Tach Attack!

Original Posted July 2009
Update posted July 2016 and updated June 2017

Eventually, I had to bite the bullet.

I'd been fighting tachometer problems in my Fly Baby since February.  It started out as a major-league shaking of the needle.  I'd pull the drive cable out, lubricate it, and the tach would work fine on the next flight and be back to its old tricks the flight after that.

And it got worse.  Pretty soon, the tach needle would shake a while, then just drop down to zero.

That got to be the litany, for about six months.  I'd make a change, and for the next flight or two, the tach would work perfect.  Then the same problems reappeared.

I tried everything, from a new tach (actuall, a borrowed used one) to a new tach cable.  With everything else pretty much eliminated, the worst-case scenario was about the only one left:  The tachometer drive on the back of the engine was bad.

An out-of-focus, grainy digital camera picture seemed to show the center drive section broken in half.  Problem confirmed.

All right…now what?

My first inclination was to get the tach drive itself fixed.  But the tach drive is part of the oil pump idler gear, buried in the accessory case in the back of the engine.  I took a few moments to contemplate what that meant:  To replace it, I'd have to remove the engine from the airplane, remove the starter, generator, and both magnetos from the back of the engine, then remove the entire accessory case to get at the oil pump gears.  And, of course, reassemble everything, with a high preference to having things working properly when I got done.  And, frankly, I'm not really qualified to break down an engine that far.  To have it done would probably run into a thousand dollars or more.

I started looking into the possiblility of an electronic tach. It was mostly disappointing.  Most installed a transducer on the engine tach drive...which I figured wouldn't work in my case, since they probably would have the same problems with my damaged drive head. Others connected to a Bendix or Slick magneto...neat option, but I have Eisemans.

Finally, I noticed the "Tiny Tach" on the Aircraft Spruce web page.  Seemed pretty good...got its signal by wrapping a wire around a spark plug cable.  From the description, I wasn't completely sure which model to buy.  But when I went to the manufacturer's web page, I found they had a universal "Commercial" model that also updated faster than the standard units.


Only $65, so I went ahead and ordered one. It had both a tach function as well as an hourmeter and two service timers. It didn't need ANY other connection other than to wrap the sensor wire around a spark plug cable. It has a built-in battery.

When it came in, I went to the airport and tried a temporary installation. No go...tach read zero. I suspected the shielding on the tach cables, and a call to the tach vendor confirmed it. The tech recommended peeling the shielding back near a plug, but I didn't want to run my tach wire out into the slipstream.

One of my spark plug wires is about 9" too long, and a previous owner had wrapped up the excess and tie-wrapped it to the engine mount.  I figured I'd modify the wire there, so if I botched it, there'd still be enough cable to connect to the mag. So I picked at the shielding with a toothpick to make some openings, then carefully plucked away at it with an exacto knife. A few minutes work, and I had a 1" long section of cable free of the braid.  (Note:  More-modern plug wires may not have this external braid!)

The tach instructions said to wrap the red wire around the cable 3-4 times. The tachometer worked, but the idle was erratic.  I eventually added a couple of extra turns to make the reading more stable.

I made a mount for the Tiny Tach from some leftover 1/8" plywood. I could have just used a flat sheet to attach behind the panel, but I ended up with a "sandwich" construction where the outer face was circular with the diameter just less than a standard instrument hole. I felt this gave a lot nicer look to the installation....it looked more like an instrument sitting "in the hole" than just a flat sheet behind.

One thing I had to do was plug off the old tach cable port on the back of the engine. Aircraft Spruce had a cap for Lycomings that had the same thread sizes, so I took a chance that it'd fit my Continental.

It did, but the hardest part was safety-wiring it in place. The cap sits between the generator and the right mag, and there aren't any good places nearby to connect the safety-wire to. I finally settled on a fitting eight or so inches away, and fed in a long piece of safety wire through pre-drilled holes in the end of the cap.

I was wiggling the wire around, trying to curve the other end back around, when I heard a crackling sound. I stepped back and saw smoke rising. The loose end of the safety wire had flipped up and had shorted a +12V terminal to ground!

My arm shot out and I grabbed the wire. Bad move. It was a 0.032 stainless steel wire with about 20 amps flowing through it. I turned it loose and grabbed a pair of pliers.

Problem solved...but now I had a nice little burn-blister across the last joint of my middle finger.

