For decades, we’ve seen mass airflow sensors (MAF) on the vehicles that we service every day. Most technicians are aware of the function of the MAF sensor, but may not be fully aware of the effect a faulty MAF sensor has on engine and transmission operation.
MAF sensors are used in both gas and diesel applications to measure the mass of the air entering the intake system. Many issues can develop if a problem occurs with the sensor such as:
- Transmission damage due to low line boost (burnt clutches)
- Stalling complaints
- Poor driveability such as a surge and misfire
- TCC and transmission shift and shudder complaints
- Check Engine light and DTCs
Mass airflow sensors are a common topic raised by technicians at ATRA seminars across the country. There are several different sensor designs that have been used throughout the years.
One design that seems to spark a lot of interest is the 8-pin Hitachi and Bosch sensors in use on many of today’s vehicles. The first 8-pin design MAF sensors were introduced around the 2012 model year.
8-pin sensors were developed to make the MAF sensor act as the “weather station” for the vehicle computers. Some manufacturers now refer to this sensor design as a multifunction intake air sensor.
This new sensor is designed to be more accurate than the previous sensors, and to measure the humidity, air temperature, and barometric pressure of the air entering the intake system. The 8-pin sensor measures:
- The mass of air entering the engine via the mass airflow sensor (MAF)
- Humidity via a humidity sensor
- Temperature of the air via intake air temperatures sensors 1 and 2 (IAT-1 and IAT-2)
- Barometric pressure (throttle inlet air pressure, or TIAP)
On some applications, such as the Duramax diesel, the 8-pin sensor is used in conjunction with the standard 5-pin MAF sensor. This means some applications actually use two MAF sensors.
The principles of how the sensor works haven’t changed. The 8-pin sensor is typically a hot-wire design. As with many other MAF sensors, the ECM monitors the current flow through the sensor to determine how much air is flowing into the engine.
What has changed is the addition of the humidity sensor, BARO sensor, and an additional temperature sensor to measure the air temperature at the humidity sensor.
The higher the air temperature, the less dense the air, so knowing the air temperature as it enters the sensor and then again at the humidity sensor is critical to proper operation.
Humidity has a major impact on the density of the air and the amount of oxygen contained in the air column traveling into the intake manifold. Humidity takes up space, leaving less room for oxygen molecules.
Standard 5-pin MAF sensors are unable to compensate for the water vapor in the air, which leads to rich air fuel ratios when high water vapor conditions are present. The MAF sensor transmits the humidity signal to the ECM using a duty cycle value. The ECM displays the intake air humidity parameter in percent (%) on your scan tool.
Barometric pressure also has a major impact on the amount of oxygen present in the intake air column. Higher elevation means less oxygen will be present in the air column, leading to rich operation. Barometric pressure also affects the amount of water vapor present in the intake air column.
To make this sensor design work, some components within the sensor share ECM circuits.
ECM 5-volt reference is shared with:
- Humidity sensor
- Barometric pressure sensor (BARO)
ECM reference low/ground circuit is shared with:
- Humidity sensor
- Barometric pressure sensor (BARO)
The 8-pin mass airflow sensor (MAF) typically produces an output frequency (Hz) signal, which varies depending on the amount of air flowing through the sensor. The ECM interprets the signal frequency and displays it in grams per second on your scan tool. The higher the sensor frequency, the higher gram per second value displayed (figure 1).
IAT-1 and IAT-2 are standard NTC-type thermistors and operate like other temperature sensors on the vehicle. IAT-1 measures the air temperature at the MAF sensor inlet. IAT-2 measures the air temperature at the humidity sensor. This value should be the same as IAT-1 during a cold start but should vary by several degrees after the engine compartment warms up.
With the key on, engine off, IAT-2 will slowly increase in temperature as the electronics in the sensor warm up. The humidity sensor and IAT-2 share the same circuit. IAT-2 is typically displayed in Hz and temperature on your scan tool (figure 2).
The lower the frequency, the lower the temperature displayed (figure 2). A reading of 20-45 Hz indicates the air temperature is very cold, while a reading of 170 Hz indicates the air is about 95ºF (35ºC) and a reading of 300 Hz indicates the air temperature is about 200ºF (95ºC) (figure 3). Readings below 13 Hz or above 390 Hz will typically set a DTC.
As the humidity of the air changes, the signal from the sensor also changes. The sensor sends a duty cycle signal to the ECM over the same circuit as the IAT-2 frequency signal. The ECM interprets the signal and displays it on the scan tool (figure 4).
ISOLATING SENSOR ISSUES
The ECM has some pretty sophisticated programming algorithms to help locate issues with the sensors The programming includes rationality checks that look at engine speed and APP values to determine engine load. Those values are compared to the MAP and MAF values the ECM is receiving to determine if a sensor problem exists.
There have been a number of problems with the new design sensors. They’ll often fool you as the scan data looks perfect with no DTCs set. At times the scan data indicates a problem, but if you aren’t aware of how the new sensor components operate, you won’t be sure whether what you’re seeing on the scan tool indicates a problem or not.
When in doubt, disconnect the sensor, restart the engine, and see if the problem changes with the system in default mode.
Keep in mind, the MAF sensor is part of an induction package, which includes the complete intake and exhaust system.
- Verify the integrity of the entire air induction system by verifying that none of the listed conditions are present:
- Any loose, damaged, or improperly installed components. Collapsed or cracked hoses, vacuum leaks, or restrictions (intake and exhaust).
- Water intrusion into the MAF
- Any snow or ice buildup (in cold climates)
- Contamination of the MAF sensor element (oil, dirt)
- Improper operation of the PCV system • Air intake modifications
- You can use a signal generator to help verify the MAF and Humidity/ IAT-2 operation.
- Connect the signal generator (Hz and Duty Cycle) to the circuit in question.
- Monitor the circuit operation with your scan tool
- Vary the signal generator output.
If the scan values match the signal control positions on the signal generator, the harness and the ECM are likely in good working order.
Dealing with the symptoms that are created by one of the new design sensors or its circuits can be a bit frustrating. Many times, you’d swear that the problem couldn’t be caused by the sensor, only to learn later that’s just what was happening. Just have some patience and remember the fundamentals.
Until next time, remember: Be grateful for luck, but don’t depend on it.