Odd problem with Aviasport tacho on 912ULS

[cscotthendry]

I've been having some strange behaviour from my tacho recently and I thought I'd share the fix on the forums here.

 

Whenever I go to full throttle or any power setting above 5,200 RPM, my tacho would imtermittently go to full scale. In cruise at 5,000 it would be fine and stable.

 

I tried a few suggested fixes and the one that did the trick was to connect a 220 ohm resistor across the leads from the pickup on the engine.

 

*geek speak alert*

 

Here's what I think is happening:

 

The tacho pickup on the 912 engine is an electromagnegtic coil. A magnet on the flywheel induces a pulse in the coil when it passes by. This produces a pulse which has a positive voltage spike at one end and a negative spike at the other end. Which polarity spike occurrs first is dependent on which lead of the coil is cconnected to ground.

 

In any case, as the engine RPMs increase, the speed at which the magnet passes the coil increases and the voltage level of the spikes increases.

 

This next part is speculation on my part. I expect that the input to the tacho gauge probably has a capacitor in series to allow the spikes to pass, but blocks the DC component of the signal. As such, the tacho looks like a very high impedance load (virtually an open circuit) to the pickup coil. With no load on the pickup coil, the spike voltages can go quite high (I've seen estimates of around 100volts). These high voltages are causing a breakdown. In the circuitry inside the tacho which is causing the erratic behaviour. With a 220 ohm resistor across the pickup, the voltages never get that high and the tacho behaves itself.

 

What I've found: the Aviasport gauges supplied by Rotax, are a D'arsonval movement meter driven by a Microchip PIC microcontroller. The microcontroller senses whatever input it is designed to sense, in the case of the tacho, it counts engine pulses, in the case of the temperature gauges, it reads the resistance of the senders, and then the microcontroller translates that measurement into a scaled analog voltage for the meter movement.

 

With benign signals like those from resistive temperature senders, the microcontroller is quite safe and reliable. However with an active signal source like the inductive tacho pickup, the micro needs to be protected from overvoltage situations as I described earlier.

 

Given that these gauges have been designed to specifically work with Rotax engines, I think Aviasport should revisit the tacho circuitry. In the mean time, if you use these gauges on your homebuilt and have the same problem with the tacho, try putting a low value (220 ohms) resistor across the tacho pickup leads and see if that fixes the problem. But before you do that, make sure you don't have a loose connection somewhere causing the problem.

 

 

[Guest mark gray]

Hi Scott, yes I had exactly the same problem and went thru two aviasport gauges before I stumbled on the solution you describe. It may not be widely known as Floods had no idea. I needed to fit the smaller 2 1/4 gage due to space requirements. My first one lasted a couple hours and pegged out. The second one worked for a while and then would go off scale after 5k rpm or so. I then tried the 3" gage and it worked perfectly, but I needed the 3" hole for something else. I imported an American 2 1/4" gage and it helpfully had the instruction that if the gauge becomes erratic at high revs, to fit a resistor as you describe and it works perfectly. I can only presume the larger 3" gage has room enough to have a resistor inside? It's interesting you should make this post as when I was having my issues with 2" gauges, I saw on your plane at Watts and noticed the smaller tacho and I was wanting to ask how you were going with it. I agree that aviasport should address the issue as the gauges are not cheap.]regards, mark

 

 

[planesmaker]

Hi Scott, yes I had exactly the same problem and went thru two aviasport gauges before I stumbled on the solution you describe. It may not be widely known as Floods had no idea. I needed to fit the smaller 2 1/4 gage due to space requirements. My first one lasted a couple hours and pegged out. The second one worked for a while and then would go off scale after 5k rpm or so. I then tried the 3" gage and it worked perfectly, but I needed the 3" hole for something else. I imported an American 2 1/4" gage and it helpfully had the instruction that if the gauge becomes erratic at high revs, to fit a resistor as you describe and it works perfectly. I can only presume the larger 3" gage has room enough to have a resistor inside? It's interesting you should make this post as when I was having my issues with 2" gauges, I saw on your plane at Watts and noticed the smaller tacho and I was wanting to ask how you were going with it. I agree that aviasport should address the issue as the gauges are not cheap.]regards, mark

I had a similar problem in that at certain rev range (2500 -4000) the tachometer would read abnormally high. Tried 3 different tachos and also changed trigger unit. After some searching the solution was to put a diode in the circuit, I put it in the positive pulse wire. Fixed!

Tom

 

 

[piecrust]

I had a problem early last year with my vdo tacho. The hr meter would glitch during flight. I was about to remove the gauge and send back to an authorised repairer. I have since had a total failure of my voltage regulating device. ATSB incident report and raa report completed after an emergency landing at Naracoorte. I no longer have any problems with my vdo hr meter since a new vrd has been installed. It's only now that I attribute the glitch to the faulty/failing vrd. Early warning sign of failure?