Wheel speed sensor transmits implausible signals or no signals

The control unit for traction control (CU) monitors the signals of the wheel speed sensors (RDF) to ensure they are present and plausible.

Possible cause(s) of fault:

• Electromagnetic irradiation
  
• Wheel blocks briefly on driving off (e.g. brake disc frozen solid when driving off on ice)
  
• Changed air gap between RDF head and impulse wheel
  
• Sensor head contaminated by metal filings
  
• Moisture has entered RDF
  
• Cable defective
  
• RDF defective
  
• CU defective
  

Storage condition(s):

• Implausible speed signal
  
• Missing speed signal
  

Troubleshooting procedure:

• Visual inspection (cables and fitting): Check RDF and supply lines for correct installation and damage.
  
• Visual inspection (sensor head): Remove RDF and check whether the sensor head is damaged or soiled.
  
• Check of wiring: Check the supply and signal wires between RDF and CU.
  

The following checks only make sense if there are faults:

Caution: If there is a break in wiring to an RDF, the CU interrupts the power supply to the corresponding RDF. Prior to a check of the signal or voltage supply, there must therefore be a check of the electric wires.

• Check of the signal voltage:
  Signal voltage OK: continue troubleshooting
  Signal voltage not OK: replace RDF
  
• Check of the voltage supply:
  Voltage supply OK: replace RDF
  Voltage supply not OK: continue troubleshooting
  
• Check of the internal resistance in the CU:
  Internal resistance OK: replace RDF
  Internal resistance not OK: Replace CU
  

Note

• The CU detects a disconnected RDF and interrupts the power supply
  
• CU replacement, see Service functions