Fuel pressure sender problem

[Wingover]

Hi All,

My fuel pressure sender in my 1987 J stopped working a while ago so based on the reading here, bought the Omega (model PX309).  It has worked fine but after a couple of months I noticed that it would not work or start fluctuating until it stopped completely.  I checked the wire connections between the sender and harness going to the panel and it seems fine.

Can someone tell me how to check if it is the sender, amplifier, broken wire or gauge?

[PT20J]

Remove the back shell from one end of the black CPC connector so that you can access the pins with a DVM with the connector connected.

With the master switch on, there should be about 10 VDC between pins 1 and 2. This voltage comes from the gauge. If it's not there, investigate the gauge and wiring.

There should be near zero volts between pins 3 and 4 with no fuel pressure. This is the transducer output. Turn on the boost pump to create fuel pressure and the voltage between pins 3 and 4 should increase. If this doesn't happen, it's the transducer.

[Wingover]

38 minutes ago, PT20J said:

Remove the back shell from one end of the black CPC connector so that you can access the pins with a DVM with the connector connected.

With the master switch on, there should be about 10 VDC between pins 1 and 2. This voltage comes from the gauge. If it's not there, investigate the gauge and wiring.

There should be near zero volts between pins 3 and 4 with no fuel pressure. This is the transducer output. Turn on the boost pump to create fuel pressure and the voltage between pins 3 and 4 should increase. If this doesn't happen, it's the transducer.

Thank you @PT20J as always!  Could you please look at the attached and help me identify the pin numbers based on the wire colours?

[PT20J]

RED is +10V (pin 1)

BLK is 10V return (pin 2)

White (pin 3) and Green (pin 4) are signal

The pin numbers are embossed on the connector next to the pin holes (but they are very small).

Do you have an AvStar fuel injector or a RSA? I'm curious because the AvStars have a tighter idle cutoff valve and allow pressure to build up in the lines after shutdown and this might be killing the transducers.

[Wingover]

17 minutes ago, PT20J said:

RED is +10V (pin 1)

BLK is 10V return (pin 2)

White (pin 3) and Green (pin 4) are signal

The pin numbers are embossed on the connector next to the pin holes (but they are very small).

Do you have an AvStar fuel injector or a RSA? I'm curious because the AvStars have a tighter idle cutoff valve and allow pressure to build up in the lines after shutdown and this might be killing the transducers.

I believe I have the RSA servo.  It is interesting you mention that.  The Omega tech support asked if there was a way to build pressure above 30 psi since that will kill the transducer.

[PT20J]

2 minutes ago, Wingover said:

I believe I have the RSA servo.  It is interesting you mention that.  The Omega tech support asked if there was a way to build pressure above 30 psi since that will kill the transducer.

Lycoming switched vendors from Precision Airmotive (RSA) to AvStar a few years ago, so if you have a Lycoming factory rebuilt or overhauled engine it might have the AvStar. Otherwise, it's probably an RSA. Transducers have three pressure specs: rated (the max normal range), overload (overpressure it can take without damage) and burst (where the case seal fails). Omega specifies overload at 3X the max normal range so a 30 psig transducer should be able to take up to 90 psi. https://www.omega.com/en-us/pressure-measurement/pressure-transducers/px309/p/PX309-100G10V

I'm not sure how high the pressure can get - on my G3X with a 75 psig max transducer it will go off scale from expansion due to the residual heat in the engine compartment after shutdown. I have a rebuilt IO-360-A3B6 with the AvStar fuel injection system.

[Wingover]

2 minutes ago, PT20J said:

Lycoming switched vendors from Precision Airmotive (RSA) to AvStar a few years ago, so if you have a Lycoming factory rebuilt or overhauled engine it might have the AvStar. Otherwise, it's probably an RSA. Transducers have three pressure specs: rated (the max normal range), overload (overpressure it can take without damage) and burst (where the case seal fails). Omega specifies overload at 3X the max normal range so a 30 psig transducer should be able to take up to 90 psi. https://www.omega.com/en-us/pressure-measurement/pressure-transducers/px309/p/PX309-100G10V

I'm not sure how high the pressure can get - on my G3X with a 75 psig max transducer it will go off scale from expansion due to the residual heat in the engine compartment after shutdown. I have a rebuilt IO-360-A3B6 with the AvStar fuel injection system.

I have a rebuilt engine but pretty sure it's RSA.  I will be doing oil change tomorrow so will check again and let you know.

[PT20J]

1 minute ago, Wingover said:

I have a rebuilt engine but pretty sure it's RSA.  I will be doing oil change tomorrow so will check again and let you know.

Unless it was recently manufactured, the RSA servo body is a casting. The AvStars are machined.

[Wingover]

7 minutes ago, PT20J said:

Lycoming switched vendors from Precision Airmotive (RSA) to AvStar a few years ago, so if you have a Lycoming factory rebuilt or overhauled engine it might have the AvStar. Otherwise, it's probably an RSA. Transducers have three pressure specs: rated (the max normal range), overload (overpressure it can take without damage) and burst (where the case seal fails). Omega specifies overload at 3X the max normal range so a 30 psig transducer should be able to take up to 90 psi. https://www.omega.com/en-us/pressure-measurement/pressure-transducers/px309/p/PX309-100G10V

I'm not sure how high the pressure can get - on my G3X with a 75 psig max transducer it will go off scale from expansion due to the residual heat in the engine compartment after shutdown. I have a rebuilt IO-360-A3B6 with the AvStar fuel injection system.

I am not sure where you see the 3x safety margin.  The link you provided is the 0-100 PSI transducer.  Looking at mine (30 PSI) I could not find the info.

[PT20J]

3 minutes ago, Wingover said:

I am not sure where you see the 3x safety margin.  The link you provided is the 0-100 PSI transducer.  Looking at mine (30 PSI) I could not find the info.

