wiring. Congrats though on your install "looks good".

Tony