Factory Tach Gauge twitching: root cause

[bcbender10]

Hi all,

What's the root cause of my factory installed tachometer twitching? This is definitely an indication error, and not a governor or prop issue. Engine data from JPI EDM730 from hours of flight recorded once a second shows rock solid RPM, fuel flow, and MAP throughout (that's 3 data points for greater than 3 hours in cruise = 3 data parameters x 3 hours x 60 minutes x 60 seconds = 32,400 data points). I felt/sensed no surging whatsoever during that time..

Further, based on this evidence, a Mooney Service Center did some testing and determine a prop governor overhaul was required. Total cost was $2000. Needless to say, the tach gauge still twitches. They did not identify or fix the root cause. Instead, they are recommending a propeller overhaul.

Please vote in the poll!

Edited March 9, 2020 by bcbender10

[toto]

I'll defer to people who know way more about this than I do, but if you have a tach generator with a panel indicator (rather than a mechanical tach), it may be an electrical issue. In my case, the shop spent an AMU worth of time investigating the problem and it turned out to be a loose connection at or near the ignition switch. It took about 60 seconds to tighten and the indicator problem has not been seen in years. 

[chriscalandro]

You don't need a MSC for this, and I wouldn't keep going to the MSC you mention based on what you are saying. It is likely just the gauge going bad.

[MikeOH]

Yeah, who is the MSC that had you replace the governor over this??? So I can stay away!

[Guest]

There are two different tachometer systems, depending on your serial number.

Clarence

[Niko182]

id say most likely its the cable running to the tach sensor might be grounding on the engine. That was my issue when I had the same problem. it was just as simple as covering the cable and the problem went away.

[thinwing]

So your shop did testing on this and determined governor was bad ,that didn’t solve problem so next it’s the prop!..than when that doesn’t solve it,what next..wait for it ..sorry gotta overhaul the engine!!!..I’ve heard of throwing parts at a problem...but these guys would do better by putting up a big diagram of firewall forward,donning blindfolds,and throwing darts at diagram.

[carusoam]

I vote all...

Did you mention that you have a JPI to compare with? Is the JPI using the same sensor? Does your JPI show the same challenge? (This could change your poll results... 

but, some things are easier to check before the others... and the sensor is probably the most expensive to replace...

Clarence answered my biggest questions...

How digital is the tach?  It is electric and picking up signals from the ignition, then interpreting and displaying them with a mechanical needle....

Mine looks similar in a ‘94 Long Body... without the oscillating needle though...

 

Looks like it Could easily be a sensor/display problem...

When compared to an acoustic tach/iPhone app... you will probably get really good readings indicating no need for gov work...

Find out what sensor you have and see if it is getting loose or dirty... or does it need replacement...?

 

There are a few similar challenges in other Mooneys... where the sensor is a bit expensive...

A JPI 900 is often a direct solution for this....

PP thoughts only, not a mechanic...

Best regards,

-a-

[bcbender10]

9 hours ago, chriscalandro said:

You don't need a MSC for this, and I wouldn't keep going to the MSC you mention based on what you are saying. It is likely just the gauge going bad.

Thanks, Chris. Agreed for this issue alone. Another MSC failed to fully fix an alternator issue and the vacuum system. This indication issue popped up during a x-country in the week in between, so asked the 2nd MSC to look into it. Savvy recommended switching to an EI R1, and I suggested that to the MSC. 

[bcbender10]

2 hours ago, thinwing said:

So your shop did testing on this and determined governor was bad ,that didn’t solve problem so next it’s the prop!..than when that doesn’t solve it,what next..wait for it ..sorry gotta overhaul the engine!!!..I’ve heard of throwing parts at a problem...but these guys would do better by putting up a big diagram of firewall forward,donning blindfolds,and throwing darts at diagram.

HA!! It's feeling that way, in spite of volumes of data (video, EDM, and pilot) pointing in other directions.

 

[bcbender10]

1 hour ago, carusoam said:

I vote all...

Did you mention that you have a JPI to compare with? Is the JPI using the same sensor? Does your JPI show the same challenge? (This could change your poll results... 

but, some things are easier to check before the others... and the sensor is probably the most expensive to replace...

Clarence answered my biggest questions...

How digital is the tach?  It is electric and picking up signals from the ignition, then interpreting and displaying them with a mechanical needle....

Mine looks similar in a ‘94 Long Body... without the oscillating needle though...

 

Looks like it Could easily be a sensor/display problem...

When compared to an acoustic tach/iPhone app... you will probably get really good readings indicating no need for gov work...

Find out what sensor you have and see if it is getting loose or dirty... or does it need replacement...?

 

There are a few similar challenges in other Mooneys... where the sensor is a bit expensive...

A JPI 900 is often a direct solution for this....

PP thoughts only, not a mechanic...

Best regards,

-a-

Yep. EDM730. I edited my question to reflect that. Would love an EDM900. Budget constraints after 2-3 months in the shops after purchase bringing it to WAAS and ADS-B and fully current on all inspections. Some day soon a full glass panel with EIS.

[carusoam]

So...

1) Is the existing JPI showing a steady rpm?

2) Does the RPM match what the iphone app says the rpm is?

3) if that is all good...  does the JPI use the same sensor that the ship’s rpm gauge is using? (Most likely)

4) If yes... the sensor is good. Find why the wire isn’t delivering the same message to the ship’s gauge...

5) If the wire is delivering the message... then the display needs help...  could be as simple as a grounding issue...

6) If all that checks out... borrow a tach from the MSC or instrument shop, or an MSer...

7) Or get a new prop, Governor, and airframe... I hear an acclaim airframe would be good for that... 

 

PP thoughts only, not a mechanic...

Best regards,

-a-

[bcbender10]

1 hour ago, carusoam said:

So...

1) Is the existing JPI showing a steady rpm?

2) Does the RPM match what the iphone app says the rpm is?

3) if that is all good...  does the JPI use the same sensor that the ship’s rpm gauge is using? (Most likely)

4) If yes... the sensor is good. Find why the wire isn’t delivering the same message to the ship’s gauge...

5) If the wire is delivering the message... then the display needs help...  could be as simple as a grounding issue...

6) If all that checks out... borrow a tach from the MSC or instrument shop, or an MSer...

7) Or get a new prop, Governor, and airframe... I here an acclaim airframe would be good for that... 

 

PP thoughts only, not a mechanic...

Best regards,

-a-

Got a million bucks for me? Hellllllooo Acclaim! This could work out for me yet....

1) yes, JPI is showing steady RPM. I'm one of those math geeks. Data & analytics is my day job and pays the bills (house & Mooney). 2) yes 3) Need to find out. If so, that would clinch it for a bad gauge or wire 4) yep 5) yep. very logical 6) I bought a replacement tach removed from another Mooney for a glass upgrade for $85 on ebay. Even if I can't use it, couldn't resist. Better than paying for a prop overhaul. 7) you buy, I fly.