RPM gauge occasionally jumpy

[gsengle]

I have a '96 Ovation.  I've started to notice that on occasion my rpm gague jumps.  The rpm is not actually surging, the prop control works fine, nothing changes when i see these jumps, and the jumps are too fast to be as near as I can tell anything other than a spurious indication.  Any thoughts?  It jumps higher never lower.  For instance, if I'm cruising at 2400, it will start to twitch higher, to 2500 and back, to 2600 or 2700 and back.  It never jumps lower than the set rpm.  Then it will go away.  My suspicion is 1) this will get worse over time, and 2) that it is the gague.  I have one more flight to get it home to where it can be looked at.

So, how does the RPM gague work?  Anything bad that could be happening that I should really get concerned about?  (yes, it will be looked at soon, but wanted this group's input)

 

thanks!

Greg

[MooneyMitch]

Hi Greg.

I do believe your tachometer is electronic, as is mine, with no actual tachometer drive cable off of the engine.  The dial is certainly analog, but the internal components are fully electronic. 

From your brief description, it is sounds as though you may have an internal component issue or perhaps even a wiring/connector issue behind the tach. unit.

If it were me, I would start at the rear of the unit, making sure the connector is secure.  Sometimes looking for the simple things first can be the ticket.

[Lionudakis]

A tach cable that is getting ready to let loose can cause that. It spins considerably slower speed than the engine, if it's binding in it's flexible housing it can catch, wind up, and release to put it simply, causing the spike. Same goes for inside the instrument too. It's just a cable housing that screws onto the tach drive on the back of the engine, with what looks like a small wound cable/spring that runs the length and spins, transmitting the rotation to the tachometer. It's simple, either can cause it, neither one cheap. 

[Lionudakis]

Oh, Unless it's electronic ! If so, check wires first as Mitch said

[MooneyMitch]

Quote: Lionudakis

A tach cable that is getting ready to let loose can cause that. It spins considerably slower speed than the engine, if it's binding in it's flexible housing it can catch, wind up, and release to put it simply, causing the spike. Same goes for inside the instrument too. It's just a cable housing that screws onto the tach drive on the back of the engine, with what looks like a small wound cable/spring that runs the length and spins, transmitting the rotation to the tachometer. It's simple, either can cause it, neither one cheap. 

[MARZ]

And for those of us with cable tachs - not a bad idea to pull the core and inspect it lube it up once in a while if it looks dry !

A judicious amount of molly lube does the cables well.

[blacknchrome]

Replace the tach.  The cable is probably going bad as they mentioned.  I had this exact problem 6 months ago.  New tach = problem solved.

[randypugh]

Greg, did you ever find the problem with your tach? Mine is starting the same thing

[jwilkins]

My 80K started doing this. The shop found a loose connection on the tach wiring that was causing readings to be higher (sorry but I don't remember where the loose connection was) We got that fixed so the readings were steady but the calibration was off by about 100 RPM, which I noticed as a difference between the ship's tach and the JPI, I had the shop check with their optical tach on the prop to see which was right. The JPI and the optical agreed spot-on. The ship tach read about 100 RPM high.

So I had a Horizon electronic tach installed. Since my 80K did not have an hour recording tach (only a Hobbs type meter buried in the co-pilot foot well) I also picked up an easily read-able hours meter on the Horizon.

My original gauges are over thirty years old now, so the decision to replace instead of re calibrate or repair is different than it might be on a '96.

[rxo188]

I have a stock 1987 J model and I've observed my analog RPM gauge surge up to 3500.  It started a year ago with the RPM going 1-200 RPM over the 2700 limit, so I would dial the prop lever back a shade to bring it back to the limit, but now the oscillations have grown to span the entire RPM gauge.  When I dial down the RPM at high power settings it will stop oscillating above the red line when the lever is at about 2500, at which point it locks on and stays unmoved at 2500. If I increase above 2600 at high power settings it will immediately oscillate around 3000 +/- 300.  The needle goes anywhere from 2700 to 3500 and oscillates 3-400 RPM in that range during high power settings with no audible change in prop pitch, no changes in egt/cht, no changes in fuel flow, and no feel of acceleration/deceleration, as one would expect from such quick changes.  This only happens at high power when the prop lever is all the way forward, such as on T/O and climbout. In cruise at when I reduce power to about 23MP it settles in the green range at the RPM I want. If I take it to 2700RPM at 23MP, then add full power there is no change in prop noise, but the rpm will again fluctuate up and down from 2700 to 3500.  In cruise flight about 10 min into a flight it works like it should.  Anyone else see this? I haven't yet taken it in.

