Fluctuating Fuel Pressure indication on 98 M20J Allegro

[Costa Leite]

Dear Mooniacs

Our 98 Mooney M20J Allegro is showing a fluctuating Fuel Pressure indication on original engine gauges.

On startup, taxi, take off and initial climb everything is fine, and then a few minutes after airborn the needle starts fluctuating, and then some times the needle is completly dead showing "zero" pressure, and afterwards sometives gets alive again... without any regular behavior.

The fuel pressure is obviously good (engine runs smooth all the time and fuel flow on Shaddin is perfectly normal), so I think the problem is with the Transducer, and my mechanic agrees with me.

I think these Transducers are hard to find... any idea how can I solve this situation without moving to digital/glass instruments?

Many thanks in advance for your comments.

Fredi

[PT20J]

An Omega PX309-030GV has the same electrical specs as the factory Kulite and will work with the Signma-Tek gauge. It has a NPT input port instead of a hose connection, so you'll need an adapter. https://www.omega.com/en-us/pressure-measurement/pressure-transducers/p/PX309

Skip

[Costa Leite]

Thanks Skip!

Do you have any idea what specs do I need to order? Output connection? Accuracy? etc...

And where can I find the adapter?

Many thanks!

Fredi

[PT20J]

The part number I supplied comes with a pigtail wire so all you will have to do is attach a mating connector to match your wiring. The Omega website link has all the specs. - click on the Data Sheet (pdf).

The Omega part has a 1/4" NPT male input and the hose is a -3 (1/8") flare. so you need a 1/4" NPT to 1/4" NPT female coupling and a -3 AN flare to 1/4" NPT male adapter. Your maintainer may have these, or you could try www.anplumbing.com.

Skip

 

 

[OR75]

I would start by making sure there is no loose electric connection 

[LANCECASPER]

On 7/14/2023 at 4:06 PM, OR75 said:

I would start by making sure there is no loose electric connection 

Exactly, these gauges are notorious for loose ground.I had the exact same issue on a '97 Encore and tapping on the gauge brought it back to life.

[Costa Leite]

Our mechanic already did some troubleshooting and didnt found any loose electric connection. He says the transducer is no longer working correctly.

[Rick Junkin]

While you're at it you may want to change the electrical connector per the attached document. I had a problem similar to yours and found this connection to be the culprit. You've isolated it to the transducer this time, but this connection is the weak link in the circuit.

Cheers,
Rick

BENZ AIRBORNE-KULITE TRANSDUCER.pdf

[PT20J]

It could also be a problem with the excitation voltage. These transducers are designed in a bridge configuration and have four wires. Two wires have the calibrated excitation voltage (about 10Vdc) that comes from the gauge and the other two wires have the signal voltage (0 to 100mV) depending on the pressure that goes back to the gauge. The fuel pressure is easy to test without running the engine since you can use the boost pump to create pressure. if you remove the backshell from the CPC connector you can get DVM probes on the pins where the wires come out and measure the voltages.