Test 7/Fault Code 21

Test point 7
Speed sensor front left Signal unplausible
Fault code 21

Control unit receives incorrect / unrealistic speed sensor signal.

Procedure
Check speed sensor signal with System Tester 9288 on "Actual values" menu. Then select Speed sub-menu and select the Speed sensor front left option on this menu. Two checks can now be carried out. Check 1 with vehicle raised. (Check for accidental interchanging of speed sensor and check of speed sensor signal quality). Check 2 during straight-ahead driving at approx. 2-4 km/h. (Signal quality of individual wheels is compared). Check 2 provides better conclusions on signal quality than Check 1.

Re: Check 1
To check, manually rotate left front wheel steadily at approx. 2 to 3km/h (observe tester display). Increase speed slowly and observe speed increase (display) at the same time.

specifications/specifications display.
Speed increments of approx. 0.06 km/h. Initial display at 1.75 km/h. This means: The subsequent value must exceed the value measured last by 0.06 km/h or must be 0.06 km/h below this value when the wheel is rotated slower. In some cases, the tester rounds the value down to 0.05 km/h or up to 0.07 km/h.

Example
First measured value - 1.81 km/h
Second value as specified - 1,87km/h
Third specified value - 1,93km/h
etc.

Further to test 2
Display all four wheels in the Jester display by additionally pressing Tester key 1 (function key 1). Drive straight ahead at a uniform speed of approx. 24 km/h, and have a second person observe the Tester display. Required display: Deviation of the four wheel speeds max. 1 km/h. Further details about test 1 and test 2 contained in Actual values menu. Actual Values (ABS and ABS/ABD)

Possible faults (cause in the event of deviation)

- Air gap between speed sensor and gear wheel (pulse wheel) too large or, due to wear (chip formation), too small (check installation).

- Pulse wheel defective or corroded.

- Wrong pulse wheel (should have 48 teeth).

- Speed sensors for front axle/rear axle interchanged* As a result, the edge position of the speed sensor is offset by 90° with respect to the pulse wheel* Distinguishing characteristic: Identification on the speed sensor wire by means of added spare part No. and position of the fastening bore with respect to the edge position.*

- Wheel bearing damage (wheel bearing not adjustable).

- Plug connection in wiring from the speed sensor to the control module or PIN on the control module connector not OK.

* Applies only to speed sensors installed before 24.11.95.