• Home
• Videos
• Dangar Marine
• How to install a tachometer in a boat

How to install a tachometer in a boat

In this video I install a tachometer in my boat. I wire up the power and then select the number of pulses or cycles my outboard has to get the correct reading.

How to install a tachometer in a boat – Video Transcript

Hey there, Dangar Stu here. Today's video is about installing a tachometer in the green machine and is proudly sponsored by MarineEngine com.

I've been asked a few times over the years about installing a tachometer in a boat and I think they're handy to have so it's about time I put one in the green machine. I think things like speedos, trim tilt angles that kind of thing is a bit marginal but a tachometer is a really useful thing. It tells you all sorts of information like has the motor started or stalled, you know obvious. Is the prop right for my boat? It's really good instrument to help you find the right prop. Is it cavitating? Is the bushing failing in the prop? Has the hull got lots of growth and I'm losing the top-end? You know I think you get a lot of information they're well worth putting on. This particular one is a Sierra one that I bought off MarineEngine.com. There is a link in the description and the part number is a 58255P.

In a bit of a misguided attempt to be organized I actually took the instructions for installation home so I could read them which I didn't do, and then promptly forgot to bring them back into the workshop. So we might wing it a bit to try and compensate. I searched the net for a PDF and I found some instructions for a slightly different brand but they may all actually just be rebranded anyway. Certainly seems to have the same diameter and the information I need about the outboard is obviously the same. The instructions for this Tachometer say to use a 3-3/8 inch hole saw. I couldn't find one exactly that size. I don't know why I left ten minutes to go shopping. But what I have here is a 3-1/2 inch so a little bit big but it's still not bigger than the outer bezel so I think it'll be okay. If it's problematic I might just do a bit of Sikaflex or something to sort of seal it in but we'll see how we go. I'd say large hole-saw this size is probably the main tool you might need to buy. The rest of it's very straightforward. What comes in the rest of the kit is simply a backing bracket that holds it against the dash and some nuts that's about it.

To start with I'll show you the back of the tachometer. What you can see here is a few things, a calibration potentiometer up here and a wire for a backlight, then here you've got the send wire. This is the signal from the outboard that is actually the AC current from the charge coil that it uses to figure out your RPM. Then a ground. Pretty straight forward and then ignition here is just power from the ignition so it gets power when the outboards on and not when it's off. You could run it straight from the battery the switch if you wanted. Then here is a switch where you select how many poles the charge coil has on your outboard, this is currently set to six poles that's how it came from the factory and as it turns out the Honda outboard is a six pole outboard so I have to change that at all.

In the instructions you'll see if you find your brand of outboard it'll tell you how many poles your outboard has, and then you just have to select that on that little dial with a screwdriver. The way these tachometers work is that as the fly wheel rotates there's a certain number of poles on the charge coil, so it knows it's going to get six pulses per one rotation. So it just counts those pulses and those get to six it's done one rotation and then it translates that to the movement of the needle. Probably the other thing you need to know, which I have a vague memory I saw on the instructions that actually came with this Sierra tachometer, which is the color of the wire that is that signal wire. So if you're looking to do one of these installs the two things you really need to figure out is the color of the wire that's your signal wire and how many poles your charge coil has. Once you get those two bits of information you're pretty much ready to go.

Alright first step for me is finding somewhere I can physically fit there so let's go look. On this side I've actually got a compass and a fuel gauge which I haven't wired up here. I just put it in there and then couldn't get the sender to fit in my tank so that video went south. But I do have a little bit of space here next to the Wi-fish. And it is clear behind it that's the other big thing to check. Having a bit of space on the dash is one thing knowing that behind it you don't have a whole bunch of wires or a structural brace or something is the other thing. So I'm thinking here is my prime candidate. Here's my hole saw so that gives me idea of the space and it will just fit there. It's a tight fit but it will fit.

Looks like I do need to take the steering wheel off to get the drill in to cut this hole. So be it. This has a 19 millimeter nut on it. When you're taking steering wheels off it's important to use a puller rather than just bashing them. They'll eventually just come off. So behind here is nice and clear of wires as well as not being obstructed by bracing. You don't want to start cutting into any wires so just make sure you have a good check before you start drilling.

For those of you that aren't that familiar with hole saws they're pretty straightforward. You get a center section like this and there's a regular drill bit in it, well it's almost regular. There's a... maybe you can't see in this slide there's a grub screw in here. That grub screw actually goes into a flat section. I've just got a regular drill bit in here that I put on the grinder just to flatten the bit off. Popped it in then the whole saw of a diameter I want or at least close to it. Screws on the outside, wind it on till it's completely tight, then I'm gonna back it off until the holes and those pins line up. Then I turn this bottom collar which makes the pins come through here and then it can't rotate any more. Then pop on the drill, we're good to go.

