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C172 tachometer problem

I fly a C172 S (G1000 version). The tachometer has started acting up in the last few days. Below about 2,100 revs it reads OK. At power settings above that it fluctuates wildly jumping up to 6,100 and back to about 2,500, all within a second or so. It continues to do this and some times a red “X” appears. It is only the reading which fluctuates, the engine continues to operate normally. I would like to get an idea of what might be causing this before I get a maintenance company to look at it so that I can understand what they might say. Could it be a bit of dirt on the reading mechanism which makes it more difficult to determine the reading at higher revs? Any suggestions?

What is the transducer?

If it is a sensor which basically counts the starter gear teeth, I would check the gap between the transducer and the gear.

If you had a problem showing up at low revs, it would be easy to check with a scope while somebody turns the prop by hand, but in this case you can’t.

One could bring out the signal to a scope (with a bit of wire) and go for a flight, which would immediately reveal whether it is the sensor or something else.

Shoreham EGKA, United Kingdom

Which tacho, the display in the G1000 or the clock? If the latter, does the drive shaft turn without strange noise? When the drive shaft shows no problem, don’t bother with the mechanical tachometers, as they are only about 200 USD to replace.

It is the G1000 display which is showing the fluctuating readings. I’m hoping its something as simple as a loose connection but checking the gap between the transducer and the gear sounds like a good idea.

It’s fairly easy, for an electronics “boffin”, to connect a signal generator in place of the transducer and just wind it up.

A few years ago, I was having loads of similar hassle with a Shadin fuel totaliser system, and I built a little signal generator (schematic). It took me maybe a couple of hours.

But those systems have a long history of spurious readings. Your case looks like a one-off so a sig gen like this £220 Ebay one would be an obvious way to check it.

Shoreham EGKA, United Kingdom
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