Air Flow Meter (AFM) Operation and Testing

Introduction

This article covers the operation of the 944 Air Flow Meter (AFM), also called the Air Flow Sensor, along with testing and repair procedures for erratic operation.

Operation

Two different AFMs were used on the 944, though they are externally similar. Both use a flapper (barn door) arrangement: as airflow increases, it forces the barn door further open. The barn door moves a wiper arm across a variable potentiometer conducting strip. A voltage is applied to the AFM by the DME computer; as the barn door opens, the output voltage from the AFM increases. This output is sent back to the DME as an airflow signal.

Differences between early and late AFMs:

• Early (pre-1985.5): DME supplies unregulated 12 VDC. Output voltage increases exponentially with barn door position but proportionally with actual airflow — the DME receives an effective airflow signal.
• Late (1985.5 and newer): DME supplies regulated 5 VDC. Output voltage increases linearly with barn door position. The DME must interpolate this signal to obtain an airflow value, simplifying the AFM's construction.

Both designs use the ratio of supply voltage to output voltage as the airflow measurement basis.

The primary problem that occurs with AFMs over time is that the wiper arm contacts wear grooves into the potentiometer conducting strip. These grooves cause erratic voltage output, resulting in hesitation or stumbling under acceleration.

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Testing the AFM (Installed)

Note: The AFM can be tested in-car, but it is significantly easier to test with the AFM removed.

AFM Connector Terminal Mapping

Looking into the end of the wiring harness plug (terminals on the AFM itself are reversed from this view):

Plug Terminal	AFM Terminal	Connection
22	1	Air temperature sensor
7	2	Output voltage to DME
9	3	Input voltage from DME
6	4	Ground

Procedure (In-Car)

1. Push back the protection sleeve on the AFM connector, leaving the connector attached. Connect a voltmeter between terminals 9 and 6 on the back of the connector. Turn the ignition to ON.

   • Early 944 (pre-1985.5): expect approximately battery voltage (~12 VDC), minimum 8 VDC.
   • Late 944: expect 5 VDC ± 0.5 V. This is the AFM input voltage.

2. Connect the voltmeter between terminal 7 and ground. Expect 150–250 mV.

3. Slowly push the sensor plate (barn door) to the fully open position using a smooth, firm object inserted through the filter intake opening. The voltage should rise steadily to slightly less than the input voltage measured in step 1, with no drops or erratic fluctuations. Any voltage drop or instability indicates dead spots in the potentiometer conducting strip; wiper arm adjustment (described later) may correct this.

4. Turn the ignition OFF and pull off the AFM plug. Connect an ohmmeter between terminals 6 and 22 on the AFM (both outer male terminals on the sensor plug). With air temperature between 15–30 °C, resistance should be 1.45–3.3 kΩ.

   • An open reading (infinite Ω) causes a rich mixture.
   • A short (near 0 Ω) causes a lean mixture.

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Test Procedure (AFM Removed)

Tools

• Multimeter
• Test leads
• 9 VDC battery
• 9 V battery connector
• 2 small female spade connectors
• Electrical tape

Making Up Test Connections

To test the removed AFM, a 9 VDC battery is used as the voltage source. Attach a 9 V battery connector to the battery and crimp small female spade connectors to the bare wire ends. If the spade connectors are slightly oversized for the AFM connector pins, wrap them with electrical tape to prevent pin-to-pin shorts.

Test

1. With the AFM removed, cut the silicone sealant bead at the bottom edge of the cover where it mates to the body (use a sharp knife or razor blade), then remove the cover.

2. The interior of the AFM:

   Component	Function
   1	Input voltage terminal (from DME)
   2	Ground
   3	Temperature sensor output
   4	Sensor output voltage (to DME)
   5	Potentiometer wiper arm
   6	Potentiometer resistor board
   7	Potentiometer conducting strip

3. Connect the 9 VDC battery negative lead to Terminal 4 on the AFM connector and the positive lead to Terminal 3.

4. Connect the voltmeter negative lead to the ground terminal (2) inside the AFM and the positive lead to the sensor output voltage terminal (4). A small voltage should be present — approximately 0.5 VDC with a fresh 9 V battery.

5. Slowly open the AFM barn door and observe the voltmeter. Voltage should increase steadily with no drops or large fluctuations. At full open, voltage should be slightly less than input voltage.

6. Any voltage drops or large fluctuations indicate dead spots in the potentiometer conducting strip — the wiper arm position adjustment (see AFM Cleaning and Repair below) may correct this.

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AFM Cleaning and Repair

Erratic AFM operation can result from dirty contacts, a dirty conducting strip, or grooves worn into the potentiometer strip. This section covers cleaning and wiper arm repositioning.

1. With the cover removed, clean the potentiometer conducting strip using a pencil eraser. Work carefully and avoid excessive pressure.

2. Clean the contacts on the bottom of the wiper arm using 400-grit sandpaper or emery cloth. The best approach is to disassemble enough to remove the wiper arm. If preferred, the wiper arm can be cleaned in place — insert a sheet of paper between the wiper arm and the conducting strip before inserting the sandpaper, so the strip is protected while cleaning the arm contacts.

3. After cleaning, retest the AFM. If erratic operation is corrected, no further action is needed.

4. If cleaning does not correct the problem and grooves are visible in the potentiometer strip, adjust the wiper arm position: place a piece of paper between the wiper arm and the conducting strip. Then push down on both sides of the back part of the wiper arm to bend it slightly inward. This effectively shortens the arm so its contacts ride on a different (unworn) portion of the conducting strip.

5. Retest the AFM. If erratic operation persists, the AFM should be replaced.