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Thread: Fuel Rail Pressure Sensor

  1. #1
    Junior Member EsRay's Avatar
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    Fuel Rail Pressure Sensor

    Dear Forum,

    Firstly, please allow me to apologise for the length of this post - I have been struggling with this one for several weeks!

    Background: I recently renewed the dreaded 'Valley Hose' (well, the 4.2 N/A version, anyway). Unfortunately, I broke the 'nipple' off my Fuel Rail Pressure Sensor (FRP) in the process. It was very brittle (as is most of the plastic in my engine). So brittle, it brought a chunk of the 'box' which contains the sensor with it and damaged the diaphragm inside. It is U/S.
    I bought a new one: https://www.britishparts.co.uk/jagua...pressure-p5020
    Prior to this, my car was running perfectly with good fuel trims etc. However, after fitting this new sensor, I got code PO193 – FRP High Voltage followed by codes PO172 & PO175 - Running Rich both banks and Restricted Performance.
    My car is pumping fuel into the inlet and the Fuel Rail pressure is through the roof, despite only 12 kPa showing on my code reader. If I disconnect the FPR Sensor, I get 483 kPa Fuel pressure, because (I think) the ECU has just maxed it out? Understandably, my Fuel Trims are also maxed out….The Sensor is telling the ECU there is no Rail pressure (when there is lots), so the ECU is telling the fuel pumps to pump harder!

    My broken sensor was a FoMoCo part - 3R3E 9F972 AA 5C28Z 734 (photo attached). The replacement has no markings. Probably Chinese? First time I have ever had a problem with British Parts UK!
    Anyway, before I decided I had been sold a pup, I conducted the FRP Pinpoint tests (see attached) to try to ascertain whether I had disturbed some wiring or something else whilst removing and reseating the Inlet Manifold. This is where it gets puzzling?
    I have the correct 5v between the FRP Connector supply and Ground with the ECU connected and the ignition on. BUT I also have 5v at the Signal and Ground wires (which should show 3v or less)?
    I have only 5,000 ohms resistance between Sensor and Ground (should be 10,000 ohms)
    There is no continuity between any of the three wires at the connector when the ECU is disconnected - (therefore no crossed wires).
    There is continuity between ECU Signal (pin 073) and Connector Signal (pin 3) BUT mysteriously I cannot find continuity between the ECU Supply (pin 12) and the Connector Supply (pin 1), despite the fact I have 5v when the ECU is connected and the ignition on?!
    I started to strip back my wiring harness, but the wires inside look pristine?
    Please, have I missed something or should I just wait for my wife (back in the UK until November 14th) to bring me yet another Sensor, but this time the original Ford version?
    Any suggestions, comments or observations will be very welcome!
    Attached Thumbnails Attached Thumbnails Ford Fuel Rail Pressure Sensor.JPG  
    Attached Files Attached Files

  2. #2
    Senior Member fat controller's Avatar
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    Could it be that the ECU is using the CANBUS to send data to the sensor to establish it's presence before it the switches the supply voltage on?

    You say you get 5k resistance between sensor and ground - I take it that is the new sensor? Where do you get the 10k figure that it is supposed to be - is that from the old sensor? If so, I would say that looks like the sensor is your problem; the actual current difference is absolutely tiny, but I would imagine that you are in hair-splitting territory with a sensor like this.
    ex - 2002 X-Type 2.5 V6; 1995 XJ Sport 3.2; 1998 Mercedes C180 Elegance; VW Passat 2.0TDI (140); Audi A4 Avant 2.0TDI; Nissan Qashqai 1.5dCi; Current - 2007 XJ 4.2 V8 Sovereign, and occasionally an Electric Wheelchair!

    It wasn't me! A big boy done it and ran away.

