Dynon Tach Bounces up and down 300 RPM in cruise

X21RV6A · #1 11-06-2013, 02:39 PM

Noticed this over the weekend after a departure from the airport. Started to level out, RPM/Manifold transitioned to 23/23 and the Dynon Tach indicated RPM's jumping from 2300 to 2600 or higher than back to 2300 then back to 2600 again. I cycled the mag switch and after the 2nd time it stopped. This happened 2 additional times that same day, each time after climbout and transiting into cruise and was stopped by cycling the mags one or two times. Engine is a 0360-A1A with CS Hartzell and standard mags. I couldn't tell if the engine was really losing RPM or if it's just the Dynon D10-A displaying artifact?

Do I have a switch problem or an early sign of a mag going south? The mags were rebuilt 120 hours ago and have 740 total time.

Any suggestions?

cfiidon · #2 11-06-2013, 03:19 PM

Brian,

Dynon specifies 30k or so resistors for the tach inputs, or more if required. My data logs sometimes showed very high tach values. I had to up the value of the resistors to 100k. Don't know if this is your set up or not.

Don (at Dunn)

Ralph Inkster · #3 11-07-2013, 12:56 AM

Also check ign switch ground.
If progressively higher resistance doesn't work, Vans sells a transducer that mounts on the engine that will solve the problem, than you only need to use the Lt standard tach input.

Walt · #4 11-07-2013, 04:46 AM

Try moving the pick up to the other mag, if that fixes it then time for the mag to go back to the shop.

vlittle · #5 11-07-2013, 05:58 AM

I had to develop an external circuit to make the Dynon tach reliable in my application (high compression IO-540). I attempted to us a trimmer potentiometer to set this, but could never find the sweep spot.

My recommendation is to use the AFS or Vans sensor. My circuit works, but it needs some electronic skills to put together. Check the Dynon forums, it's all there.

Stu McCurdy · #6 11-07-2013, 07:48 AM

I have an AFS 5500 installed. At one point I developed a 200-300 RPM oscillation in the Engine Monitor portion. After going through all the possible problems and fixes, I finally used a meter to check plug wire continuity. Sure enough, I had an Electronic blue plug wire that showed no continuity. I pulled the cap off to find the center core carbon wire broken where it doubles over on top of the blue insulation and the cap crimps on top. The spark from the tower had to be increased so much to jump the gap that it was putting an electronic input into the RPM wire inducing the oscillation. As soon as I fixed the broken plug wire end, the oscillation went away. Probably started when I pulled and rotated the plug wire cap to remove it to clean plugs during Condition Inspection.

vlittle · #7 11-07-2013, 08:26 AM

Quote:

Originally Posted by Stu McCurdy

I have an AFS 5500 installed. At one point I developed a 200-300 RPM oscillation in the Engine Monitor portion. After going through all the possible problems and fixes, I finally used a meter to check plug wire continuity. Sure enough, I had an Electronic blue plug wire that showed no continuity. I pulled the cap off to find the center core carbon wire broken where it doubles over on top of the blue insulation and the cap crimps on top. The spark from the tower had to be increased so much to jump the gap that it was putting an electronic input into the RPM wire inducing the oscillation. As soon as I fixed the broken plug wire end, the oscillation went away. Probably started when I pulled and rotated the plug wire cap to remove it to clean plugs during Condition Inspection.

Good point. One of the advantages/disadvantages of using the p-lead as a tach source is that it provides an indirect measurement of ignition health. Unfortunately, I think I would prefer a reliable tach source AND and independent ignition analyzer.

As it turns out, I have just purchased a portable oscilloscope for this very purpose (ignition diagnostics), plus other day-to-day uses. I plan on having a p-lead post in the cockpit so that I can easily check the ignition health at Annual time or even in flight. It should get me invited to more cocktail parties as well.

Eventually, I will publish waveforms on this forum as a reference. I have been unable to get this information anywhere on the 'net. I will do both my O-320 and my IO-540.

X21RV6A · #8 11-07-2013, 07:25 PM

Quote:

Originally Posted by Stu McCurdy

I have an AFS 5500 installed. At one point I developed a 200-300 RPM oscillation in the Engine Monitor portion. After going through all the possible problems and fixes, I finally used a meter to check plug wire continuity. Sure enough, I had an Electronic blue plug wire that showed no continuity. I pulled the cap off to find the center core carbon wire broken where it doubles over on top of the blue insulation and the cap crimps on top. The spark from the tower had to be increased so much to jump the gap that it was putting an electronic input into the RPM wire inducing the oscillation. As soon as I fixed the broken plug wire end, the oscillation went away. Probably started when I pulled and rotated the plug wire cap to remove it to clean plugs during Condition Inspection.

Stu,
Your suggestion is in my opinion the most plausible. I have both P-Leads connected to the keyed switch/Dynon and recommended resistors in place since the installation a year ago and just now having the issue. I recently had all the plugs out to be cleaned and gapped and it's possible one of the blue wires was damaged. I did notice that one had a pretty tight bend as a result of coming into contact with the baffle and cowl. Will check that first and report my findings.