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Mass Air Flow Sensor Testing
 
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Pelican Technical Article:

Mass Air Flow Sensor Testing

Nick Czerula

Time:

1 hour1 hr

Tab:

$100

Talent:

**

Tools:

T25 Torx driver

Applicable Models:

R56 MINI Cooper Hatchback (2007-11)
R56 MINI Cooper S Hatchback (2007-11)

Parts Required:

Mass air flow sensor, engine air filter

Hot Tip:

Work with a cool engine. Clear engine fault codes when done

Performance Gain:

Car will run well

Complementary Modification:

Replace engine air filter, inspect intake air ducts for cracks

MINI R56 digital engine management (DME) systems use a hot-film mass air flow sensor, installed between the air filter housing and the intake manifold, to monitor air flow into the engine intake. Inside the sensor, there is a thin metal film, which is maintained at a constant temperature via electrical current from the ECM. As ambient intake air flows over the film, it is cooled down and therefore additional current is needed to maintain its constant temperature. The ECM monitors this additional current to determine the mass of the incoming air and to adjust fuel supply accordingly. Note that as the temperature of ambient air increases its volume expands. Since the air flow sensor hot film monitors heat-loss to the passing air, it automatically adjusts for the increased volume (= lower density) of the incoming air. The HFM6 style sensor sends a digital mass air flow and temperature signal to the DME.

Mass air flow sensors accuracy skews over time and can lead to fault codes or drivability problems. When replacing, inspect the plastic intake air ducts for cracks or dry rot. If the intake air ducts are faulty, replace them first and see if the problem you are having is remedied. An un-metered air leak downstream of the mass air flow sensor will set codes similar to a faulty mass air flow sensor. When you suspect a mass air flow sensor as the culprit for setting a fuel trim fault code, check when the fault code was set. You can do this using OBD II freeze frame data. Normally, a vacuum leak would set a fuel trim fault code at idle or very low engine RPMs. A mass air flow meter would usually set a fault code when you are driving, or at a higher RPM. You can check your fault code freeze frame data using an OBD II scan tool, similar to how you would check for fault codes.

In this tech article I will show you how to test the mass air flow sensor on MINI R56 models with a 4-cylinder engine.

Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Do not work on your vehicle if you feel the task is beyond your ability.

Vehicle models change and evolve, as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

The mass air flow sensor (red arrow) is located at the air filter housing (red arrow) outlet on the left side of the engine.
Figure 1

The mass air flow sensor (red arrow) is located at the air filter housing (red arrow) outlet on the left side of the engine. The turbocharged engine is shown. See our tech article on normally-aspirated engines for the photo of the location.

In the past mass air flow sensors provided an analog signal to the DME.
Figure 2

In the past mass air flow sensors provided an analog signal to the DME. With the HFM6 sensors, they now send a digital signal. The old voltage testing methods no longer work. The yellow wire at terminal 5 on the sensor is referred to as the signal. If we read this signal with a DVOM it would display as a constant 2.5 volts. If we looked at the frequency, it would display about 60 Hz, not as expected from a signal.

In the past mass air flow sensors provided an analog signal to the DME.
Figure 3

In the past mass air flow sensors provided an analog signal to the DME. With the HFM6 sensors, they now send a digital signal. The old voltage testing methods no longer work. The blue wire at terminal 54 on the sensor is referred to as the correction signal. If we read this signal with a DVOM it would display as a constant 2.5 volts. If we looked at the frequency, it would display 1500: 2500 Hz depending on load.

This image shows the voltage and frequency of the correction signal over different RPM ranges.
Figure 4

This image shows the voltage and frequency of the correction signal over different RPM ranges.

Lift the intake air housing lid up and detach the duct from the mass air flow sensor.
Figure 5

Lift the intake air housing lid up and detach the duct from the mass air flow sensor.

If you have a circuit fault code (2B5F) for the correction signal, that is the blue wire, or terminal 4.
Figure 6

If you have a circuit fault code (2B5F) for the correction signal, that is the blue wire, or terminal 4. If you have a circuit fault code (2B51) for the air mass signal, that is the yellow wire, or terminal 5. 

In the past, we tested analog mass air flow sensors using a scope and raising engine rpm, looking for the response from the sensor to the airflow increase.
Figure 7

In the past, we tested analog mass air flow sensors using a scope and raising engine rpm, looking for the response from the sensor to the airflow increase. This style of test holds true with the HFM6. Instead of monitoring analog voltage, measure frequency over time, or graph it. At idle, the frequency will be in the range given above and smooth as shown here.

When RPM is raised or throughout the operating range, the frequency will move up on the graph, reflective of the changes in air flow.
Figure 8

When RPM is raised or throughout the operating range, the frequency will move up on the graph, reflective of the changes in air flow. You should have an immediate response from the sensor.


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