4.0TDI MAP Sensor Help

[HPsauce]

Typical sea level pressure is 1013mb so both of those readings are probably wrong.
Sea-level pressure can drop below 900 in extreme weather but is rarely if ever above 1050 (except in the Dead Sea and similar places).

[Mechcanico Lee]

Most of the atmospheric readings I see on the sensors are around the 1000 mb Mark ...it doesn't have to read excatly 1013mb just there abouts but be a steady reading at the same elevation .

If you go into the data blocks now after the boost pressure replacement what is your ignition on reading on the sensor that you say has some variation on .

Is the car in limp ? if it's still driveable , drive it and watch that reading have you got a reasonable bit of change when you go under some load .

So again ,...fixed value is atmospheric .....variable value is the actual boost pressure .

One step at a a time, don't get confused with other things ...air mass just measures air mass and air temp on most kinds of sensors .

If the boost pressure is showing the 1111.4 mb reading at ignition on that is enough to flag the incoherence code .

This is where we end up back at the boost pressure wiring checks if sensor supply voltage or earth is wrong so any signal outputs from that sensor will be wrong .
Is the replacement boost sensor you have a got a good make ? Not ting tong ?
Another test...... un plug the boost sensor then see what the default value is on the boost sensor data block .

[Artermis]

Yep, she is in limp mode at the minute.

If I unplug the sensor I get these:

16497 - Intake Air Temp. Sensor (G42)
P0113 - 000 - Signal too High
Freeze Frame:
RPM: 0 /min
Torque: 27.3 Nm
Speed: 0.0 km/h
Voltage: 11.10 V
Absolute Pres.: 999.6 mbar
Temperature: 1.8°C
Temperature: 15.3°C
Raw, Form. N/A: 000 000 000

16622 - Manifold Pressure / Boost Sensor (G31)
P0238 - 000 - Signal too High
Freeze Frame:
RPM: 0 /min
Torque: 27.3 Nm
Speed: 0.0 km/h
Voltage: 5.10 V
Voltage: 11.10 V
Absolute Pres.: 1020.0 mbar
Raw, Form. N/A: 000 000 000
Absolute Pres.: 999.6 mbar

16620 - Manifold Pressure / Boost Sensor (G31)
P0236 - 001 - Implausible Signal
Freeze Frame:
RPM: 0 /min
Torque: 27.3 Nm
Speed: 0.0 km/h
(no units): 2.0
Raw, Index/Key Err.: 255 000 006
Raw, Index/Key Err.: 255 000 083
Absolute Pres.: 1020.0 mbar
Absolute Pres.: 183.6 mbar


Readiness: 0 0 1 1 1

Not sure I know what that's telling me though, other than they're down to 11v so the battery is now flat. Had to give up for today and stick it on charge.

[Artermis]

Also not sure how I'm going to actually test the wiring on the ECU as the plugs are clamped in place with some kind of security bolts so I can't just take the ECU connectors off to test for continuity between pins.

Its impossible to tell whilst still on the car which ECU is what and what plugs are what so I can't just poke into the back of it.

[Mechcanico Lee]

Hello Ben I will just ask some of the questions again ....

If you go into the data blocks now after the boost pressure replacement what is your ignition on reading on the sensor that you say has some variation on ?

Is the replacement boost sensor you have a got a good make ? Not ting tong ?

You are using vcds ??
On the first page is advanced measurements highlighted ? So you can go into the readings like this ......




**This is not a A8 its just for an example** ......113 ambient pressure 115 actual boost pressure .


I just want to be sure we are reading the correct values for actual boost and atmospheric pressures .


Regards Ecms ......yes they have the pain in the arse security plates over the top of them normally with the snap head bolts ......no easy way round this I'm afraid , I either grind a deep slot in the snap head bolt and use a big flat screwdriver to undo ...or if you cannot get at the bolts just grind through the plate which ever way you can .


Before you do that.... with ignition on you need to check at the boost sensor ( disconnected )plug that you have the ' strong' 5 volts supply and a ' strong ' earth
Use a volt Meter and a normal bulb test light to test these wires like I explained in the earlier posts ......there's good reasons behind this method ....and for not using led Type test lights or just using a volt meter .

There's a reason why you want to test the 5 volts and Earth at the boost sensor plug also .....see ....you could test at ecm end and they could be present ......doesn't mean that they are present at the boost sensor plug though .....if the loom has corrosion or chaffed through wiring the volts or earth pathway could be compromised from Ecm to loom plug .


So let's say the 5volt supply and earth is good , the other two wires ... boost signal and air temp signal measure the voltages off them wires ...just as an example let's say them wires are 2 volts at the sensor end ....they should be 2 volts at ecm end also .

Hopefully some of our other members could pull a wiring diagram off the Vag Ekra site

My gut feeling is that this will be a poor earth at the sensor ....but carry out the tests ......... You have heard me say this before ......test ...rule out move on ....test not guess .

Ben if you need more help i will pm my number to you .

[Artermis]

Thanks Lee

My dad has charged my battery over night for me. Heading in to town with him and mother to sort her mobile phone out.

I'll test the plug this afternoon when everything has defrosted a bit and report back.

Appreciate the support - you've been very helpful (and patient) with me!

[Mechcanico Lee]

No worrys , make a note of the wire colour codes and what volts you have on each one , timber to test the earth wire with the bulb test light .

[Artermis]

Hi Lee,

In answer to your questions. Hopefully I've tried to cover as many of them as I can before running out of time and daylight for today.


1. If you go into the data blocks now after the boost pressure replacement what is your ignition on reading on the sensor that you say has some variation on ?