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    Junior Member EsRay's Avatar
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    Hello Fat Controller, Thank you for your reply. The 5v is present at the Sensor Connector's Supply Pin and Ground when the ECU is plugged in and the ignition is on; this is the correct Pin test result. 5v is also present at the Connector's Signal wire and Ground with the ECU plugged in and the ignition on; this is wrong - It should read no more than 3v (see the Pinpoint Tests I attached to my previous post).
    The 5k resistance I get between sensor Connector and Ground is with the ECU connected but obviously with the ignition off. It should read 10k? (see Pinpoint test G206886t9, the last one on the attachment). In all cases the Sensor itself is irrelevant to these tests and their results, because it is not connected?
    Unless I am doing something wrong?

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    Senior Member fat controller's Avatar
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    Sorry, I should have read the attachment before - I didn't see it.

    The way that I read that, your failure is at test G206886t8 - the voltage is higher than 3v, which means you have a short somewhere. Until you find it, don't progress to test G206886t9. Where the short is, is anyone's guess. Before you go opening up wiring harnesses. I would be inclined to do more simple testing - - if you have 5v, you could use that 5v to light a bulb (bound to be able to pick up a 5v bulb somewhere) - - with the bulb lit, just wiggle the wiring loom where it bends over panels, goes through holes, or sits around parts of the engine bay; don't forget to check the connectors themselves for contamination causing shorts.

    Bear in mind, electricity is like water - it will take the shortest route it can to get to where it wants to be, even if that is through something that doesn't look like a conventional conductor (grime, dirt, fluids etc)
    ex - 2002 X-Type 2.5 V6; 1995 XJ Sport 3.2; 1998 Mercedes C180 Elegance; VW Passat 2.0TDI (140); Audi A4 Avant 2.0TDI; Nissan Qashqai 1.5dCi; Current - 2007 XJ 4.2 V8 Sovereign, and occasionally an Electric Wheelchair!

    It wasn't me! A big boy done it and ran away.

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  7. #5
    Junior Member EsRay's Avatar
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    Thank you. Good idea; I will definitely give it a try!
    It is still very puzzling - No continuity between the three Sensor wires, so the Sensor Signal wire cannot be getting the extra 2 volts from the Sensor Supply wire? Or the Ground?
    A couple of questions when you have time: In the attached Wiring Diagram the Ground wire (pin 2 BG) for the FRP Sensor appears to be shared by the Grounds for the MAP Sensor and the Throttle Position Sensor (TP Sensor) and then to go to Pin 19 at the ECU; is the point where they intersect likely to be a splice or is it a Ground Point? Should I be able to find continuity between the FRP Sensor Connectors Ground pin and ECU pin 19? How about (say) the TP Sensor Ground and ECU pin 19?
    I don't know how to test a Ground?
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    Senior Member fat controller's Avatar
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    For some reason that PDF does not show well on my screen and is barely readable I'm afraid - I would guess that the intersection is a ground point, although quite why they would run that back to the ECU is beyond me; there are known issues with grounds on our cars, and I believe the three that give trouble live at the front behind the headlights (have a search and I am pretty sure you will turn something up). BG being repeated concurs to be that it is the ground, and there should be continuity between it and the battery negative. The problem with testing continuity is that you may very well get continuity on a multi-meter (3v, no load) but as soon as you ask for a meaningful current to pass, the circuit fails.

    Also be careful about disappearing up rat-holes wondering where a voltage has come from - these are complex cars, and there is data going around in addition to operational voltages.

    To maybe clear the waters a bit, I would be inclined to put a length of wire onto the battery negative terminal and use that as the ground for your meter, then measure the voltages on each pin; that way, you have a known good ground and are working with actual voltages from each pin then, rather than measuring voltages across a circuit that is already under suspicion. Following that, check the earth points at the front of the car and make good as needed, then go from there.
    ex - 2002 X-Type 2.5 V6; 1995 XJ Sport 3.2; 1998 Mercedes C180 Elegance; VW Passat 2.0TDI (140); Audi A4 Avant 2.0TDI; Nissan Qashqai 1.5dCi; Current - 2007 XJ 4.2 V8 Sovereign, and occasionally an Electric Wheelchair!

    It wasn't me! A big boy done it and ran away.