The cap was finally safetied, and I ran the tach wire ran over the same route as the old tach drive cable.  Wrap the red sensor wire around the  open area between the braid four times, then lay the sensor wire parallel to the spark plug wire and wrap the whole area in copper tape.   I was concerned about the open braid causing radio noise.

No radio noise...but no operation of the tach, either.

I figured it was the copper tape, so I stripped it away and straightened the sensor wire.  That got things working again.

I'd brought a headset with, and noticed that the open section in the braid produced only a very minor bit of popping into the radio.  But I'd started to wonder if maybe my copper tape wrapping HADN'T been the problem, earlier.  I had laid the rest of the sensor wire horizontally along the plug wire under the copper tape.  The diagram from the tach manufacturer shows the sensor wire coming straight away from the plug wire, instead.  So I re-wrapped the section with copper tape, taking care to let the sensor go directly away from the plug wire instead of wrapping it in the copper tape.

And lo and behold, it worked.

I wrapped the area with electrical tape for a bit of durability.  Finally, the loop of spark plug wire was tied back the way it had been, against a tube of the engine mount with tie-wraps.

A test flight produced very nice operation.  There's still a bit of instability at low power settings, but once the RPM crosses 1000, the tach is rock-steady.  The copper tape shielding is working great; no ignition noise is audible.

Since it's connected to the left magneto, conventional "Mag checks" are a thing of the past.  When the mag switch goes to "R", the tach goes to zero...it can't register if the magneto isn't firing.

I don't see this as much of a drawback.  I've been less concerned with the actual RPM drop during a mag check than with rough running, instead.  I figure that any real problem will manifest itself as missing and stumbling, rather than hitting 155 RPM on a mag drop.

In any case, I own a small handheld tach, and will use it at annual time to make sure things are hunky-dory.

The hourmeter function is pretty slick; it registers hours until the engine starts, then automatically switches to the tach mode.  However, the hour meter shows hours and minutes only until the tach reaches 199:59; after that, it reads whole hours only.

Not a real drawback, as the tach also includes a secondary counter that can still be used to keep track of minutes.  However, 200 hours is about five years of flying for me, and the built-in battery of the Tiny Tach is only good for 5-8 years.  At that point, I'd better hope the company  is still operating...or come up with some other solution.

Update - July 2016

Well, all good things must come to an end.  I installed the original Tiny-Tach in July 2009, and by late 2014, the display was starting to get a bit faded.

But...shoot, it still worked.  So I ordered another one as a stand-by, but left the old one in place.  Surprisingly, it held on for another year and a half.  Two weeks ago, it was very, VERY faint (but readable).  But by the end of my last flight, it was completely dead.

Pretty good, really.  The company guarantees five years (which is about when it started to fade), but I got seven years' service out of it.

First decision was whether to install the one I'd ordered earlier.  They were good for five years, and this one had already used up almost two years of life.

I decided to make life easier and install a connector at the spark-plug-wire-wrap.  That way, I figured, I could just quickly set up and install a new one if this one goes bad.

Plans shattered with trying to run the cable for the new tach.  I could NOT get it to feed from the cockpit.  I eventually had to remove the top cover forward of the cockpit to get access to the run.  With that, though, it was easy.

Super easy to finish the run, here.

But when I got things forward, I started to re-think the connector idea.  Replacement of the Tiny Tach wasn't going to be as simple as I desired.  Why take a chance on the connector messing things up totally.

As it turned out the old plug-wire wrap was easy to remove.  The new one was easy to apply, but for some reason, the gauge didn't work!

Not sure what happened.  Maybe the application of the copper tape interfered with it.  Or...maybe the Tiny Tach design had changed a bit.  I tore everything open again, and started from scratch...and found I needed only half the number of wire turns I'd had before.  The instructions caution that too MANY turns can cause issues.  Don't know if it was just my installation, or the new gauge is more sensitive. 

In any case, the fewer turns appear to have done the trick.  It's working now.

On Tiny Tach Battery Replacement

(added June 2017)

I had originally intended to cut open the old Tiny Tach and investigate methods of making the battery replaceable.

Decided against it for one main reason:  As I mention above, when the hourmeter on the tach passes 200 hours, it counts ONLY whole hours rather than hours and minutes.  Frankly, that's of little use to me, and my tach had enough hours that I'd have to deal with it in a year or so.

I do have an old Hobbs meter; considered using that to keep track of hours instead.  But I like the cleaner approach of just replacing the whole unit.

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