[carusoam]

Expect that a worn cable, or not well lubricated cable can do this. If left unchecked the needle on the rpm gauge may flop off.

Good news is, you can buy a very nice digital replacement for not a lot of money...

For back-up ideas, in the event the instrument is lost....

This is where the advice to check the position of the blue knob vs rpm of the engine while you still have instrumentation.

Knob full in 2700 rpm. How many turns = 2500 rpm? And again 2400 rpm?

You can purchase an iPhone app that senses rpm using engine sounds.

Best regards,

-a-

Experience came from an M20C, ymmv...

[fantom]

Why are you not getting it checked out? Time to go digital and the sooner the better.

[Marauder]

I have a stock 1987 J model and I've observed my analog RPM gauge surge up to 3500. It started a year ago with the RPM going 1-200 RPM over the 2700 limit, so I would dial the prop lever back a shade to bring it back to the limit, but now the oscillations have grown to span the entire RPM gauge. When I dial down the RPM at high power settings it will stop oscillating above the red line when the lever is at about 2500, at which point it locks on and stays unmoved at 2500. If I increase above 2600 at high power settings it will immediately oscillate around 3000 +/- 300. The needle goes anywhere from 2700 to 3500 and oscillates 3-400 RPM in that range during high power settings with no audible change in prop pitch, no changes in egt/cht, no changes in fuel flow, and no feel of acceleration/deceleration, as one would expect from such quick changes. This only happens at high power when the prop lever is all the way forward, such as on T/O and climbout. In cruise at when I reduce power to about 23MP it settles in the green range at the RPM I want. If I take it to 2700RPM at 23MP, then add full power there is no change in prop noise, but the rpm will again fluctuate up and down from 2700 to 3500. In cruise flight about 10 min into a flight it works like it should. Anyone else see this? I haven't yet taken it in.

If the prop did over speed, it is a serious event. Based on what your ancillary data is telling you, it probably is just the tach. My mechanical tach ended up getting destroyed because the shop (avionics) who replaced it during my avionics upgrade thinking they were doing me a favor, ended up using a replacement cable that was too long and it took out the internal mechanism.

I went with the EI primary replacement and really like it.

Is now:

[MisfitSELF]

My RPM gauge is "jumpy" too.  But only in "both".  Once I select either R or L mag, it becomes rock steady.  My MX manual says it could be one of the 15K OHM resisters going from the ignition switch to the RPM gauge could be bad.  I've look under the "hood" but couldn't find said resistors but was able to trace the whole length of the wire.  I'm curious if anybody knows where those resistors are typically located.  It could be as simple as just getting new 15K ohm resistors since mine are likely 35 years old.

Yes, I now realize that I'm replying to a 6 year old thread....

Bruce

Edited October 21, 2019 by MisfitSELF
Added my "6 year old thread remark"

[carusoam]

Bruce,

What tach do you have? Or what year is your J?

Got a pic?

Best regards,

-a-

[MisfitSELF]

It's a 1984 (built in 1983).  S/N 1425 I believe.  I have a pic somewhere, just not with me at the moment.

[MisfitSELF]

Just an update on my particular issue:

I found and replaced the 15K Ohm resistors leading from the ignition switch to the RPM gauge.  Turns out they were right next to the ignition switch as described in the MX manual.

The result:  little to no change.  Maybe a bit less jumpy in both but it didn't fix the problem.  Remember, in my case, it's jumpy in "BOTH" but rock steady in "LEFT" or "RIGHT" and I have the 'electronic' not mechanical RPM gauge.

Could the ignition switch but the culprit?   It's a Bendix "push to start" style and unfortunately a replacement off aircraftspruce is $600!

[carusoam]

Sounds like...

 a call to the manufacturer of the instrument could be helpful...

Can you post a pic, better a video on YouTube with a link here?

It picks up electronic signals and displays them as rpm... sounds like some electrical interference is being picked up...

There are also filters for that... see if you have any capacitors related to your electronics installation... capacitors act as filters for DC electronics... capacitors can be age limited over a few decades...

PP thoughts only, not an instrument guy...

Best regards,

-a-