Next thing I do is use a center punch to mark the exact center of the hole on the dashboard that way the center drill bit here won't wander around. Once the center bit has gone through then it's a case of just being patient. Here we get firm and even pressure leave the hole saw to do the work. Oops! I cooked my hand drill. The hole drilling didn't go well. This is the cover plate but once I've cut through there, there's a dash section that I no longer have a center hole for so the hole saw went everywhere. It's a mess. So what I'm gonna do is I'm going to make a new cover plate. I've got the hole through there - now that's great. Making your cover plate put that on put the Wi-fish back on. Put the tachometer back in start smiling again.

I'm going to drill this new plate on the bench just by clamping it to a block of wood. We'll drill through it then we'll install it. It occurred to me now what's gone wrong with this one a bit and definitely more so with the original one, and that's that the original center drill bit for this hole saw had broken. So I replaced it with a six millimeter drill bit I put on the calipers and was actually a six point two then I just ground a flat side on it so I could put the grub screw on to it, etc. but it's actually not true it's wobbling inside the hole saw which is making the hole larger than it should be so the thing I've learned out of that is don't fake it. Buy the proper center bit for the hole saw put it in it'll be fine. So I'll definitely buy a stock of those in the future but live and learn.

If this still ends up being too large I'm just gonna put the Sikaflex bead around it. Here's the finished product, I've still got the Wi-fish off but I'll screw that back in later. And yeah it's okay, actually I think it's close enough to not need the Sikaflex which is nice. So we're going to take this out to the bench and we'll make up some wires for now then we'll do the full install.

Two wires from the tachometer are going to go to the four controls. Now the loom comes from the outboard to the forward controls and then several wires come back out. These are the wires designed to go to your dash instruments. I'm going to take some twin core wire put two ring terminals on the end for the pins here, and then the other end has actually got bullet connectors so I'll see what I've got. Actually don't think I've got the same sizes there which is a shame otherwise we plug in. So I might do something up for now and then I'll buy some plugs later but if you can get some correct sized male bullet connectors that's ideal. For the other end of the wires I don't need a huge length of wire because I'm simply going from the right hand side of the dash to the forward control so it could be pretty short. I should measure it but I can't be bothered climbing back up into the boat so I'm going to call it yay bit of extra.

The instructions say to use at least 18 gauge wire and this particular wire is a tinned marine wire. These are the lugs I'm going to crimp on they're about the right size for my posts. The color of the lug dictates the internal diameter and the blue ones seem about right for this wire. Now there's a light pin here and I can't see any reason that I can't just have a little jumper from the ignition power to the light pin so that when their ignitions on the lights on. I can't see why not. I'm gonna try it first if it's a problem I'll change it but that's what I'm thinking. The ground is going to go to the terminal block ground that's under the dash so I'll put another ring terminal there on that wire. But I'll just make it, you know, once again yay long and you can just go into the terminal block nice and easy. This single coil wire is slightly thicker so I'm going to use the yellow terminal cap for this one.

Okay otherwise I'm rigged up now. I'll show you what we've got. Single wires going to ground then I just got a jumper from the ignition power to the light I think that should be fine. And then we've got the ignition wire and the sender wire coming here. The other two here, so this will go to 12 volt ignition power, this will go to the gray wire which is the signal. I've pushed it through. Wires just here now on the back I'm just going to slide this bracket and in the final two nuts onto those longer posts just to secure it in. So I'll secure it first then we'll do the wiring.

The next step in the instructions is to disconnect the battery before you do the wiring so I'm just going to switch my battery switch off. This here now is the negative just going to a negative terminal block so that's all taken care of. For the other two wires I've decided to scale new heights of dodginess and cut off some bullet connectors from an old ignition coil and here they are. I put the brown to red and then the white is to a blue which will go to a gray. So here they are on the loom the signal wire going to the gray and then the power the ignition wire coming to these red wires out of the four controls. So I'll switch the battery switch on them we'll give it a whirl.

