Air Flow Meter (AFM) Operation and Testing

Overview

This guide explains how the Air Flow Meter (AFM), also called the Air Flow Sensor, works in your Porsche 944 S / S2 with its 16v engine. The AFM measures the air entering the engine, sending this info to the DME computer to control fuel delivery. We’ll also walk you through testing it and fixing common issues like hesitation during acceleration, which can happen when the AFM’s internal parts wear out. Keeping it in good shape is key to smooth engine performance.

Pink and blue text represents links to additional resources, tools, parts, and partner websites.

Tools Required

• Multimeter (voltmeter and ohmmeter functions)
• Test leads for multimeter
• 9 VDC battery
• 9 V battery connector with leads
• Small female spade connectors (2)
• Electrical tape
• Sharp knife or razor blade
• Pencil eraser
• Emery cloth or 400-grit sandpaper
• Sheet of paper (for cleaning)
• Smooth, soft item (e.g., plastic tool) to push barn door

Step-by-Step Procedure

Step 1: Understand AFM Operation

1. The AFM in your Porsche 944 S / S2 uses a “barn door” that opens as air flows in, moving a wiper arm across a potentiometer (a variable resistor). The DME computer sends a steady 5 VDC to the AFM, and the AFM sends back a voltage that rises smoothly as the door opens. This tells the DME how much air is entering the engine.
2. Wear over time can create grooves in the potentiometer, causing erratic signals and poor engine response. This guide helps you test and fix that.

Step 2: Test the AFM (Installed in Car)

3. Locate the AFM connector near the air intake. Push back the protective sleeve but keep the connector plugged in.
4. Connect your multimeter’s positive lead to terminal 9 (input voltage from DME) and negative lead to terminal 6 (ground) on the back of the connector. Turn the ignition to ON (don’t start the engine). You should see 5 VDC ± 0.5 VDC. This checks the DME’s supply voltage.
5. Move the positive lead to terminal 7 (output voltage to DME) and keep the negative on ground. With the barn door closed, expect 150–250 mV.
6. Insert a smooth, soft tool through the air filter intake to slowly open the barn door fully. Watch the voltage rise to just under 5 VDC (e.g., 4.5–4.8 VDC). It should climb steadily without drops or jumps. If it fluctuates, the potentiometer may have dead spots.
7. Turn the ignition off and unplug the AFM connector. Use an ohmmeter between terminals 6 and 22 (outer terminals) on the AFM plug to test the air temp sensor. At 15–30°C (59–86°F), expect 1.45–3.3 k-ohms. An open circuit (infinite ohms) means a rich mix; a short (0 ohms) means a lean mix.

Step 3: Test the AFM (Removed from Car)

8. Remove the AFM from the car. Follow this link to ensure this step – Air Flow Meter Removal – of the process is followed accurately.
9. Cut the silicone sealant around the AFM cover’s bottom edge with a sharp knife or razor blade. Lift off the cover to access the internals.

10. This image shows the AFM’s internals: 1) Input Voltage Terminal, 2) Ground, 3) Temp Sensor Output, 4) Sensor Output Voltage, 5) Wiper Arm, 6) Resistor Board, 7) Conducting Strip.

11. Attach a 9 VDC battery’s negative lead to AFM terminal 4 (ground) and positive lead to terminal 3 (input voltage). Use spade connectors wrapped with electrical tape if they’re loose.

12. Connect the multimeter’s negative lead to the internal ground terminal (2) and positive lead to the sensor output terminal (4). With the barn door closed, expect around 0.5 VDC (e.g., 0.463 VDC with a 9.41 V battery).

13. Slowly open the barn door fully. Voltage should rise smoothly to just below the battery voltage (e.g., 8.45 VDC with a 9.41 V input). Drops or jumps mean worn spots on the potentiometer.

Step 4: Clean and Repair the AFM

14. With the cover off, gently rub the potentiometer conducting strip (7) with a pencil eraser to clean it. Don’t press too hard.
15. Clean the wiper arm (5) contacts with emery cloth or 400-grit sandpaper. Slide a sheet of paper between the wiper and strip, then sand the contacts through the paper to protect the strip.
16. This image shows the wiper arm and grooves. If grooves are deep, cleaning may not help.

17. Test the AFM again (repeat Step 3). If erratic voltage persists and grooves are visible, adjust the wiper arm.
18. Slide paper under the wiper arm. Press down on both sides of the wiper’s back end to bend it slightly, moving the contacts to an unworn part of the strip.
19. Retest the AFM. If it still fluctuates, replace it with a new AFM (944 606 123 00).

Post Procedure Checklist

• Reinstall the AFM and ensure all connections are secure.
• Start the engine and check for smooth idle and acceleration.
• Listen for odd noises from the AFM indicating a loose barn door.
• Verify no check engine light appears related to air flow.

Sources

• Clark’s Garage
• Pelican Parts
• Porsche 944 Workshop Manuals
• Youtube