Battery powered digital tach

[Collin Campbell]

I am finishing up the rebuild on Bob's LSA. Been having some issues with the tachometer he wants to use. It is PN# TLP14 from Aircraft Spruce. So far I have tried everything Bob and I can think of to get it to work. It is a lightweight unit (only 65 grams...) has an internal battery, simple to hook up...only two wires, one is ground to the engine and the other (a induction/signal wire) wraps around a plug wire. The thing acutally does work, BUT the readings are all over the place....at mid range throttle, I am seeing readings bounce around from 700, 1200, 2100, 3000 rpm.

I have tried moving the ground wire to various locations, Bob even had me attach the induction wire to a plug wire with a short section of the shielding removed (thinking the shielding was blocking the signal). Same results... We tried attaching the induction wire directly to the spark plug itself...still the same. I tried a temporarily routing ot the wires outside the window away from any potential interference.

I called the tech service rep from the company that makes them. He said possibly I was getting interference from some source or the unit was defective. Well, I ordered another unit and it gives the same results. And intererence??? On Bob's planes??

At wits end at this point, does anyone have any experience with this kind of instrument?

[gregc]

That is a puzzler. My only idea is that it should work on any gasoline engine so try it on a lawnmower or something and see what happens.

[David Swartzendruber]

Collin, I don't have any experience with the tach you are using but I thought I'd pass along something that helped solve a similar problem with another type of digital tach. Someone with a Rotax 912 was having an issue with the tach going crazy at higher rpms but was okay at lower rpms. The issue ended up being resolved by adding a resistor in series with the signal going into the tach. Evidently there was enough noise on the signal that the tach was picking up additional random pulses. Adding the resistor damped out the lower amplitude noise enough for the tach to ignore it and then the tach only picked up the real pulses. If you wanted to try this with your issue, I would think adding the resistor in the ground wire would have the same effect as adding it in the induction wire and then you wouldn't have to cut any wires to try it. The resistor we used to resolve the tach problem with the Rotax engine was a 10K, 1/4W resistor.

[jcowgar]

I wonder if it is worth the hassle to figure it out. Not to sway your decision on that tach, but I looked at it and was interested, but then I saw in the Q&A "While the battery life is on average 4 years, it can sometimes last up to 5 years but the battery is built in and not replaceable." I have seen others say it lasted 3 years. Once the battery is dead, I understand you throw it away and buy a new one. While I don't know the weight, there are similar models that work on a coin battery such as https://www.aircraftspruce.com/catal...?clickkey=4246 for about the same price. I would imagine about the same weight as well.

[Bcone1381]

am seeing readings bounce around from 700, 1200, 2100, 3000 rpm.

I have no experience with that tach. But once upon a time I flew a non-electric Taylorcraft with Non-resistor spark plugs. We could hear ignition noise....a crackle every time a plug fires....through a hand held com radio. The noise reacted to change in RPM. It was pretty bad, as the engine turned faster the noise/whin increased. The radio worked well other aircraft. We learned about resistor/non-resistor plugs. I am wondering if that tach is designed to pick up signals like that handheld did in the Taylorcraft but with non-resistor plugs it's getting signals from other cylinders too?

If the engine was sitting at 700 RPM and picking up some other signals from those plugs, I'd think you would see values of 700, 1400, 2100, 2800. Those are very close to what you are seeing.