I'm now out on the water to calibrate this tachometer. I'll quickly show you though I wasn't getting power out of the forward controls out of the red lead which should have power when the ignition is on. I know it's not a fuse problem because the trim tilt works, the starter works and they all run off the same fuse so at some stage you'll have to pull it apart. But what I've done instead is just run direct power under the dash I'll show you that all I've done here is take a red power lead from the positive bus under the dash and instead of using this ignition switch wire which doesn't seem to work at all I'm gonna get power straight so that's the only change I've got at the moment. I'll sort that out later.

I've got my tachometer unplugged at the moment because I'm going to be using this multimeter for calibration. So this gray wire which is the signal wire from the stator coil I'm going to put the positive lead in. These ends are great on the multimeter because they plug straight into these bullet connectors. And then there's an earth here so we've got positive to the gray wire which on the Honda's is the signal wire and then negative just to an earth on the multimeter itself.

I've got it set to Hertz here so we're looking for a frequency and then we'll fire it up and see what value we get. Here you can see we're getting about 35 Hertz so that's our idle speed 35 34 I'm actually going to do it as 33 to make the math simple. So we'll do this on paper and I'll show you how you convert the frequency of the signal into RPM the way we convert a frequency in Hertz to rpm is we say 33 Hertz is 33 cycles per second and because RPM is in minutes we say 33 times 60 to find out how many cycles per Minute. We're getting now the cycles aren't one per rotation in the motor. This is where the poles of the stator is really important, now this is a six pole motor and each pole is a positive negative so we're getting three full cycles per rotation of the motor. So if we come down then we're basically saying 33 Hertz times 60 seconds in a minute. Three full cycles of the stator per rotation which should be - a shortcut we just sort of rationalize and say it's 11 times 60 so 660 rpm is what this motor is doing at 33 Hertz presuming the six pole figure is correct. What I'm going to do now is accelerate until the frequency gets to a hundred and then I'm going to plug the tachometer back in and we'll see what a hundred Hertz is being displayed on the tachometer as and that will give us an idea of where we stand with the calibration. I think we can call that close enough 150 Hertz was giving us 1200 rpm on the taco which is way out , it's nowhere near, it's got to be at least three or something.

So we'll do the math see what it should be and then see what we can do to the tachometer to get the expected result. So if we work through the math again we've got 150 Hertz which is 150 cycles a second so we run times that by 60 get 150 per minute then we're dividing by three the number of cycles per rotation so 9000 divided by three which gives us 3000 rpm original eight Hertz but that's Wrong. So 3,000 rpm is what we should be reading on the tachometer when we've got 150 Hertz showing on the multimeter now because that math is way out you know it's basically a factor of two or close to it. I figure what I'm gonna do is just change the poles I don't think the little calibration potentiometer is going to have enough variation. I imagine that's more in the five to ten percent range so I'm going to swap it from six poles to three poles that may be because this has already done the division so this is saying look it's a six pole engine so it should be three cycles something on those lines. If you're getting a figure way off what it should be I think you've got a suspect the selection of poles before any final calibration.

So what you can see here is I've swapped now from six P round here to three P which is also listed as six C so I like to look up what the P and C stand for I presume it's poles and cycles but I would have thought it's the other way around. Six poles giving you three cycles so I'll do it at home. Already we can see now that this tachometer is sitting on about 600 rpm which is exactly what the math said that we should be at four idling. So I think this is much closer to what we should be. What I'm gonna do now is plug the multimeter back in get it up to 150 Hertz then plug the tach back in and see if we're somewhere near 3000. So here we are close enough to 150 then I'll unplug the multimeter for the tachometer in. You can hopefully see there we're just above 3000 and we were just about 150 Hertz so I think we can confidently say this is dialed in now.

The figure I'm getting on the tachometer is matching the number I'm getting from the multimeter. I was dividing by 3 when I was doing the math for the multimeter and that's now set to 3 pulses and 6 cycles. I think there's a bit of a terminology issue going on here that probably warrants a bit of further investigation but at the end of the day the math for the multimeter did correlate to this 6 pole motor divided by 2 and eventually when I found a setting on the tachometer that gave me the correct result it gives me something to be confident in. Obviously when you install this if you go by the recommended setting and it's wrong by a factor of 2 or 2/3 or something then just change that setting to get the right result.

Not all multimeters have a frequency setting on them so you may need to borrow one or whatever to do that particular test. I do think it's worth having when you first install it folks. I think it's a really great way of seeing what's coming out of the outboard and how that number is being represented on the dial of the tachometer.

All right well that wraps this video up I hope it helps you if you're looking to install at a tachometer like this in your boat. It's not really rocket science but it can just take a little bit of configuring to get it right for your outboard. Alright, well, thanks for watching. Take care, see you.