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Thread: Bad fuel pressure sensor or bad calibration?

  1. #1

    Bad fuel pressure sensor or bad calibration?

    I recently ordered and fitted a new replacement VDO fuel pressure sensor part # 41215, for my 3500EM non-S. I did this because the fuel pressure reading became erratic after about 270 hours.

    The new fuel sensor reads way too high, over 10 psi running on the electric or mechanical pump.

    I know the non-S systems don't allow you to pick the sensor type, but the calibration values are in the ENGINE.AFC file.

    Here are the appropriate numbers from mine:

    FUEL_PSI.NUM_CAL_PTS, 11
    FUEL_PSI.DISPLAY_VALUE_1, 0
    FUEL_PSI.AD_VALUE_1, 48
    FUEL_PSI.DISPLAY_VALUE_2, 10
    FUEL_PSI.AD_VALUE_2, 55
    FUEL_PSI.DISPLAY_VALUE_3, 20
    FUEL_PSI.AD_VALUE_3, 91
    FUEL_PSI.DISPLAY_VALUE_4, 30
    FUEL_PSI.AD_VALUE_4, 118
    FUEL_PSI.DISPLAY_VALUE_5, 40
    FUEL_PSI.AD_VALUE_5, 141
    FUEL_PSI.DISPLAY_VALUE_6, 50
    FUEL_PSI.AD_VALUE_6, 175
    FUEL_PSI.DISPLAY_VALUE_7, 60
    FUEL_PSI.AD_VALUE_7, 214
    FUEL_PSI.DISPLAY_VALUE_8, 70
    FUEL_PSI.AD_VALUE_8, 244
    FUEL_PSI.DISPLAY_VALUE_9, 80
    FUEL_PSI.AD_VALUE_9, 272
    FUEL_PSI.DISPLAY_VALUE_10, 90
    FUEL_PSI.AD_VALUE_10, 304
    FUEL_PSI.DISPLAY_VALUE_11, 100
    FUEL_PSI.AD_VALUE_11, 353

    Do they look correct for the 41215 VDO sensor please and if so, do you think the new sensor could be bad?

    I’ve checked the ground connection at the sensor and the back of the 3500EM and both are good.

    My airplane is an RV- 9A with a carbureted O-320. The previous sensor read about 2.5 psi on the mechanical pump and about 5.5 psi when the electrical pump was running.

    Colin
    Last edited by Cojaken; 09-11-2021 at 04:28 PM. Reason: Added info

  2. #2
    Advanced Flight Systems Shawn McGinnis's Avatar
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    The calibration is correct for a VDO 0-30 41,215.

    If you disconnect the measurement wire and measure the voltage that the EFIS is providing what are you getting?

    I've also seen people that have clogged reducer's causing issues.
    Shawn McGinnis

    Advanced Flight Systems
    support@advanced-flight-systems.com

  3. #3
    Thanks Shawn. I measured the voltage supplied and the resistance across the disconnected sensor at zero pressure.

    The resistance is 9.5 ohms.

    The voltage on pin 8 of the Engine Sensor connector on the back of the 3500EM is 4.157 V

    The voltage at the sensor end of the brown wire is 4.15 V

    All measurements with the engine not running.

    Do any of these measurements explain the high pressure readings?
    Last edited by Cojaken; 09-17-2021 at 10:05 AM.

  4. #4
    Advanced Flight Systems Shawn McGinnis's Avatar
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    Those numbers seem fine. Can you verify that 0-2bar stamped on the hex of the sensor?
    Shawn McGinnis

    Advanced Flight Systems
    support@advanced-flight-systems.com

  5. #5
    0-2 bar is stamped on the hex of the sensor

  6. #6
    Advanced Flight Systems Shawn McGinnis's Avatar
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    Is the ground path shared or directly back to the EFIS?
    Does the gauge read 0 psi before the engine is started or after cracking open the fuel system to release pressure?
    How high does the pressure go if you just turn on the boost pump without the engine running?
    Shawn McGinnis

    Advanced Flight Systems
    support@advanced-flight-systems.com

  7. #7
    It is not easy to physically follow the ground wire, so to eliminate this as a potential problem, I ran a new temporary ground wire directly from the sensor to the 3500EM case. The behavior was the same.

    The gauge reads 0 psi both before the engine is started and after cracking open the fuel system to release pressure.

    The pressure goes to 9.4 psi with the boost pump on without the engine running. This is a little lower than before because I adjusted the shift, which is now 3.6.

    Also, the 0.3 psi AD_VALUE read from the calibration screen is 25 and the 9.4 psi AD_VALUE is 207. That surprised me a little because it doesn't seem to correspond with the values in the ENGINE.AFC file. Maybe I am interpreting that incorrectly.

    I sourced another sensor locally to try to eliminate the sensor as the problem and the behavior is the same, so I went back to the one I purchased from AFS.

  8. #8
    Advanced Flight Systems Shawn McGinnis's Avatar
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    Seeing an adjust above 10% of the sensor range is normally a bad sign. If you drop the adjust back to 0 your previous 5.5 psi on electric boost lines up with AD_VAL 207 which is about 5.5 psi. The only odd one is the 0 psi being 25 AD_VAL.

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    FUEL_PSI.DISPLAY_VALUE_* happen to be tenths in the file.
    Shawn McGinnis

    Advanced Flight Systems
    support@advanced-flight-systems.com

  9. #9
    Thank you for the information Shawn.

    I just made another observation. As you know when shutting off the master, the 3500EM switches to the internal backup battery for thirty seconds before shutting down.

    I noticed that when this happens, the fuel pressure reading on the screen jumps from 0 psi to 2.4 psi and stays there until it shuts down.

    Does that suggest anything?

  10. #10
    Shawn, thanks for our telephone discussion. As a result of our conversation, I got a Mityvac MV8510 Silverline Elite Hand Pump, adapters and a gauge and used them to check the calibration of the new VDO sensor.

    These are the numbers I came up with and loaded on the 3500EM:
    FUEL_PSI.DISPLAY_VALUE_1, 0
    FUEL_PSI.AD_VALUE_1, 26
    FUEL_PSI.DISPLAY_VALUE_2, 10
    FUEL_PSI.AD_VALUE_2, 80
    FUEL_PSI.DISPLAY_VALUE_3, 20
    FUEL_PSI.AD_VALUE_3, 120
    FUEL_PSI.DISPLAY_VALUE_4, 30
    FUEL_PSI.AD_VALUE_4, 165
    FUEL_PSI.DISPLAY_VALUE_5, 40
    FUEL_PSI.AD_VALUE_5, 190
    FUEL_PSI.DISPLAY_VALUE_6, 50
    FUEL_PSI.AD_VALUE_6, 210
    FUEL_PSI.DISPLAY_VALUE_7, 60
    FUEL_PSI.AD_VALUE_7, 250
    FUEL_PSI.DISPLAY_VALUE_8, 70
    FUEL_PSI.AD_VALUE_8, 275
    FUEL_PSI.DISPLAY_VALUE_9, 80
    FUEL_PSI.AD_VALUE_9, 300
    FUEL_PSI.DISPLAY_VALUE_10, 90
    FUEL_PSI.AD_VALUE_10, 320
    FUEL_PSI.DISPLAY_VALUE_11, 100
    FUEL_PSI.AD_VALUE_11, 340

    I now have confidence that my fuel pressure is reading correctly.

    Thank you for your insight.

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