Erratic RPM indication

I've been enjoying my dual AFS 5600's over the past 70 hours in the RV 10. Along the way, I've noticed the occasional erratic RPM indication. The indication may show high above redline and then a second later, too low. Aural alarms get the BP raised but the actual engine RPM itself is not changing at all leading me to believe it's a sensor or wiring issue. Upon inspection under the cowl, the only indication of something out of place was the black RPM sensor cylinder was loose within the red anodized housing that was screwed into the magneto. That was corrected with RTV and I thought the problem was solved until today when I noticed another erratic reading. Inspection of the sensor revealed it was snug. Has anyone experienced this? Faulty sensor ? I checked continuity of the wires and everything checks out although the unpredictable nature of the problem may indicate a loose connection somewhere. Ideas?

David

Make sure the tach wire is not running with the p leads anywhere in the wire bundle, it will give you erratic readings.

Thanks Gary. I was also worried about interference and checked the P-lead today. The wires don't run together and the closest they come to one another is at the mag itself (couple inches apart). Nevertheless, I put some aluminized tape on the wires and went for a flight. Erratic RPMs again with no change. I'm suspecting an issue at the sensor itself but I won't know until I replace it. Arrggg. Frustrating, especially when Bitchin' Betty doesn't leave you alone and continues to yell in your ear

D

I had the problem as well. I finally put two and two together and determined it to be a heat problem on the sensor. It would only happen on long flights. Short duration flights no problems. I put a blast tube on it and the problem has no longer appeared. This included a 5 hour flight to Osh. Prior to that, I tried replacing the connectors at least three times, no joy.

As Alton said, I have had the problem with an overheating sensor. A blast tube solved the problem.

AltonD said:

I had the problem as well. I finally put two and two together and determined it to be a heat problem on the sensor. It would only happen on long flights. Short duration flights no problems. I put a blast tube on it and the problem has no longer appeared. This included a 5 hour flight to Osh. Prior to that, I tried replacing the connectors at least three times, no joy.

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The magneto pick-ups are sensitive to temperature for sure. I used the p-leads for the one magneto in my Rocket to avoid this, but that started out as a disaster as well.

Turns out that the p-lead waveform is sensitive to just about everything...rpm, map, plug wires, plug condition, magneto age, compression ratio and a few other things I've forgotten.

Worse yet, every firing event is slightly different, so on a six-cylinder engine, there is no single ideal waveform with which I could deal. By varying the input resistors to my engine monitor, I could get proper indication some of the time, but it was unreliable. Eventually, I designed a device called the P-TACH that I put up on the MakerPlane site as open source hardware.

This device stabilises the p-lead pulses and provides reliable digital signals into the engine monitor. Better yet, it sits on the cold side of the firewall so it can't experience the harsh engine environment.

Of the three main rpm sensor types (tach drive sensor, magneto sensor, p-lead sensor), the p-lead sensor is the only one that avoids the effect of heat on it's performance.

Follow up

I wanted to mention that I tried the following interventions:
1) Ensuring the P lead and sensor wires were far apart
2) Wrapping the wires in aluminized tape to avoid RFI
3) Installing a blast tube
4) Placing high temp RTV to secure the sensor plug into the housing
5) Placing epoxy to secure the plug into the housing (per AFS advice)

Nothing worked. So, I opted to replace the sensor. AFS was kind enough to provide a replacement under warranty since the sensor only had ~60 hours on it. The new sensor has worked like a charm now for ~10 hours with no signs of trouble. Had I remembered that the sensor was only $40, I may have opted to replace it sooner

David

Hmmmm. You forced me to look. I was thinking much more for the sensor. Mine is $55 from AFS. It is the only sensor I do not have a spare for. Guess I will be ordering one.

Update

After the RPM sensor replacement around 70 hours, things have been flawless until last week when the erratic RPM indication returned. The plane is now around 250 hours so I doubt it's an installation issue seeing as how it ran 180 hours without a problem. Seems to add credence to the possibility of the sensor being heat sensitive but dunno. New sensor to be ordered shortly.

I saw almost he exact same problem. AFS had a system to "filter" the signal and the problem hasn't returned.

Tach Indications

I had a similar problem early on with my AFS 5500 with wandering tach indications. Finally traced it to the ends of the blue electronic ignition leads where the center carbon wire folds over on top of the blue insulation before clamping on the snap connector to the plug. Too tight a bend will break the carbon leaving a minute gap, not enough to cause misfiring, but enough so the impulse is raised so high to jump the gap it puts out a huge EMP. That gets picked up by the AFS 5500 RPM sensor and caused small erratic jumps. I found the problem by checking the wire continuity. I solved the problem by using an awl to role the carbon wire around to the top of the blue insulation before clamping the connector. Continuity checked good and no more erratic RPM indications.

Jesse said:

I saw almost he exact same problem. AFS had a system to "filter" the signal and the problem hasn't returned.

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Jesse - I haven't heard of this. I'll ask Rob at AFS about it. Thanks!

Stu McCurdy said:

I had a similar problem early on with my AFS 5500 with wandering tach indications. Finally traced it to the ends of the blue electronic ignition leads where the center carbon wire folds over on top of the blue insulation before clamping on the snap connector to the plug. Too tight a bend will break the carbon leaving a minute gap, not enough to cause misfiring, but enough so the impulse is raised so high to jump the gap it puts out a huge EMP. That gets picked up by the AFS 5500 RPM sensor and caused small erratic jumps. I found the problem by checking the wire continuity. I solved the problem by using an awl to role the carbon wire around to the top of the blue insulation before clamping the connector. Continuity checked good and no more erratic RPM indications.

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I'm running dual mags but I wouldn't be surprised if it's not the sensor but rather, some electrical gremlin causing the issue.