EDM 930 Fluctuating Indications

[Ragsf15e]

I noticed my oil pressure fluctuating a while ago.  Now my fuel pressure decided to follow suit.  MP has always jumped, it needs a snubber.  JPI says to look at the wire routing and make sure it’s not with noisy wires... however that hasn’t changed since this started.  Anyone else have ideas?
 

Video below:

https://photos.app.goo.gl/anNav39fGesFmdyaA

[carusoam]

See if you can find where @Bob_Belville discussed getting snubbers for his JPI930...

Bob was the ever-diligent engineer that had to bring JPI up to speed with the mechanical issues that come with measuring MP and FP... and noisy signals like that...

The noise is typically related to the engine rpm, and all the engine valves that are opening and closing...

The slow speed of data collection makes a mess out of the noise signal, making it harder to recognize...

The sensors might not be able to keep up with the variations either...

Remember to follow the procedure when installing snubbers...  to allow the right amount of air to be present in the line...

Have a look at your FF number...  that is varying a lot as well... that data doesn’t typically have the variation as the FF is quite steady in overall...

Two types of snubbers get used... mechanical, to quiet actual variations in the line...  electrical, to digitally filter the data being presented...

PP thoughts only... not an instrument guru...

Best regards,

-a-

[Ragsf15e]

13 minutes ago, carusoam said:

See if you can find where @Bob_Belville discussed getting snubbers for his JPI930...

Bob was the ever-diligent engineer that had to bring JPI up to speed with the mechanical issues that come with measuring MP and FP... and noisy signals like that...

The noise is typically related to the engine rpm, and all the engine valves that are opening and closing...

The slow speed of data collection makes a mess out of the noise signal, making it harder to recognize...

The sensors might not be able to keep up with the variations either...

Remember to follow the procedure when installing snubbers...  to allow the right amount of air to be present in the line...

Have a look at your FF number...  that is varying a lot as well... that data doesn’t typically have the variation as the FF is quite steady in overall...

Two types of snubbers get used... mechanical, to quiet actual variations in the line...  electrical, to digitally filter the data being presented...

PP thoughts only... not an instrument guru...

Best regards,

-a-

Yeah it used to be stable without snubbers.  Something has changed.  Other than the manifold pressure, I don’t think this is a snubber issue.  I think it’s electrical.  Unfortunately, I have no idea how to hunt it down.

[Ragsf15e]

I asked JPI support to look at the video as well... this is what I got:

”Make sure ignition harness shielding and p-leads are terminated correctly. EDM harness no tied up with any high noise wiring.”

So the harness hasn’t moved in 9 years.  Is he talking about the magneto P leads?  If those aren’t shielded right can they mess up stuff not near them?

[carusoam]

9 hours ago, Ragsf15e said:

I asked JPI support to look at the video as well... this is what I got:

”Make sure ignition harness shielding and p-leads are terminated correctly. EDM harness no tied up with any high noise wiring.”

So the harness hasn’t moved in 9 years.  Is he talking about the magneto P leads?  If those aren’t shielded right can they mess up stuff not near them?

Looks like...

he is detailing the common issues of having the EGT wires run along the spark plug wires... and other noise making wires...

This situation comes up with people that are not familiar with what the wires are carrying... and electromagnetic  interference...

 

In your case, going from good to bad over time...

Something has changed.

So...  JPI is saying check the same sources of noise again...  to see if the shielding has failed to shield... because of a broken ground wire on one end...

Electronic noise is always being generated.  JPIs are always sensitive to noise.  Find the typical noise makers and shield/ground them out... properly.

Also...

Drifting digits on a lot of displays is often caused by grounding issues... the display is comparing against its ground, and assuming a value of zero... if it’s ground isn’t very good... the assumption of zero isn’t very good...

You may want to confirm the instrument being grounded to the panel.  And.. the panel being grounded to the plane... and the engine being grounded to the plane...

PP thoughts only, not an electro-magnetic specialist...

Best regards,

-a-