Auto Wipers

[njc021073]

My Audi A3 has the automatic wipers on it, with a control to set the sensitivity of the sensor in intermittent mode.

Ever since having the windscreen replaced by Audi to cure a leak when the car was a couple of month old, the auto wipers have not been functioning correctly.

At them minimum sensitivity setting the wipers never wipe, and at the maximum setting, the wipers only wipe when the screen has been completely obscured by rain.

I have had it checked out by two different garages, who say there is nothing wrong (after squirting it with a high pressure hose ).

I know from comparing it with other VAG cars I have driven (and my own Passat) that there is deffinately something wrong.

Is there anyway of changing the sensitivity of the sensor in VAGCOM?

Audi don't seem to want to help. The most annoying thing is that before the windscreen change, the system was really good!

[Chris_B]

Is the new screen an OEM Audi one, or 3rd party? Does it have a shade band at the top? AFAIK, the sensor is in the mount for the rear-view mirror; does it look unobscured from the front? Can you compare your windscreen to a stock one on another A3 at the dealer?

You might just have a windscreen with different infrared light transmission characterstics.

[njc021073]

It was an OEM fitted by Audi. It has the black banding in the correct place for the rear view mirror to stick onto. And looking from outside, the sensor is not obscured in any way.

The car was only 2 months old when they changed the screen, and it worked really well before. It does still work, but the sensitivity is way too low on all settings.

I downloaded the bosch blurb on their sensor (asuming VAG used Bosch) and it seems to work off the principle that if there is no reflection from sender to reciever there is no wipe, but if there is reflection (beam reflected by water droplet on outside of window) it sends a signal to the computer, which then works out the wipe.

I am sure that it is the correct screen.

[Chris_B]

Hmm, perhaps they buggerd the sensor up somehow refitting it? Got dirt in it or something? Perhaps they got the bonding glue in it when sticking the rearview back on?

I think the other make to Bosch is TRW or somethig like that, but I think they mainly service the US automakers, so Bosch kit seems likely.

[Vic]

Neil

The sensor works in the opposite way to how you thought!

Right, lets get technical.

There is a transmitter and a receiver optical sensor angled such that with a dry screen, there is maximum light reflection off the inside of the front surface of the windscreen. At this point, the receiver gets a 'maximum' light input level. Lets call this the datum

If a droplet of water appears on the surface of the glass in the sensor window it 'optically fuses' with the glass and causes some of the light that would normally be reflected to the receiver to be dissipated in other directions and hence the signal level to the receiver drops. The amount of signal received at the receiver is inversely proportional to the amount of water on the windscreen so, the lower the received signal, the faster the wipers go!

Now the manufacturers must assume that no two installations of it's sensors are ever going to be exactly the same. Factors like the closeness and optical clarity of fit of the sensor to the screen, (angle etc), glass thickness, laminate layers, optical clarity of the screen and the reflective property of the inside surface of the glass are all going to affect the maximum reflected signal (The Datum). In your case Neil, with the replacement screen, it is clear that the receiver in your sensor is now getting a higher level of signal than before so it is taking more water on the screen to drop this level and cause the wipers to activate

That's the theory. Now my assumption...

To take into account the above factors, there must somewhere be the facility to calibrate the rain sensor and set the datum level when the screen is clean and dry. This must be being done at the factory and as yet the dealers are not being made aware of it. It will be something like the throttle body calibration in VAG-COM.

If anyone knows a way to find out more, please let us know.

Vic

[njc021073]

Spoke to Audi again yesterday when I picked up my car, and they said that there is no setting in VAGCOM to adjust sensitivity, it is all done onboard in the sensor itself.

I agree with you vic, as the output from the sensor feeds some kind of algorithm in the management system somewhere, there should be an adjustment possible,

Anyone, know