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Know How – Expert Edition – ABS Dropout

11 Aug 2021


With Tim Stock

Tim is well-known in the automotive aftermarket for his wealth of experience and expertise in all things repair-related. In this Expert Edition article, he will be recounting a recent customer issue brought to the NAPA technical line concerning a cyclic drop in a vehicle’s Anti-Lock Braking System (ABS) signal and how it can be rectified. To find out more about NAPA braking products, contact the NAPA braking helpline at 0117 428 8090.


The ABS may be tremendously helpful for giving you more control over a vehicle’s handling and braking, but – as with any vehicle system – there is always a chance of an error that seems more difficult to diagnose than it is. In a recent case, a cyclic drop in the ABS signal gave an experienced installer some pause. Fortunately, NAPA Know How was able to find a solution to the problem.


In this particular instance, the ABS error originated in the rear speed sensors, though any speed sensor could be responsible for signalling the fault. A survey of the rear sensor signals revealed a cyclic signal drop every 44 teeth in the right-hand rear sensor with a “tone ring missing tooth” error message. Upon attempting a traditional mechanical repair, however, the installer encountered two key obstacles:

  • The tone ring was enclosed, making visual inspection impossible.
  • Replacing the driveshaft did not resolve the signal dropout.


When dealing with this type of error, a close inspection of the bad sensor trace will likely reveal a disparity between its duty and the duty of the correct sensor trace. If this is the case, take the following steps:

  1. Swap the sensor producing a bad trace to the loom of the sensor producing the correct trace.
  2. If the bad trace transfers with the swapped sensor, you have found the fault – replace the sensor with a new one.
  3. Finally, perform a signal reading (and possibly a road test) to ensure the replacement fixed the issue.

In our case study, the ABS module was detecting the signal error and resetting the ground every 44 teeth. The routine of the dropout suggested that a module would have been responsible. Approaching ABS faults from this perspective can speed up diagnosis.


Passive sensors generate their own voltage to produce an alternating current, while the more common active sensors switch a direct voltage. In diagnosing either sensor type, it is essential to understand modern oscilloscopes.

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