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MAF sensor voltages?

Tiny
GéZA KOVáCS
  • MEMBER
  • 2006 FORD S-MAX
  • 1.8L
  • 4 CYL
  • TURBO
  • 2WD
  • MANUAL
  • 168,000 MILES
Hi. If I unplug the MAF sensor connector, 5v can be measured on the signal line. According to Autodata, there should be 0 Volts in this case. If I reconnect it, 1.77 volts are displayed on the signal wire when the ignition is turned on. The 5-volt sensor supply drops from 5 volts to 2.57 volts with the engine running, the voltage of the signal line rises to 2.1 volts at full throttle. The MAF sensor has 4 wires. Signal - 12 Volt supply - ground - 5 Volt sensor supply

Wire break? ECU problem? Connection error?

Thank you
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Monday, August 28th, 2023 AT 12:08 PM

31 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,356 POSTS
It sounds like the MAF sensor is bad causing the voltage discrepancies. I would swap it out to see if the problem goes away, did you have a code or?

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Please go over this guide and get back to us.
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Monday, August 28th, 2023 AT 12:26 PM
Tiny
GéZA KOVáCS
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I forgot to write that the sensor is new. And there is voltage on the signal line even when the connector is disconnected. (5 Volts)
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Monday, August 28th, 2023 AT 11:53 PM
Tiny
GéZA KOVáCS
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There is no error code.
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Monday, August 28th, 2023 AT 11:54 PM
Tiny
BORIS K
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Hello

Pin out at MAF sensor(4 wire)

Pin 1, not occupied
Pin 2, grey/orange wire IAT signal to ECM pin AB1
Pin 3, violet/grey wire 12V MAF supply from fuse F9
Pin 4, blue wire ground to ECM pin AB3
Pin 5, yellow/violet wire MAF signal to ECM pin AC4

The MAF sensor incorporates the IAT(Intake Air temperature sensor)
With MAF disconnected and ignition on you should see between 4.8-5.2V on pin 2
If voltage is not ok next measure the voltage at pin AB1 at the ECM(Engine Control Module)
If voltage at ECM is in order next check the grey/orange pin to pin.
Expect to see less than 2 ohm resistance
This can be done using a voltmeter

How to use a voltmeter
https://www.2carpros.com/articles/how-to-use-a-voltmeter

How to check wire
https://www.2carpros.com/articles/how-to-check-wiring

There are 3 connectors on ECM, A-B-C
The pins are on connector A, small
The ECM is located in the engine compartment, left front
See image 3

Cheers, Boris
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Tuesday, August 29th, 2023 AT 2:06 AM
Tiny
GéZA KOVáCS
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Okay. I will check it tomorrow. Thanks
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Tuesday, August 29th, 2023 AT 5:58 AM
Tiny
GéZA KOVáCS
  • MEMBER
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Hi,

I measured the 3 wires from the MAF to the ECU (resistance), it showed 1-2 ohms. The resistance of pin2 and pin5 was 25k ohm. 21k ohms between pin2 and IAT. Is it possible that these wires are in contact with each other?
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Wednesday, August 30th, 2023 AT 10:48 AM
Tiny
BORIS K
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Hello,

Did you measure these resistance value when measuring into the MAF?
These values sound plausible as they would have internal connection in the MAF.

Cheers, Boris
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Thursday, August 31st, 2023 AT 9:31 AM
Tiny
GéZA KOVáCS
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No, I measured it when the wire was disconnected from the MAF and connected to the ECU.
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Thursday, August 31st, 2023 AT 10:57 AM
Tiny
GéZA KOVáCS
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What I don't understand is that when the wire is disconnected from the MAF, why is there 5V on both pin2 and pin5? If it is connected, then pin2 is 2.57Volt, pin5 1.77Volt. In my first article, I wrote the voltage of the two wires interchangeably.
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Thursday, August 31st, 2023 AT 11:01 AM
Tiny
BORIS K
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Hello,

First back to basics.
What fault codes do you read using a suitable scan tool?

Next
Pin 2 is the signal wire for the IAT resistor. This will be a 5V supply when not connected.
Pin 5 is the signal wire for the MAF signal, this usually is a 5V reference voltage from the ECM when not connected.
When connected you are then reading a reduced voltage caused by the internal resistance of either sensor.
Suggest to disconnect ECM connector and MAF connector.
Check that neither wire has any voltage with ignition on.
Check each wire pin to pin for any resistance issues, you should get less than 2ohm when measured pin to pin.
Also check that neither wire has any continuity to any other wire.
This can be done using a voltmeter.

