Test Point 6

DTC 21

Front Left Wheel Sensor

No signal from speed sensor reaches control unit, or signal is incorrect / unrealistic.

Important: This fault is also indicated if the PDAS/ABS control unit for the 911 Carrera 4 (964) is installed.

If assembly work has already taken place, check that the correct control unit has been installed.

Distinguishing feature: part number.





928 control unit with PSD = 928 618
911 Carrera 4 control unit = 964 618





1. Pull off the control unit plug. Measure the internal resistance / check continuity between PIN 50 and PIN 48 on the plug.

Nominal value 600...1600 Ohm. If the nominal value is not attained, check the wires and plug connections in the circuit from the front left speed sensor.

If the nominal value (600...1600 Ohm) is not attained although the wires / plug connections are in working order, replace the speed sensor.

Important: Perform measurement directly on the speed sensor before replacing the sensor.

2. Checking the speed sensor signal.





For this check, use the ABS 2-LED tester or an oscilloscope. Connect the ABS 2-LED tester to the control unit plug with the 55-pole adapter lead.

If an oscilloscope is used, separate the speed sensor plug connection at the wheel suspension or on the spring strut dome in the engine compartment. For distinguishing features, Description of On Board Diagnostics

Connect the oscilloscope to the plug at the speed sensor side. Depending on the tester, select special input or secondary screen.

Check by turning the left front wheel by hand. ABS tester in program switch position 6 / rotary switch for wheel selection to front-left wheel. Turn the wheel until the LED (No. 2) lights up without flicking.

If an oscilloscope is used, turn approx. 1 revolution per second.

Nominal values / nominal display

ABS 2-LED tester = > 2.0
Oscilloscope = sine wave > 2 volt
(measured value: peak to peak)

Possible causes of deviations:

- Air gap at speed sensor too large / too small (check installation).

- Pulse wheel damaged or corroded.

- Wheel bearing play too large.