4.0TDI MAP Sensor Help

[North2007]

Ben, check the wiring, looks like the most likely area.

[mattylondon]

Originally Posted by North2007

Ben, check the wiring, looks like the most likely area.

Agree, check the wiring from the engine ECU. Pins A7 A56 A26 A41 - this also includes the temp sensor wiring as it's also in the sensor.

Sometimes with logged G31 faults it's the N75 bypass valve (wastegate) that is actually at fault. The error code is with boost at 1000mb, or roughly 14psi, seems high to me. I don't know much about this engine and I couldn't find an N75 in the 4.0tdi current flow or repair manual, so I'm wondering if it's controlled by the turbo's own control unit?

That's where I would continue my search now knowing that the sensor is good.

[Artermis]

Originally Posted by mattylondon

Agree, check the wiring from the engine ECU. Pins A7 A56 A26 A41 - this also includes the temp sensor wiring as it's also in the sensor.

Sometimes with logged G31 faults it's the N75 bypass valve (wastegate) that is actually at fault. The error code is with boost at 1000mb, or roughly 14psi, seems high to me. I don't know much about this engine and I couldn't find an N75 in the 4.0tdi current flow or repair manual, so I'm wondering if it's controlled by the turbo's own control unit?

That's where I would continue my search now knowing that the sensor is good.

Thanks Matty / all

I guess wiring is most likely suspect, seeing as the fault is at 0rpm the car can't be generating any boost.

I'll run a continuity check between the sensor plug and those ECU pins when I get chance.

Thank you.

[mattylondon]

I've since read that the waste gate is controlled by an electronic turbo actuator on the 4.0tdi. Manufactured by Hella, part number: 6NW 008 412. If this is causing overboost or underboost it can be overhauled by a few companies. The same part was used in fords and jags. But let's hope it's a fixable wiring issue

[Artermis]

I hope it's wiring. Something must have either failed spectacularly or its wiring. It was fine one journey and then lit up with warnings the moment I turned it on for the next.

No actual mechanical symptoms at all

Hope it's easy fix as since my wife encountered employment problems (the company shut the store she managed) funds have been more than a little bit tight.

These things never come at a good time!

[carmel120]

hi you can use vagcom to check the actuator on the turbo there is a bulliten in the workshop manuals on this site it was service page.

[carmel120]

page 52

32-Service%20Manual%20Maintenance%20(2).pdf

[Mechcanico Lee]

Here's how it normally works with most systems ......

The car will have an atmospheric sensor ....this sometimes is a separate sensor or it can be internal to the ecm .....this at sea level will read very close to 1000 milli bar ( atmospheric pressure )with the ignition on.

The boost sensor in the intercooler ignition on will also read very close to atmospheric ( absolute pressure ).... the ecm will do a signal compare at ignition on , if I remember correctly these readings have to be within 50 millibar of one another ......any pressure above atomsperic is classed as boost .

Am I correct in thinking if you clear the code it comes straight back without even running the engine ? .......if yes .....this is already negating checking the turbos for boost ,as ecm is seeing the fault without it even running .


In the fault code you have ,the details show two pressures .... I reckon one is the atmospheric reading the 999.6 mb and the other the 1111.3 is the incorrect reading from the boost sensor .

After you have replaced the boost sensor if you go into data blocks and look at boost pressure and atmospheric pressure what are the readings with just ignition on . If the data blocks don't give a description of what they are if you look at the blocks and give it a good blip ....the boost one will change but the atmospheric will always stay the same .


If the boost reading is still at the 1111.3 mb ignition on , next checks at the boost sensor plug .....Matty says air temp combined in the sensor so it will have 4 wires .......
One wire 5 volt sensor supply
One wire air temp signal
One wire boost signal
Last wire earth

Have you a volt Meter and a basic bulb type electrical test light ?

So with ignition on and sensor plug unplugged find the 5 volts supply then with test lamp crock clip on earth dip on the same pin as the 5 volt supply and make sure it stays at 5 volts .....the bulb just loads the 5 volt supply a little so we know the 5 volts is a ' strong ' supply .
The other two wires will probably have 3 or 4 volts on them ....this voltage is a limp value implemented by ecm as a default because the plug is in plugged from sensor
Then lastly put the test lamp crock clip on a positive supply and find the earth pin .... the lamp should light at full brightness .

I actually have a 21 watt test light ...it just puts the feeds and earths under more load so you know they are 'strong ' , using led type test lamps can be very miss leading .....they will light at anything thus not proving the wiring is strong

If 5 volts and earth ok , would be a case of load testing the other two wires and checking for short to ground or short to positive .

Let us know how you get on .

[Artermis]

Originally Posted by Mechcanico Lee

Here's how it normally works with most systems ......

The car will have an atmospheric sensor ....this sometimes is a separate sensor or it can be internal to the ecm .....this at sea level will read very close to 1000 milli bar ( atmospheric pressure )with the ignition on.

The boost sensor in the intercooler ignition on will also read very close to atmospheric ( absolute pressure ).... the ecm will do a signal compare at ignition on , if I remember correctly these readings have to be within 50 millibar of one another ......any pressure above atomsperic is classed as boost .

Am I correct in thinking if you clear the code it comes straight back without even running the engine ? .......if yes .....this is already negating checking the turbos for boost ,as ecm is seeing the fault without it even running .


In the fault code you have ,the details show two pressures .... I reckon one is the atmospheric reading the 999.6 mb and the other the 1111.3 is the incorrect reading from the boost sensor .

After you have replaced the boost sensor if you go into data blocks and look at boost pressure and atmospheric pressure what are the readings with just ignition on . If the data blocks don't give a description of what they are if you look at the blocks and give it a good blip ....the boost one will change but the atmospheric will always stay the same .


If the boost reading is still at the 1111.3 mb ignition on , next checks at the boost sensor plug .....Matty says air temp combined in the sensor so it will have 4 wires .......
One wire 5 volt sensor supply
One wire air temp signal
One wire boost signal
Last wire earth

Have you a volt Meter and a basic bulb type electrical test light ?

So with ignition on and sensor plug unplugged find the 5 volts supply then with test lamp crock clip on earth dip on the same pin as the 5 volt supply and make sure it stays at 5 volts .....the bulb just loads the 5 volt supply a little so we know the 5 volts is a ' strong ' supply .
The other two wires will probably have 3 or 4 volts on them ....this voltage is a limp value implemented by ecm as a default because the plug is in plugged from sensor
Then lastly put the test lamp crock clip on a positive supply and find the earth pin .... the lamp should light at full brightness .

I actually have a 21 watt test light ...it just puts the feeds and earths under more load so you know they are 'strong ' , using led type test lamps can be very miss leading .....they will light at anything thus not proving the wiring is strong

If 5 volts and earth ok , would be a case of load testing the other two wires and checking for short to ground or short to positive .

Let us know how you get on .

Cheers Lee - I have been thinking along the same-ish lines as you, although I believe it's the other way round.

Whatever is reading the 999.6mbar i think is what is faulty. Every time I clear the error only for it to return this value never ever changes. I've been hunting this for a few days now and it's constant. The second absolute pressure value varies slightly, which I think is largely reflective of reality given that i'm probably about 1/4 mile from the sea and more or less at sea level.

The 999.6mbar absolute pressure - where ever this is coming from - never ever changes. If only I know what it was reading this from.

[Artermis]

Should add at this point I'm having a gut instinct that MAF sensor is involved somehow. Don't know why but I do...

These cars are a git to work on though aren't they! Had a look inside bank 1 airbox the other day and it looks like it's incorporated into a moulded unit in the airbox half. Nice and easy to remove and inspect, then!