916 fuel pressure problem

What size tubing are you using for line D?  Also, differential pressure will be the same as gauge pressure with the engine NOT running since the intake plenum is exposed to atmospheric pressure. So you can temporarily remove the differential pressure sender and install a manual gauge to verify the accuracy of the sender. 

All lines in green are AN6 Aluminium and lines in blue are AN6 PTFE.  So pretty much identical diameter throughout the circuit.

I understand the N1EU07D is only seeing atmospheric pressure when engine is off.  But the pressure regulator (also connected to Plenum) is seeing the same and should be compensating to correct range, I think.

Agreed on next step.  I have an accurate analogue gauge ordered to measure fuel pressure.

I did do a crude test of the sensor by connecting it to an adjustable compressed air line.  The readings were following the changes in pressure applied. But the pressure gauge on the compressor was too inaccurate to give definitive results.

Also worth noting that the N1EU07D was initially incorrectly wired to the Garmin with (Garmin)12v -> GND, (Garmin)GND-> SIG, (Garmin)SIG-> 12V.   The company who built the harness assured me this would not damage the sensor.

Ian,

Are you using the N1EU07D or the N1EU70D?  The former would be a 0-7 PSI sender and the latter would be 0-70 PSI (correct), so I assume this is a typo in your post.  

The N1EU70D can be pre-calibrated from the factory for 0-5V, .5-4.5V or .2-4.7V.  Do you know which you have?  The Garmin interconnect drawings for the fuel sensor shows a calibration of .2-4.56, so the calibration of your sensor should be .2-4.7 volts. In other words, a return voltage from the sensor at 0-PSI should be .2 volts, and 4.7 volts at 70-PSI. 

If your sensor calibration is correct....

Assuming you are using the Garmin GEA-24 Engine Module, you might also want to check the reference voltage that the module is sending to the N1EU70D.   The reference voltage should be 5V. If it's off, then the return voltage will be incorrect also. So, a reference voltage of 5.5 would return a 10% error on the high side. I have seen the reference voltage off as much as .5 volts, and Garmin will replace it if that is the case.  

Hi Jeff,

Yes, I mistyped.  It's a N1EU70D.  It also has EU-20,926 stamped on it, but I don't know what that means.  

I'll test it with a multimeter tomorrow to see which calibration it has.

Yes.  Using a GEA-24. But there isn't a 5v reference voltage connected to the sensor and I can't see that documented in the UMA installation doc.

The wiring is documented as follows:

Orange/White 9 - 12V.   This is connected to GEA24 pin 3 and 12V

Blue/White - Ground.  Connected to GEA24 pin 1 and GND

White - Signal.  Connected to GEA24 pin 2 and floating I think

Shield - GND.  Connected to GND

Looking at the spec sheet, I see you are correct that it has a 9-12v dc input rather than a regulated 5v.  Sorry, I remembered it differently.  So that means the supply voltage from the GEA-24 is not critical for the sender accuracy. It will be interesting to connect the manual pressure gauge and see if the sender is accurate.  The spec sheet says +/- 2.5% full scale so that should be within 1 PSI at half scale.  Regarding the initial incorrect wiring, I don’t think you can be sure the sensor (or the GEA-24) was not affected. I guess you will know once you do the manual gauge test.