Rotax 912 ULS

#1
Here is a nice little conundrum for all you 912 fans:

Background: Engine & aircraft 849.5 Hobbs Hours (doing 50 hr oil change & inspection this coming weekend).

Symptom: Within the last two flight hours, my tacho (a Rotax original) had developed an intermittent hiccup/pulse (clockwise or if you like increased rpm by about 100-200 rpm) right through the rev range.

Other: Nil other symptoms. All engine gauges indicate normal. Have noticed no change in engine performance. Runs very smoothly without apparent hesitation. Ignition check L/R - Both normal.

I had thought a loose connection but wouldn't the gauge rpm drop, rather than rise??

Any ideas/suggestions????
 

cscotthendry

Well-Known Member
#3
Connect a 200 ohm resistor across the signal and ground leads at the tacho. Should fix your problem. Also search the forums here for threads on this where it is discussed at length.
 

kasper

Well-Known Member
#4
Connect a 200 ohm resistor across the signal and ground leads at the tacho. Should fix your problem. Also search the forums here for threads on this where it is discussed at length.
Had the same tacho response when I changed the 912 out on my trike years ago - old engine into flydat was perfect, new engine went from 2000rpm to full flip at 2001rpm. its just a rotax thing - their long held rep for quality can be a bit iffy on electrics.
 
#5
Connect a 200 ohm resistor across the signal and ground leads at the tacho. Should fix your problem. Also search the forums here for threads on this where it is discussed at length.
Thanks Scott - checked out JayCar; they do a 180 ohm & a 220 ohm in 1 Watt - I assume the 220 would be the way to go ???
 

cscotthendry

Well-Known Member
#6
Thanks Scott - checked out JayCar; they do a 180 ohm & a 220 ohm in 1 Watt - I assume the 220 would be the way to go ???
Yes, the actual resistance value is not critical. Most tachos have a high impedance input design. The tacho coil on the rotax engine is an inductive pickup (basically a coil of wire around a metal core) that is triggered by a magnet on the flywheel whizzing past it. This can generate very high voltages when coupled to a high impedance. The high voltages overwhelm the semiconductors in the tacho's input circuit. Putting a resistor in parallel with the tacho input lowers the impedance and drops the volts down to the point where they don't cause "breakover" in the input transistors of the tacho.
 
#7
Purchased the 220 ohm 1 watt carbon resistor ( x 2 /pack) from JayCar - will fit and report back shortly.

Perversely, just did a 2 hr round trip - didn't notice the tacho "jump" even once
 
#10
Got another question for you Scott; Why isn't a resistor incorporated into the tacho by the manufacturer?? Its not like it would add significantly the cost, would prevent these little erroneous readings and add to the overall perception of reliability of the device..
 

cscotthendry

Well-Known Member
#11
Very good question Skippy! Especially relevant to tachos specifically marketed for Rotax engines.
My only guess would be the Heisenberg uncertainty principle. It is impossible to measure something without affecting it. So my theory is that, tachos are made like multimeters, with the highest input impedance possible, so they present less loading to the signal source, thus not affecting it so much
 
#13
Sorry Scott - poor excuses to follow - been very very busy with non aviation activities AND the fact that the tacho didn't so much as wobble on my recent 2 hr trip has lessened any earlier urgency (to correct problem) that I might have felt. Will try to get to it sometime next week
 

johnm

Well-Known Member
#14
its interesting when a tacho fails on you in flight............ a decreasing tacho read

your brain can easily imagine you are having an engine failure (irrespective of engine noise)

........ been there done that
 
#15
its interesting when a tacho fails on you in flight............ a decreasing tacho read

your brain can easily imagine you are having an engine failure (irrespective of engine noise)

........ been there done that
Yep the brain sure can play tricks on you - I regularly think my engine develops an intermittent miss over tiger country. Runs as smooth as silk at all other times.
 
#17
Okay - have (temporarily) fitted the 220 ohm/1 watt resistor. Test flight was supposed to be a 2 hr round trip up the coast but a near stationary rain cell over the Sydney Lane of Entry/Exit caused me to return to base - entire exercise including some touch -&-goes etc etc was 1.1 hrs - and the tacho ? no untoward fluctuations. Hard to say if the resistor has done the job cause the last (pre resistor) flight had no tacho anomalies. I guess time will tell if this little mod has done the trick.
 
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