EOS/Golf MkV fault code P1592 - Altitude/Boost Press. Sensor. Implausible Correlation - VW Eos Forum : Volkswagen Eos Forums

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Old 08-07-2019, 12:55 PM   #1
voxmagna
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Default EOS/Golf MkV fault code P1592 - Altitude/Boost Press. Sensor. Implausible Correlation

Post 1 of 2 long posts
This fault has been stored a few times on my scans. It can be reset and doesn't seem to come back often or affect the running. It puzzled me because I replaced the boost pressure sensor G31 and there was no immediate fault code. I could sometimes fix it by cleaning the sensor connector with contact cleaner. Now I have all the answers and fix for this nasty intermittent fault that can cost a lot of time. In worst cases it can put on the MIL light and set limp home mode. In other cases Stealers might charge you up for a new sensor, see the fault go away and think they fixed it.

Most V.W electronic sensors work over the range 0-5V. Well, their active range is usually from 0.9-4.75V. This is deliberate by design to enable diagnostics to detect invalid 0V and +5V as sensor or wiring faults.

P1592 is an unusual fault code and hard to trace because it is often intermittent. The language description doesn't explain a lot either. There are 2 sensors in the V.W engine management control system measuring air pressure - The MAP (Manifold Absolute Pressure) sensor G70 and the Turbo boost pressure sensor G31. The MAP sensor sits on top of the inlet pipe just right of the battery. The G31 boost pressure sensor sits on top of the intercooler hose, on the right of the engine bay towards the front right corner of the radiator.

So what does 'Implausible Correlation' mean? When you first turn on the key both sensors measure the static atmospheric pressure. There is no inlet air and the engine and turbo are off so you would expect both sensors to read the same value of atmospheric pressure. But remember they are different sensors and some error in pressure between them is expected. However, the V.W software is clever and checks both pressure sensors read about the same at key on. If they are different by a predetermined error amount, one or both cannot be relied on for measuring pressure and a P1592 implausible correlation fault code is set. The magnitude of difference error determines whether it's just a fault code and the engine runs normally (my case) or a large error when the MIL light is put on. Our cars don't have barometers so they use this simple logic that if there's a big difference from 2 separate sensors at key on, something is wrong.

O.K so we could have a fault in either or both pressure sensors when tested at key on? Probably unlikely because the most likely fault is related to sensor grounding. Both sensors output a DC voltage corresponding to pressure and this DC voltage checked by the ECU is WITH RESPECT TO GROUND/EARTH. At key on for atmospheric pressure, both sensor output voltages will be a hundred or so millivolts above the low threshold, relative to ground. If the ground reference moves, then so does the low reference point. If both sensor grounds move by the same amount of error, there will be correlation and no fault, even though the absolute atmospheric pressure measurement is off compared to a barometer. The bad news is when the ground potential voltage is different for each sensor. The errors caused by unequal and high resistance ground wires on sensors can be hard to find. Easy errors are static and can be measured by an ohmmeter. But the worst errors are dynamic and change as current is drawn from high amperage devices through the grounding system. Keeping the ground loop resistances to the battery as low as possible ensures sensor outputs voltages near to ground are less affected by ground currents.

So how did the V.W wiring design screw this one up? The boost pressure sensor is located and wired from the front right corner of the engine bay to a nearby ground located under the washer fill hose. The MAP sensor ground is diagonally opposite and furthest away at the rear, with its loom grounded to a nearby earth point. If they had used the same length wire for each sensor connected to the same clean earth point near the battery, there should not have been a problem. But the wire selection least cost algorithm chooses the smallest wire and shortest route!

I removed both sensor connectors and identified the ground pins with a multimeter on ohms. For the boost sensor G31 it is the brown wire (on pin 1 or 4?). For the MAP sensor G70 it is the green wire connected to pin 3. I am going to measure the ground resistance (voltage drop) from each ground pin to the negative terminal of the battery. I know what to expect because the ground path for the boost sensor will be longer and higher resistance since it is furthest from the battery. I'm not using the ohmmeter because I want resolution for very small resistance. I used a 22 ohm 10W resistor, one end is connected to battery + USING A THICK JUMP LEAD. The other end is slid into the ground pin of each sensor plug and the voltage (mV) then measured between the resistor/sensor pin and battery (-) terminal.

