No fuel pressure indication on first flight

[Ron B.]

Tried swapping the fuel pressure sensor wires with the oil pressure sensor wires and we now have fuel pressure indication with the boost pump on. Seems like problem solved but. We always have a pressure reading of between 60 and 71 psi on the oil pressure indicator before the swap was tried. We have a G3X and VP-X system. Panel and harnesses by reputable advertiser on VAF. If the swap correct the issue the harnesses were labeled backwards. Weather does not allow a ground run for testing.
Question, is there any way we could get a true oil pressure if the fuel pressure leads to the sensor were incorrect?

 

[planenutz]

I'm not sure you've provided sufficient information to be able to fully assist, however in the first instance be aware that the Fuel Pressure sensor and the Oil Pressure sensor normally supplied with the G3X installation kits are not the same. They have different operating pressure ranges. The reason I highlight this is in case you've swapped the wires over and in doing do are now using the wrong sensor for the wrong function.

It sounds like you have a wiring problem that might be as simple as a pin in the incorrect place on the connector to the VP-X. If this is the case then you'd be best to grap a multimeter and check the continuity of the wiring between the sensors and the connector.

One other suggestion - is your fuel pressure line coming off the left side of the Mechanical Fuel Pump on the engine, or off the right side of the pump? It should be on the left side in order for it to provide fuel pressure when the engine is running and the electric pump if turned off. If its on the right side of the mechanical pump you will only get fuel pressure when the electric pump is on and no fuel pressure reading when the electric pump is off. Which is kinda what you've said, but not quite.

 

[Ron B.]

Mike you are correct, I didn’t even disclose which RV I am talking about. It’s an RV -14A. The fuel pressure sensor is coming off the pressure side of the mechanical fuel pump as it is intended to be. I also forgot to mention that it reads high when it does have a value 70 psi ( they’re about) and CNN most of the time it is “X “ out.
I am an hour and a half ( RV time) away from the plane and I have been told today both sensors are rated at 75 psi( the fuel sensor has a service bulletin on it and a new 150 psi one is on order).
I think it’s an electrical connection issue also.
Thanks

 

[planenutz]

It sounds like you have more than one thing going wrong there, Ron.

As you've understood, the Fuel Pressure sender should be the one rated to 75 PSI and the Oil Pressure Sensor should be the one that is rated to 150 PSI. From what I can understand this installation was incorrect before?

If you have the fuel pressure line on the outlet of the pump and routed to the correct sensor then there should be no issues from that portion of the system as its pretty foolproof. That leaves the sensors, the wiring and the setup to consider and for that you're going to need to systematically trace the wiring, confirm the correct pin-outs and then ensure the G3X is set up correctly. There's no shortcuts to doing this.

You could of course place a test gauge on the fuel pressure line to confirm the pressure you have in that system however its unlikely to be anything totally unexpected as the fuel pump typically only does what a fuel pump does best and nothing more.

In the setup of the G3X is there an integer or multipler setting that could be wrong? You say that Fuel Pressure reads about 70 PSI and my expection is it should roughly be about a third of this... so is the system needing a correction factor to reduce this by a factor of 3?

And finally - how good are the grounding points? Any chance you've missed a ground wire somewhere? A poor crimp? A loose wire?

 

[Ron B.]

Problem was resolved, the harness was labeled incorrectly and the wrong pressure sensor was installed on the oil side. They are now as Mike says, 150 on oil and 75 on fuel. Both are sending correct information now. Thanks