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Tachometer or Engine Issue

FlyingBanker

Well Known Member
Patron
Help me troubleshoot, please. My RV6A recently started reading about 500 rpm too low pretty consistently. I have mechanical cable from the engine with a transducer and Vans IE VTACH GEN 3500 with transducer. I?ve cleaned and lubed the cable and still reading low. The engine sounds like normal performance, has normal performance numbers, all other engine instruments reading normal. If I knock on the face of the tachometer, it has sometimes jumped up a couple of hundred rpms on the reading. How can I be sure if the issue is a tachometer issue (which seems the likely culprit) or an engine issue? Is there some sort of diagnostic testing I can perform?
 
. How can I be sure if the issue is a tachometer issue (which seems the likely culprit) or an engine issue? Is there some sort of diagnostic testing I can perform?

Assuming an FP prop, at the same altitude and temperature, a given RPM should net a consistent TAS. Therefore, if your RPM reads 500 low for a known to you airspeed/altitude/RPM combo, you know it is an indication error.

Larry
 
I haven't had the plane that long and don't have an outside air probe, so hadn't yet calculated any TAS standards to measure against. Other thoughts on how I can troubleshoot this?
 
I haven't had the plane that long and don't have an outside air probe, so hadn't yet calculated any TAS standards to measure against. Other thoughts on how I can troubleshoot this?

This applies to IAS as well. You can get an rpm reader at harvor freight. It uses a sticker on ther prop.
 
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There is no way to diagnose this without a digital tachometer, but my guess is that it is a reading issue. The transducer is prone to failure, as is the gauge. I had similar symptoms. I replaced the gauge with no improvement, so I replaced the transduce and that fixed it.
But "fixed" is a relative term. I never found the readings to be very stable, so I replaced it with a MGL RPM-1. It costs about the same as replacing the gauge and transducer.

https://www.aircraftspruce.com/catalog/inpages/mgl-blaze-11-16591.php

With the mag sensor you will get rock solid readings

https://www.aircraftspruce.com/catalog/inpages/mgltachslick.php?clickkey=1900508
 
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