My Results: Battery voltage 13.2V, Resistor 22 ohm, Boost sensor ground pin 240 mV, MAP sensor ground pin 45mV. Therefore, the ground return resistance from the boost sensor is 5 times that of the ground return from the MAP sensor. Bad news when trying to correlate the same pressure values and I'm sure it could get a lot worse.

We need to know where the ground points are in the engine bay and which are used by the Boost and MAP sensors. The ground wires for both sensors are very thin and we can expect a high resistance but not different. For the boost sensor G31, the vertical wiring loom next to the washer filler has a bulge near the bottom and a brown wire exits the loom and attaches to the right side ground under the fill hose. The MAP sensor has a much shorter loom and I think its ground is a large ground point under the air box? Look at the EOS ground location - it is a very revealing diagram.
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Old 08-07-2019, 01:00 PM   #2
voxmagna
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Default EOS/Golf MkV fault code P1592 - Altitude/Boost Press. Sensor. Implausible Correlation

Post 2 of 2 long posts

........There are two solutions to this sensor problem - The Half Monty takes about an hour, but leaves out an important unknown. Then the Full Monty which fixes everything sensor ground related now and forever in the future. Most sensors are using the ground return as a zero voltage reference and improving engine bay grounding for the loom will benefit all.

The Half Monty solution requires the covers on both sensor plugs to be removed to expose their ground wires. Each ground return is cut about 30mm back from the connector shell and an additional ground wire 0.75 sq. mm. solder spliced and sleeved. For the boost sensor plug, the wire can be run down the inside of the convoluted sleeve to the bottom and ground point 614 under the washer pipe. Crimp a 6mm dia. eye on this end, remove the dome nut (10mm 1/4" socket on a universal joint), fit the extra ground and re-tighten the dome nut.

Unfortunately, there isn't enough room inside the MAP sensor sleeve to take the additional 0.75mm sq. ground wire, but this can be neatly taped to the sleeve with cloth tape and then run around the rear of the battery to ground point 615 on the left suspension strut. Fit a larger (8mm?) terminal eye, remove the 13mm dome nut and fasten down.

With luck the ground return through the chassis to the battery -ve will be good. Re-measure the resistances or voltage drop from each ground pin on the sensor connectors to the battery -ve post. The resistances/voltage drops should be lower and of similar value. But the Half Monty shortcut has flaws!

The Full Monty takes about 2 hours and includes an inspection of the main ground terminal under the air box and the addition of 16 sq. mm copper wire between key ground terminals to ensure these grounds stay good irrespective of what happens to the chassis.

1. Remove the washer fill bottle and connecting pipe. Remove the top screw holding down the right headlight. This allows it to move forwards about 20mm. Locate earth point 614 and remove its dome nut.

2. Solder splice the 0.75 sq. mm extra ground to the brown wire 30mm back from the shell, run it down the sleeve and crimp an eye on the end.

3. Crimp an eye on the end of 16 sq. mm wire (I've used electricians green/yellow earth bonding wire for both sizes). Fasten the eye loosely to terminal 614 and form the wire to run over the right headlamp and under beneath the top panel above the radiator. There are some holes along its length that can be used for tywraps - this is thick heavy copper wire! Leave the far end loose until you have exposed earth point 652 under the air box.

4. To remove the air box , disconnect the MAP sensor topside towards the rear. Remove all (8) screws securing the top cover & remove the air filter. Push the top cover over to one side - no need to unclip the hose.

5. Remove one hex screw on the inside corner of the air box lower half. Pull off the 'U' shaped plastic inlet pipe from the air box , unclip and lift the front box top cover and its 2 half connection pipe.

6. Pull the lower half of the air box up hard - it is held by 2 pegs and rubber grommets.

7. Now have a good look at earth point 652 which grounds the motor/alternator/starter/ and has a large loom ground wire. If it looks like my photo all green and in a poor state, you will be glad you went for Full Monty fix!