Pressure Overload: -15 to 50 psig and 0 to 1000 psia: 3 times rated pressure or 20 psi whichever is greater, 100 to 10,000 psig: 2 times rated pressure

[Wingover]

I’m wondering if leaving the mixture and throttle pushed in after shutdown can eliminate the pressure spike?

[PT20J]

1 minute ago, Wingover said:

I’m wondering if leaving the mixture and throttle pushed in after shutdown can eliminate the pressure spike?

The throttle isn't necessary because it won't completely shut off the mixture even if closed, but cracking the mixture control after shutdown works. If you merely crack it, it will bleed off the pressure but still be lean enough so it's unlikely to start if someone were to turn the prop on a hot engine. You can tell that it's cracked enough by watching the fuel pressure gauge.

[Wingover]

@PT20J  I can confirm that I have the RSA fuel servo.  I also checked the voltage, and I have zero.  A couple of days ago when I checked (while the transducer wires were connected as well), I saw 7v between the red and black but noting today.

I understand that there is an amplifier between the gauge and transducer. Not sure how to check that....

Edited October 22 by Wingover

[PT20J]

 There is nothing between the transducer and the Sigma-Tek fuel pressure gauge. The gauge itself generates the 10 VDC excitation voltage for transducer. All the electronics are in the gauge. The individual gauges remove from the cluster from the front. However, I found it easier to remove the cluster and open it to get the gauges out. Sigma-Tek repairs them if you cannot find a used one.

[Wingover]

1 hour ago, PT20J said:

 There is nothing between the transducer and the Sigma-Tek fuel pressure gauge. The gauge itself generates the 10 VDC excitation voltage for transducer. All the electronics are in the gauge. The individual gauges remove from the cluster from the front. However, I found it easier to remove the cluster and open it to get the gauges out. Sigma-Tek repairs them if you cannot find a used one.

My electrical guy (not available for a few months) said there is an amplifier (my J is 28v) like this one:

Mooney M20J TRANSDUCER AMPLIFIER P/N 950D0311-001 - 800331-984 FUEL PRESSURE AMP | eBay

Here is the wiring diagram:

Edited October 22 by Wingover

[PT20J]

35 minutes ago, Wingover said:

My electrical guy (not available for a few months) said there is an amplifier (my J is 28v) like this one:

Mooney M20J TRANSDUCER AMPLIFIER P/N 950D0311-001 - 800331-984 FUEL PRESSURE AMP | eBay

If there is, it will show up on the schematic. Searching the Service Manual, that part number only shows up for SN 24-3000 through 24-3153. 

[Wingover]

4 minutes ago, PT20J said:

If there is, it will show up on the schematic. Searching the Service Manual, that part number only shows up for SN 24-3000 through 24-3153. 

Yes, mine is 24-3028

[PT20J]

43 minutes ago, Wingover said:

Yes, mine is 24-3028

The Omega transducer is a direct replacement for the Kulite transducer used by Mooney with Sigma-Tek gauges. 

According to the Service Manual Electrical Equipment Chart, your S/N has a "Mooney" cluster and transducer. Since the amplifier was made by IAI, perhaps the gauges and transducers are also made by IAI for Mooney. Anyway, IAI is still in business and does repairs. https://www.internationalavionics.com/  You might call them and see if they have information that would help you figure out what's going on with your setup. I might also contact the Mooney service department.

[A64Pilot]

On 10/21/2024 at 3:41 PM, Wingover said:

I’m wondering if leaving the mixture and throttle pushed in after shutdown can eliminate the pressure spike?

Mine will build pressure, it’s easy to check just turn the Master on after engine has been off for 10 min or so, if it’s building pressure it will pin the pressure on the gauge

What I do is after shutdown is I push mixture in with the Master still on, pressure quickly goes to zero, pull mixture fully out again and turn Master off. I also always leave keys on instrument panel except when I’m away, then they go in my pocket.

Two weeks ago, the day after I did his condition inspection on his Pitts, a neighbor got hit in the back of his neck by the prop, he apparently moved it and one or both mags were hot. It would have killed him the Dr said if his neck hadn’t been fused with steel rods in it, but he was hospitalized for several days, hot mags with any fuel in the cylinder are dangerous, and when I push the mixture in to bleed off the pressure some fuel is going into one or more cylinders probably not much, but maybe enough if the mags are hot to kill you.

[PT20J]

I’ve tried opening the mixture after shutdown to bleed off the pressure and then closing it again. But the problem is the residual heat in the engine compartment will cause the pressure to increase again. If I check it again ten minutes later, the gauge will be pegged again. I don't know how high it gets. The factory gauge pegged at 30 psi. The Garmin G3X EIS will read to 75 psi and it will peg that. I haven’t seen that it hurts anything, and I don’t like leaving fuel in the hot cylinders after shutdown, so I just ignore it. With the mixture in ICO, the pressure can’t reach the servo internals. Someone posted that theirs apparently forced the mixture control shaft out against the internal spring and it became disconnected from the mixture plate, but I’ve never had that happen. 

[A64Pilot]

I’m going to have to see if mine builds pressure a second time, I can see the logic if it’s refilling with fuel it should as that little bit of fuel flow and 10 sec or so hasn’t cooled much of anything.

As far as fuel in the cylinders it doesn’t hurt anything, nearly every other engine gets shut off with ignition, except aircraft.

Edited Tuesday at 06:53 PM by A64Pilot

[PT20J]

4 hours ago, A64Pilot said:

As far as fuel in the cylinders it doesn’t hurt anything, nearly every other engine gets shut off with ignition, except aircraft.

Agree. For me, it’s the safety issue since I’m around the prop putting the plane away.