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  10. #7
    Junior Member EsRay's Avatar
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    Thank you again - All makes very good sense - I will rerun all the tests, but using a battery earth, as you suggest.

    I've attached the wiring diagram as a word doc - Just to see if that is any better; it is fig.03.3 in the 2005 MY XJ Electrical Guide.
    Attached Files Attached Files

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    Quote Originally Posted by EsRay View Post
    Hello Fat Controller, Thank you for your reply. The 5v is present at the Sensor Connector's Supply Pin and Ground when the ECU is plugged in and the ignition is on; this is the correct Pin test result. 5v is also present at the Connector's Signal wire and Ground with the ECU plugged in and the ignition on; this is wrong - It should read no more than 3v (see the Pinpoint Tests I attached to my previous post).
    The 5k resistance I get between sensor Connector and Ground is with the ECU connected but obviously with the ignition off. It should read 10k? (see Pinpoint test G206886t9, the last one on the attachment). In all cases the Sensor itself is irrelevant to these tests and their results, because it is not connected?
    Unless I am doing something wrong?
    that doesn't ring true to me.......

    you have 5V at the supply and 5V at ground - and 5V at the read-back from the sensor.

    You also state, however, that 5V at supply and ground is the correct pin test result - I haven't looked it up, but this surely can't be right - ground should always be 0V, surely

    Ground should be 0V all of the time, so it sounds like you're missing an earth - which would most likely give you a read-back of 5V instead of the expected.

    regarding the sensor, have you tried feeding it 5V in the correct place and correcting the ground (to 0V) somehow and then testing the output and resistance at both pressurised and un-pressurised ?

    You may also be able to obtain a characteristic curve for the sensor, particularly if you Google the Ford part number

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    Junior Member EsRay's Avatar
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    [QUOTE=2woody;1138901]that doesn't ring true to me.......

    you have 5V at the supply and 5V at ground - and 5V at the read-back from the sensor.

    You also state, however, that 5V at supply and ground is the correct pin test result - I haven't looked it up, but this surely can't be right - ground should always be 0V, surely?

    Hello 2Woody, Thanks for posting. No, I am saying that I have 5v between the FRP Connector's Ground and Supply pins AND 5v between Ground and Signal pins; I go on to say that the Connectors Ground to Signal voltage should be no more than 3v (as it says in the Pinpoint Tests attached to my very first post, above). Like you, I have no idea why even 3v should be present at the Signal pin, but that is what it says?
    I have also drawn attention to the fact that the FRP Sensors Ground is shared by the MAP and the Throttle Position Sensors' Grounds and then they all appear to be connected to pin 19 at the ECU? I have also tried to speculate a short from Supply to Ground yielding the 5v at both Supply and Signal, but if this was the case, surely I would have continuity between Ground and Signal and I do not?
    I admitted in my post No. 5 above that I do not know how to test this Ground, because I do not know where it is; pin 19 at the ECU? (there is no continuity between ECU pin 19 and the FRP Connector's Ground? The Engine Block?
    2Woody, as you have gathered, I am working outside of my knowledge base!

  14. #10
    Senior Member fat controller's Avatar
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    Yeah, when testing any ground, always go for a good ground. I base this on my training on hot chassis like old TV's, Hi-Fi components etc - - often, when you try and test a voltage between a supply (+volts) and an in circuit ground, you can get some odd results; if you hook the ground probe on your meter to a known good ground (often the metal chassis itself on electronic products) and then use your other probe to test voltages at each point in a given circuit.

    Cars are no different really - hence my suggestion to use a long bit of cable attached to the battery negative as a known good ground and then probe about with the other lead to establish what the voltages actually are.
    ex - 2002 X-Type 2.5 V6; 1995 XJ Sport 3.2; 1998 Mercedes C180 Elegance; VW Passat 2.0TDI (140); Audi A4 Avant 2.0TDI; Nissan Qashqai 1.5dCi; Current - 2007 XJ 4.2 V8 Sovereign, and occasionally an Electric Wheelchair!

    It wasn't me! A big boy done it and ran away.

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