8. Remove the 13mm dome nut and clean up the eye connections. MY07 was so bad I started with 80 grit paper then went for the needle files to remove the crud.

9. Bring around the 16 sq. mm additional ground wire, fit a large terminal eye (8 or 10mm?) and route it neatly to earth point 652, keeping it flat so it won't foul under the air box.

10. Now take a second (shorter) length of 16sq.mm wire, fit a large eye and check you can still fit it and the dome nut to earth point 652. Poke this second large wire through the slot under the battery case alongside other ground wires, then up behind the battery and over to the left suspension strut earth 615. Safety note: V.W deliberately keep + and - wires separate, do not run alongside or over the red battery +ve wire!! To recap, your earth point 652 should have the OE engine earth, the brown OE loom earth, 1 long 16sq.mm earth wire from terminal 614, and a short 16sq.mm earth wire running to terminal 615 on the left strut. You have thick additional bonding between earth points 614,652, and 615. You have also added the two 0.75 sq. mm wires to the sensor grounds on the connectors and connected these to terminal points 614 and 615

11. Smear dielectric grease over the earth points and new connections.

12. Refit the lower half air box, connect up the pipes and front inlet box. Fit air filter and cover (8 screws). Fit the MAP sensor plug.

13. Now test your work! Measure and compare the resistances, or voltage drops on the ground pins of each connector using my method. My results after earthing modifications were: Boost sensor 10mV, MAP sensor 10mV. That works out at 0.017 ohm each sensor ground return and both are equal.

14. If you have vcds, hook it up with ignition on, engine off and look for the Charge (Boost) pressure value and the atmospheric pressure read from the MAP sensor. My results were 979.2 mb for the Boost sensor and 989.4 mbar for the MAP sensor. That's a 1% difference in value. When you replace a pressure sensor as I did, the dealer can use their software tools to apply small corrections to the ECU sensor tables and in theory reduce the difference error, as long as they know which is correct! I shan't bother with a Stealer visit , 1% error between pressure sensors seems o.k to me?

15. V.W have taps inside their wiring looms where several ground (or other) wires are terminated to a common point. There could still be problems with those but it's not an easy job to unwind the loom wrap to get inside. Passing a current (e.g. 0.6 Amp like I did) and flexing the wire whilst measuring voltage drop should reveal a bad wire or internal join.

16. Finally, there is an EOS wiring loom change mod. to fix this (see the vcds Wiki link). I haven't bought or seen it, but I think a couple of hours spent with some heavy copper earth bonding wire doing before and after tests will have done a better job?

http://wiki.ross-tech.com/wiki/index...0/P1592/005522

Correction: Photos referring to Earth point 12 mean Earth point 652, the large engine ground.
.

Last edited by voxmagna; 08-07-2019 at 07:41 PM.
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Old 08-13-2019, 06:39 PM   #3
RobSlaterFSI
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Hi vox

Love it. Very informative , going to check my earth points at the weekend . Don't think they look as bad as yours but I will clean them up .I did have some spray 'sealant' for use on electrical earths kicking around . Will take a pic when I get a moment
Regards

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Old 08-13-2019, 08:49 PM   #4
voxmagna
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The most important earth point is under the airbox which I had never completely removed before. I suspect few others have? The diagram showing the earth point locations in the trunk is important for those that have had water leaks which they cured, but not thought to check and clean afterwards?

The problem with most sensors is they use the ground as reference and over time you only need a little increase in resistance to throw up these hard to track down gremlin faults. Making matters worse they use skimpy small guage wires for a lot of sensor logic interconnection and once these wick some water their resistance goes up. The boost and map sensor isssue was followed up by V.W with a wiring harness mod./change and a TB was issued. I suspect Stealers don't do anything until there's a problem. I was wrong to say they are the only two pressure sensors, because there are also exhaust gas pressure(and temperature) sensors and improving loom grounding integrity should avoid any future problems with those.
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