Tach Attack

A month or so back, I asked about re-setting the reading of a standard
aircraft tach to reflect the time the airplane spent with a temporary
replacement.

Unfortunately, as it turns out, there was nothing wrong with my
tach...or with the tach cable. The tach-drive section on the engine
itself (Continental C85) was bad.

Once I was convinced the engine-side tach drive was munged, I started
looking into the options for electronic tachs. 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 company's web page, I found they had a
universal "Commercial" model that also updated faster than the standard
units.

http://www.tinytach.com/tinytach/commercial.php

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.

The tach instructions said to wrap the red wire around the cable 3-4
times. I gave that a shot, and the tachometer worked.

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
the panel.

http://www.bowersflybaby.com/tech/tach_done.jpg

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 across the last joint
of one of my fingers. Got a beau-ti-ful blister, now.

http://www.bowersflybaby.com/tech/tach_owie.jpg

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.

The tach worked now, but was very erratic at low RPM. I wrapped the end
of the sensor wire around the plug wire one more time, and the problem
got better. So I wrapped the rest of the red wire around the plug
(about six turns). The idle was now stable, except it would glitch
momentarily every five or ten seconds.

http://www.bowersflybaby.com/tech/tach_coil.jpg

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.

http://www.bowersflybaby.com/tech/tach_copper.jpg

Wrap the area in electrical tape for a bit of durability, then re-secure
the spark plug wire to the motor mount with tie-wraps, as before.

http://www.bowersflybaby.com/tech/tach_wrap.jpg

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.

Ron Wanttaja