1980 Merc 115 HP Tachometer install?

[Fowl_language]

Hello all,
Thanks for having great responses to all questions on the forum. I always look up answers on Iboats first unfortunately I can't find a solid answer for this question.

I just bought a "new to me" boat with a 115 hp 1980-81 ELPT (Electric start, Long shaft, Power trim) outboard. The guy had a hour counter and a voltage meter already installed. My buddy (who knows a lot about outboards, suggested that I get a tachometer so I don't push the outboard past it's limits, especially if I do not have the right prop on the motor). Right now it has a prop that says :90 hp Merc: on it. He says that the 90 and up props will fit a certain gap of HP motors from that era. So the 90 or 115 hp rating is meaningless but the pitch on the prop is important. Which I have read about before and believe. He says if the pitch is off for the weight of the boat the motor could spool faster than it should when at full throttle leading to problems with the motor in the future. Unfortunately, I cannot tell the pitch of the prop as the markings are not visible any more.

This leads to my first question, is it possible to add a tachometer that will accurately read the RPM for me? I have looked at the Tiny Tach (tm) and the reviews are hit and miss. Plus I would need the wire extension which makes me think there is a possibility that these models came with a tach from the factory (possibly from the throttle control?)

If anyone knows the best way to throw an aftermarket tach on this motor I would be very appreciative. Thank you for your time!

FL

 

[Georgesalmon]

Re: 1980 Merc 115 HP Tachometer install?

The tach wire comes off the stator. It should also be at the controls. My 1980 115 controls have a plug on the front with three wires. Ignition (purple), ground (black) and tach (gray). You can buy almost any 2-cycle outboard tach and hook it up using those wires. They are probably using the black and purple already for the other gauges you have. Purple is 12vdc with the key switch on. Teleflex, faria and most others come with very good directions to set the gauge for your engine properly and wire them.

 

[Fowl_language]

Re: 1980 Merc 115 HP Tachometer install?

The tach wire comes off the stator. It should also be at the controls. My 1980 115 controls have a plug on the front with three wires. Ignition (purple), ground (black) and tach (gray). You can buy almost any 2-cycle outboard tach and hook it up using those wires. They are probably using the black and purple already for the other gauges you have. Purple is 12vdc with the key switch on. Teleflex, faria and most others come with very good directions to set the gauge for your engine properly and wire them.

Thanks for the information, unfortunately my controls have a spot for the wires but no wires are coming out and there aren't any connections. I'll trace the wires from the hour meter and volt meter to see if I can find the gray and black wires. I've been working on the boat for the last few days haven't had the chance to look at the control box carefully.

 

[Georgesalmon]

Re: 1980 Merc 115 HP Tachometer install?

The gray wire will not be connected to anything, its only for the tach. There might be a plug over the connection on the front of the control box. If you don't have the actual harness you can use spade connectors and make your own. I actually think the the tach wire on my motor was brown or tan where it came off the rectifyer in the motor. You can trace it all the way from the motor to. The rectifyer has two yellow wires (from the stator) a red wire for the battery charge and a black ground. One wire that is also attached to one of the yellow wires is for the AC signal to the tach and it should be brown, or tan, or gray. I have newer motors and the tach wires are all gray but I may have misspoke myself on the older 115 I have about the tach wire being gray. Boats not here, is at my sons house. I will look next week if you can wait that long. Whatever color it is it does come off one of the yellow wires on the rectifyer you can look to be sure. The rectifyer is a about a 1.5" square plastic box located on the stbd side of the engine high almost under the flywheel above the switch boxes with three terminals on it. The ground wire (black) is attached to a base mounting screw not one of the actual terminals.

 

[Fowl_language]

Re: 1980 Merc 115 HP Tachometer install?

Thanks GS for the replies. I recently bought an older boat with a 1980 80hp mariner on it, lucky for me it has a Tach on the motor. So I will try and rip that off of the old boat and replace the hour meter on the 115 with the tach. I tried looking at the wiring on the 115 merc but I got cross eyed. I will check again when I get back home. Right now i'm researching in the field and currently i'm dealing with a "gas in oil" crisis on my king cobra 454 in the research boat. I wish I had the dough just to throw a nice 250 OB on the thing!

Thanks for the help i'll try my best to keep the topic updated.

 

[Fowl_language]

Re: 1980 Merc 115 HP Tachometer install?

I tried the install with the old tach from my other boat, I found a wiring diagram for the 115 and the rectifier is like the diagram, one yellow, one yellow/red and one gray, but the gray is only on a nut by itself. I wired the tach on the gray and set up the temporary lines to test it tomorrow on the water I will report back. If it's not the gray i'm not to sure on which wire it could be. On these old 115's I can't even seen the strator so I have no idea which wires run up there. The tach seems to work when I give power to it, the needle 0's out anyways. I'll report tomorrow night more than likely.

Thanks again for the help.

 

[Fowl_language]

Re: 1980 Merc 115 HP Tachometer install?

got the tachometer installed to the gray wire on the rectifier, works great. Problem is my top end speed is max 32 MPHs at 5000 RPM. I have a three blade 19 pitch prop on there. Boat is a 15 foot fiberform surf-rider, I'm thinking it should at least go 40-50 not sure what is going on.