OK so today the two values are slightly different - we have a different weather system over us now with much more settled weather and colder temps.

Therefore today we have an absolute press of 1030.2 (this is the fixed value, 999.6mbar in earlier post).

The variable value, i.e. the one that does seem to change is at 1122.0mbar, when car is running. When the fault is generated, at first ignition on, this value increased to 1132.0mbar.

If I clear the fault, engine on or off, the fault doesn't repeat until ignitionoff and on.

2. Is the replacement boost sensor you have a got a good make ? Not ting tong ?

It's unbranded I'm sorry to say. You know I have two of the replacements, as I mistakenly bought two. I also have a 'spare' map sensor off an Alfa 156 which is BOSCH. I still have the genuine BOSCH that came off the A8.

When I've checked the resistances between the pins both BOSCH sensors show about 1ohm lower than the spare generic sensor I've got. Both BOSCH sensors are comparable with each other. I've not tested the sensor on the car but I can't see it being different to it's twin.

Unfortunately the Alfa sensor plug isn't compatible with the A8 to test.

3. You are using vcds ??
On the first page is advanced measurements highlighted ? So you can go into the readings like this ......

Yes I am using VCDS and HEX-CAN cable. I don't have the advanced measurement option highlighted, it's greyed out.

When I go into the measurement blocks I can look at group 10 and see the following:

15:02:48 Group 010
0.0 mg/str Mass Air / Rev.
1030.2 mbar Absolute Pres.
1122.0 mbar Absolute Pres.
0.4 % Duty Cycle
Wednesday,28,December,2016,15:11:36:33490
VCDS -- Windows Based VAG/VAS Emulator
VCDS Version: 12.12.3.0 (x64)
Data version: 20140905

The second of the two absolute pressure values, 1122.0mbar responds to throttle. For a car in limp mode it responds as I've expect, i.e. vac in response to throttle input. I can only test at stationary as the front end is in bits to access the pressure sender and it's on jack stands!

The other absolute pressure value, 1030.2 is fixed / constant and doesn't react to engine or input whatsoever.

Groups 10 and 11 show this (don't know what difference between 10 and 11 is). This is five mins later so engine is a bit warmer than above.

15:07:36 Group 010
552.3 mg/str Mass Air / Rev.
1030.2 mbar Absolute Pres.
1111.8 mbar Absolute Pres.
0.4 % Duty Cycle

15:07:36 Group 011
672 /min RPM
938.4 mbar Absolute Pres.
1111.8 mbar Absolute Pres.
1030.2 mbar Absolute Pres.



4. Before you do that.... with ignition on you need to check at the boost sensor ( disconnected )plug that you have the ' strong' 5 volts supply and a ' strong ' earth

OK, so I've found out my test light is broken, I'll have to ask my dad for help to test that much.

I have tested the other pins at the plug. Haven't broken into the ECU you.

The pins are as follows:

pin 1 - brown w/blue = 0.04v
pin 2 - white w/green = 5v
pin 3 - yellow w/red = 5v
pin 4 - white w/yellow = 5.6v

stands to reason that pin 1 is the earth.

[Mechcanico Lee]

Great good work Ben ....sorry I should of said this in last post , with the orginal boost sensor with the plug plugged in check them voltages again .

Im happy with the atmospheric reading 1030mb or 999.8mb it's close enough it not an exact science ....it's only a tiny aneroid sensor internal to ecm .

See how with sensor unplugged there is 3 pins at 5 volts or thereabouts , one of them 5 volts is the sensor supply the other two 5 volts are coming from the ecm ....this system is called '5 volts pull down ' .......so plug the boost sensor back in and in therory 2 of them values will change make a note of this .

So the brown / blue judging from the colour code must be the earth ....you need to load test this wire with a bulb so we know it can carry some load .

So if we know 5 volt supply good ...earth good and the other two signal wires when you measure voltage at the plug with it connected and that is X you need to measure that at ecm plug .

The bit that clouds the whole diag is the unknown brand boost sensors ..... see the orginal boost sensor could be faulty but both the new sensors could be giving in correct readings ....there's a lot of Chinese parts out there that can cloud the whole diagnostic process and send you off on a tangent , with sensors we tend to use dealer parts , it just saves alot of ' re diagnosis ' .

Try these tests Ben and see how we get on .

[Artermis]

Originally Posted by Mechcanico Lee

Great good work Ben ....sorry I should of said this in last post , with the orginal boost sensor with the plug plugged in check them voltages again .

Im happy with the atmospheric reading 1030mb or 999.8mb it's close enough it not an exact science ....it's only a tiny aneroid sensor internal to ecm .

See how with sensor unplugged there is 3 pins at 5 volts or thereabouts , one of them 5 volts is the sensor supply the other two 5 volts are coming from the ecm ....this system is called '5 volts pull down ' .......so plug the boost sensor back in and in therory 2 of them values will change make a note of this .

So the brown / blue judging from the colour code must be the earth ....you need to load test this wire with a bulb so we know it can carry some load .

So if we know 5 volt supply good ...earth good and the other two signal wires when you measure voltage at the plug with it connected and that is X you need to measure that at ecm plug .

The bit that clouds the whole diag is the unknown brand boost sensors ..... see the orginal boost sensor could be faulty but both the new sensors could be giving in correct readings ....there's a lot of Chinese parts out there that can cloud the whole diagnostic process and send you off on a tangent , with sensors we tend to use dealer parts , it just saves alot of ' re diagnosis ' .

Try these tests Ben and see how we get on .

Will try the above.

Also have a genuine BOSCH part on order from Eurocarparts. Will report back in the morning.

Thanks Lee