A good guide on testing MAF sensor output using live data from a suitable scan tool:
https://www.alldata.com/sites/default/files/file-attachments/maf_sensor_testing_110918.pdf

Cheers, Boris
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Friday, September 1st, 2023 AT 4:12 AM
Tiny
GéZA KOVáCS
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Two error codes are written by the diagnostics (with VCX nano, ids software).
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Friday, September 1st, 2023 AT 7:47 AM
Tiny
GéZA KOVáCS
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The resistance of the wires was below 2 ohms and there was no continuity in relation to each other.
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Friday, September 1st, 2023 AT 7:50 AM
Tiny
BORIS K
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Hello,

None of these fault codes are engine related.
BCM - Body Control Module
PDM - Passenger Door Module

What makes you think that there is a problem with the engine management system?
Do you have running issues?

Cheers, Boris
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Friday, September 1st, 2023 AT 7:59 AM
Tiny
GéZA KOVáCS
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Having startup problems after several hours of idle time. It is hard to start, when it starts, it stops immediately. After 4-6 starts, it does not stop. One day the power went out, it wouldn't accelerate even though I stepped on the pedal, after a while the power came back. The turbocharger already emits a "whooshing" sound above 2000 rpm, there is also a little oil under the blade in the housing.
The ones that were bad were replaced - injector, high pressure pump, glow plugs, leak off pipe, fuel filter, pipes and connectors. + Hand pump - many people said that it gets air - but then why does it start once for the first time and sometimes after several attempts. E.G. I cleaned the intake manifold, put it back together - I even took out the fuel filter housing - and it started in first gear. It didn't start anymore, just a few times.
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Friday, September 1st, 2023 AT 8:33 AM
Tiny
BORIS K
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Hello,

Looking at the video you are back probing pin 5 which is the signal wire for the MAF.
A virtually static voltage of around 2.11V.

On live data, the previous post the air mass was at 11g/sec, which seems a bit low.

Have you checked the operation of the EGR valve? Not blocked and moving freely.
Another option would be to blank the EGR valve for testing purposes.
See image 1+2 below.

An eBay link:
https://www.ebay.co.uk/itm/121106889703

Cranking and non-starting on a diesel can be caused by a stuck open EGR valve.
Another aspect to check when vehicle is non start is the fuel rail pressure, ensure that while cranking the rail pressure is more than 200bar in live data. The ECM will not trigger the injectors if the rail pressure is below this value.

Cheers, Boris
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Sunday, September 3rd, 2023 AT 1:06 AM
Tiny
GéZA KOVáCS
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Yes, the EGR is blanked. The rail pressure is more than 200 bar. "Air mass 11g/s at idle". Still not enough?

Thank you very much for your help so far.

Cheers, Géza
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Sunday, September 3rd, 2023 AT 4:03 AM
Tiny
GéZA KOVáCS
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Can't a faulty MAP sensor produce such a symptom? Or IAT sensor?
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Sunday, September 3rd, 2023 AT 1:19 PM
Tiny
BORIS K
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Hello,

A faulty MAP/Boost pressure sensor can cause all sorts of issues.
Check the sensor in live data.
Ignition on/engine off you should see ambient pressure, about 1000 mbar.
Engine idling the reading is usually slightly lower, around 920-950 mbar as there is no turbo boost at idle.
A faulty temperature sensor can cause starting and running issues.
The easiest way to check these sensors, IAT and ECT, is to check live data when the engine is cold.
Ideally stood over night. The live data values should equal ambient temperature.

Cheers, Boris
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Monday, September 4th, 2023 AT 1:25 AM
Tiny
GéZA KOVáCS
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Hi,

I watched it live data. I'm attaching pictures and video. The ECT is new. At idle pressure is 100kPa. (Is the IAT measured by the MAF or the IAT? Because I disconnected the IAT and it didn't change intake air temp, but when I disconnected the MAF it showed 0°C on the intake air temperature)
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Tuesday, September 5th, 2023 AT 1:17 AM
Tiny
BORIS K
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Hello,

The IAT sensor is part of the MAF, pins 2+4

Watching the video, the starting behavior is as if the glow plugs are not being activated.
As soon as there is a bit of combustion temperature the engine starts well.

A good video on testing:
https://youtu.be/fvSiZ5xtBwE?si=zCNFyLmI5u8Wwf3E

A good glow plug has an internal resistance of about 0.5ohm.

Cheers, Boris
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Wednesday, September 6th, 2023 AT 12:53 AM

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