My heroic Johnson outboard motor took all my attempts at maintenance and repair in stride, so I decided to finish the project off with a flourish by adding a tachometer.
Tachometers serve many important functions, but my main reason for wanting to install this one was that it looked spiffy:
I’d already used an inexpensive, battery-powered induction tachometer to get my engine idle settings approximately correct.
(note: some of these are affiliate links. This means that if you click the link and buy the related product, the vendor may pay me a fee. Your price remains the same)
The way these gizmos work is, you wrap the attached wire around the #1 spark plug lead and start the motor. It counts the spark pulses, then translates that number to engine RPM–assuming you have it on the right setting, which I’m not sure I ever did.
There were a few drawbacks: for example, the wire was too short to reach the driver’s seat, so if I wanted to check RPMs I had to abandon my post and crawl to the back of the boat.
This was problematic at anything above a fast idle since there was no one else to take the wheel. Clearly not a safe situation–as a responsible boater I had an obligation to upgrade to a new tachometer!
Choosing a new tachometer
This was going to be easy because my heart was already set on the spiffy one.
My engine even came with a “System Check” electrical harness that was designed to plug directly into the new tachometer. Easy!
In addition to RPM, it checks oil pressure, oil level, fuel pressure, and engine temperature!
Hang on…
Since I’d bypassed the VRO (variable-ratio oiling) system on my engine, the two oil monitoring functions wouldn’t work.
The fuel pressure check probably wouldn’t work, either–not with my new, “vanilla,” non-oil mixing fuel pump.
Suddenly I had to decide if I wanted to pay twice as much for an advanced tachometer when–despite the easy plug and play design–most of its features wouldn’t work on my boat.
After much internet research and hand-wringing, I decided to order the “basic” model instead.
Other than the missing–and mostly nonfunctional–idiot lights, this one looked very similar. Definitely spiffy.
I still wanted to use the existing System Check plug on my engine harness to connect to my new tachometer, so I researched the wiring. Of the eight wires on the plug, I would only use three: purple (for power), black (for ground), and gray (for tachometer signal).
Unlike the induction models, this kind of tachometer counts pulses generated by the stator as it spins with the flywheel. You set a switch to tell it how many poles your engine has (if you have no clue, like me, you can simply refer to the enclosed cheat sheet) and it does the requisite math to determine RPM, magically moving the needle to the appropriate place on the dial.
Since I didn’t have any gray wire handy, my home made Deutsch plug fitting used brown instead of gray for the tachometer signal wire. I told myself I’d replace it with the right color one day soon…
Testing
In a rare burst of foresightedness, I decided to test the tachometer before installing it. I plugged the Deutsch connector together and fired up the motor.
Nothing.
“Hmmm,” I thought, “perhaps the System Check plug only works when all eight wires are live.”
No worries: attached to the same harness was a simpler tachometer connection: just the three wires–purple, black, gray–with ring connector terminals. I hooked them up to the tachometer and started the motor again.
Still nothing.
Granted this motor harness was 31 years old, but surely all that wiring wasn’t bad?
Before doing anything rash I called Paul at Faria Beede customer service, and he verified that yes, the RPM needle should move to zero whenever power is applied.
I hooked the tachometer power leads directly to the battery–nothing.
Apparently the problem was with the tachometer, not the wiring. I promptly returned it and ordered another.
When the replacement arrived I went through the same steps as before: neither of the engine harness connections worked for this one, either. But when I hooked it directly to the battery–Huzzah!–the needle zeroed out.
Now all I needed was a good tachometer signal. I traced the gray wires through the questionable harness back to the engine bus. They led to a yellow and gray wire coming out of the stator.
This, I decided, must be the source of the tach signal. A quick online search confirmed my hypothesis, so I hooked up a test wire and ran it to the tachometer.
I held my breath and started the motor once more:
Success!
Installation
Since I was hotwiring the gauge directly to the stator output, I wouldn’t have to chase down faults in the old wiring.
I briefly considered replacing the entire harness, but the wiring I needed–engine start and now, tachometer–was working. And those harnesses are EXPENSIVE.
So I rolled it up, stashed it back under the deck where I’d found it, and forgot about it.
I bought a mounting cup and bracket so I could put the tach on top of the dash, next to the fish finder.
My outboard repair spirit guide made a video about installing a tachometer and gave me a couple of ideas: not least of which was jumping the power lead for the tachometer instrument light to the main power post.
This elegant solution saved me running an extra wire to the switch: any time I switched on the tachometer, the light in the case would also come on.
While I was thinking about the light, I replaced the original GE 194 bulb with a red LED version.
I put a quick disconnect onto the tach power wire where it plugs into the back of the dashboard switch. I put another onto the ground wire, doubling it with a line that runs back to the negative bus on the fuse box.
I drilled holes in the dash to hold down the bracket and a 7/16″ hole for the wires. After filing the sharp edges off the latter, I found a rubber grommet that fit.
I ran a length of wire–the correct gray color so future investigators could guess what I’d done–from the stator output on the engine bus all the way forward to the tachometer. I used 3/8″ plastic wire loom to protect it from sharp edges, UVs, etc.
The tach–and associated light–powered up nicely when I flipped the switch, so I fired up the engine to double check gauge function.
This time I got no RPM indication at all. I may have used colorful language.
After double-checking my connections, I went inside to cool off and think about this new problem.
My first guess was that the new wire from the stator to the gauge was too long, somehow attenuating the RPM signal.
This didn’t really make sense, since my tachometer was designed to work with the factory engine harness, which uses similar gauge wire and is significantly longer.
Nonetheless, I researched how to amplify the signal by adding a pull-up resistor to the circuit, all of which was Greek to me. However, the repeated use of the word “circuit” struck a dim chord…
In this final installation I’d powered my tachometer, via a switch, off of the “house” circuit. However, the engine electronics–including the tachometer signal–all live on their own, separate circuit.
When I’d tested the tachometer wired directly to the engine starter battery, everything worked.
So maybe the gauge needed to be powered by the engine circuit for the gray wire (RPM signal) to be properly grounded?
My grasp of electricity is shaky, but I decided it wouldn’t hurt to try.
I took the gauge power and ground wires off the house circuit and connected them from the switch directly to the starter battery terminals. Since the tach was no longer connected to a fuse box, I put a 5 amp inline fuse on the positive battery lead for protection.
Sure enough, that did the trick.