VitoCorleone08
New Member
Had a persistent ABS/ESP light fault recently. Scan tool showed RPM sensor open circuit. Turned out the wheel bearing had collapsed and pushed the inner grease seal/magnetic ring out the back of the knuckle and decapitated the sensor 
Managed to replace the bearing, which was a headache. Fitted an aftermarket RPM sensor and cleared the fault code, only to get a new code saying RPM sensor air gap too large.
Knowing there is no adjustment to the sensor in the knuckle, and despite all the online advice to only use a genuine Mercedes sensor, I decided to have a crack at getting my much cheaper aftermarket one to work
For future googlers, the fix process was as follows:
On the ‘bolt side’ of the sensor, just below the mounting tab that sits on the top of the knuckle when it’s installed, take a small flat file and file a flat edge into the plastic casing of the round part of the sensor…I filed it until it was almost flush with the thinner flat part that extends into the knuckle. This will allow a couple of mm of movement towards the wheel bearing when it’s all in situ. Once that’s done, use a round file, or a milling bit in a drill to elongate the bolt hole of the sensor mounting tab in towards the body of the sensor by a couple of mm, this will allow for the movement in the direction of the bearing and allow it to still be bolted down. If you have access to a scan tool that displays live data, select the ABS sensor readings and it should give a wheel speed when the wheel is rotated…spin it by hand and check the wheel speed is displayed. It should also show an air gap test result…adjust the newly adjustable RPM sensor in towards the bearing until it reads a consistent ‘OK’. Once it does, snug the mounting bolt up tight to lock it in place, clear the DTC’s and you’re good to go
Managed to replace the bearing, which was a headache. Fitted an aftermarket RPM sensor and cleared the fault code, only to get a new code saying RPM sensor air gap too large.
Knowing there is no adjustment to the sensor in the knuckle, and despite all the online advice to only use a genuine Mercedes sensor, I decided to have a crack at getting my much cheaper aftermarket one to work
For future googlers, the fix process was as follows:
On the ‘bolt side’ of the sensor, just below the mounting tab that sits on the top of the knuckle when it’s installed, take a small flat file and file a flat edge into the plastic casing of the round part of the sensor…I filed it until it was almost flush with the thinner flat part that extends into the knuckle. This will allow a couple of mm of movement towards the wheel bearing when it’s all in situ. Once that’s done, use a round file, or a milling bit in a drill to elongate the bolt hole of the sensor mounting tab in towards the body of the sensor by a couple of mm, this will allow for the movement in the direction of the bearing and allow it to still be bolted down. If you have access to a scan tool that displays live data, select the ABS sensor readings and it should give a wheel speed when the wheel is rotated…spin it by hand and check the wheel speed is displayed. It should also show an air gap test result…adjust the newly adjustable RPM sensor in towards the bearing until it reads a consistent ‘OK’. Once it does, snug the mounting bolt up tight to lock it in place, clear the DTC’s and you’re good to go
