Measuring a hot wire mass air flow (MAF) sensor with frequency output

Sensor information

Type: Hot wire with frequency output
Power supply: 12 V
Signal type: Frequency varying
Signal level: 0.2 V to 5 V or 12 V (battery voltage)

Workings of the hot wire mass air flow sensor with frequency output

The hot wire air intake sensor with frequency output measures the amount of air going into the engine in the same way as the normal hot wire mass air flow sensor. The difference between the sensors is found in the output signal: the sensor in this article has a frequency changing output. The frequency changes according to the amount of air passing the sensor. A low frequency corresponds to a low air flow, a high frequency corresponds to a high air flow.

Connecting the lab scope

Correct functioning of the hot wire air mass sensor sensor with frequency output can be checked by measuring the following signal voltages, see figure 1:

Channel Probe Voltage Range
1 Red probe Sensor output signal 20 V
Black probe Ground on battery
2 Yellow probe Positive side of sensor power supply 20 V
Black probe Negative side of sensor power supply
Measuring diagram
Figure 1: Measuring diagram
Measuring on a working hot wire sensor with frequency output
Figure 2: Measuring on a working hot wire sensor with frequency output

The lab scope is connected to the hot wire mass air flow sensor with frequency output via a Measure lead TP-C1812B and Back Probe TP-BP85 and is set to recorder mode.

In recorder mode a streaming measurement is performed, continuously displaying the signals live on screen. Because the measured signals vary slowly, the Automotive scope ATS5004D is set to a slow measuring speed.

Measuring

Figure 3 shows a waveform of a hot wire mass air flow sensor sensor with frequency output. The signals are measured under the following conditions: key on, cranking, idle, 2000 RPM, 3000 RPM and back to idle again with the engine at operating temperature. This signal can be downloaded and used to correctly set up the lab scope or as reference signal.

Download hot wire sensor with frequency output measurement

Lab scope measurement of hot wire mass air flow sensor with frequency output

Figure 3: Lab scope measurement of hot wire mass air flow sensor with frequency output

In figure 3, the sensor output signal on channel 1 is converted to a frequency signal (purple signal) by using an RPM I/O and channel 2 (yellow) shows the power supply of the hot wire mass air flow sensor. After the measurement is started, the key is switched on which powers the sensor. While the sensor is warming up the hot wire, the frequency momentarily spikes. When the engine is started a drop in the power supply is visible along the individual intake strokes in the frequency signal. When the engine has started and stabilized to idle speed the frequency is stable around 3660 Hz. The throttle is pushed to reach 2000 RPM and 3000 PRM after which the throttle is released to let the engine reach its idling RPM again. With the raising and falling air flow the frequency changes accordingly which is visible in the frequency signal.

Diagnosis

Signal values may differ on different types of engine control units and hot wire mass air flow sensors with frequency output. Consult ATIS for information on specific engine control units and hot wire mass air flow sensors with frequency output.

The following signal deviations can indicate a problem:

  • No signal:
    Cause: back probes have no connection (perform a connection test), poor or no power supply to sensor, sensor defective

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Disclaimer

This document is subject to changes without notification. All rights reserved.

The information in this application note is carefully checked and is considered to be reliable, however TiePie engineering assumes no responsibility for any inaccuracies.

Safety warning:

  • Before measuring, check that sources of dangerously high voltages are switched off or shielded from contact. Voltages considered to be dangerous are voltages over 30 V AC RMS, 42 V AC peak or 60 V DC.
  • Keep a clean working environment when doing measurements.
  • This measurement and procedures are a examples / measuring suggestions and are no prescribed protocols.
  • TiePie engineering can not anticipate the safety actions that need to be taken to protect persons and appliances. Before starting a measurement, check which safety measures need to be applied.