Rain and Light Sensor Fault Diagnosis

There are a couple of tests you can perform to check that your sensor unit is performing properly. This is useful to do before removing a windscreen so you can make changed before re-installing, or to simply check whether the issue is just a sensor pad needing to be updated - as we've said in other posts they can age, harden and discolour all of which affect the unit performance. 

We've written this short guide to walk through tests for the rain and light sensors. 

For the Rain sensor: Ensure that the vehicle’s hand brake is on and it is in neutral, or Park (P) position for an automatic transmission. Turn the vehicle on and leave the engine at idle speed and ensure that all doors and windows are closed and make sure the wiper switch is in the ‘Automatic / On’ position. Spray water onto the windshield at the area where the sensor is located and check that the wipers operate automatically.

For the Light sensor: Ensure that the vehicle hand brake is on and it is in neutral, or Park (P) position for an automatic transmission. Turn the vehicle on and leave the engine at idle speed and ensure that all doors and windows are closed and make sure the wiper switch is in the ‘Automatic / On’ position. Block out any light by placing your hand or a dark piece of paper over the area of the windscreen where the sensor aperture is located and check that the lights switch on automatically.

If either or both of the above tests fail, then it is likely that the sensor or control unit is faulty, or that it needs to be re-calibrated. This should be checked and rectified by an authorized OEM vehicle dealer, before the windscreen is replaced.

Useful tips on removal and installation of all sensor units:

The rain sensor on the Mercedes or Jaguar for example, will only activate the wipers when all doors are closed – a safety/courtesy feature which stops water from being 'swept' off the screen into the front of the cabin when the wipers are operating.

In fact, the rain sensor will only work when all the doors and windows are closed and the engine is running. Also before removing the vehicle windscreen and disconnecting the sensor unit, it is important to ensure that both the light and wiper switches are in the ‘OFF’ position so that neither the wipers nor lights can automatically function by mistake.

Electric windows should also remain closed until the sensor unit is re-connected.

It is also important not to turn on the ignition switch or the radio during the time the sensor is disconnected, as the electronic system will measure differences in voltage and may change the calibration settings of the sensor unit. This may lead to a malfunction of the sensor unit when re-connected and a possible failure to operate either the lights and/or wipers correctly.

The only way to rectify such an error would be to take the vehicle to an authorized dealer with the appropriate equipment to re-calibrate the system.