Rain Sensor not working on E39
 

I have a 2000 BMW 528i E39 with rain sensing wipers. When I turn the wipers on in rain sensing mode the wipers intermittently wipe every 8 seconds weather it is raining or not. I replaced the relay but this did not help. They had been working perfectly so I know it is not user error. The windshield has not been replaced nor have any modifications been made to the vehicle since it has quit working. Has anyone seen this issue?

The rain sensor is a led and photo cell that picks up a reflection off of the windshield, its located right at the rear-view mirror mount. If anything like dirt, stickers, cracks in the glass happen there, it can wreak havoc with the sensor. Did anything change between working and not working? new stereo install? other accessory?

No modifications
 

Thanks for the reply. No modifications have been made to the car. I checked the sensor and windshield and I can't see any noticeable damage. Didn't know if there was a fuse or relay I didn't know about.

I am having similar problems with my '02 E39.

The wipe every 8 seconds is the failsafe mode for the intermittent wipers -- my car will do this even if I completely DISCONNECT the rain sensor. AFAIK, a bad relay would keep the wipers from working at all.

My rain sensor works for a short while, maybe a few minutes, before going into failsafe. You can tell if the rain sensor is still alive by adjusting the sensitivity on the wiper stalk with the wipers in the intermittent/auto position: Any time you click to a new sensitivity step, the wipers should wipe once. If they don't, and the sensitivity knob doesn't do anything, then the rain sensor is off-line. I have to turn the car off and back-on or unplug and replug the rain sensor for it to start working again, but then it only works for a few minutes.

If your rain sensor works at all (e.g. the sensitivity adjustment does something or it responds to rain, even if only initially), then the sensor may be good. Try making sure the glass is very clean and free of coatings. RainX supposedly causes problems, but I had RainX on my windshields for years without any issues with the rain sensor. My problem seems to be getting worse -- the sensor used to work for an entire trip about half the time, now it always goes dead within a few minutes.

In my case, I suspect a failing sensor or slow age/sun degradation of the plastic lens, adhesive, or other components that keeps the sensor from reliably detecting the difference between a clear windshield and rain. From there, the software probably forces it in to failsafe.

FWIW, the rain sensor is a complex little gizmo, with an array of 4 or 5 reflective photo gates and a microcontroller. It sits on the K-bus with the light control module, your IKR (hvac), windows, seats, and all that stuff. It has its own diagnostics and probably reports its own fault codes if you want to pay somebody to plug your car in and read them...

I might try picking one up on e-bay to see if that helps. Replacement is an easy 2-3 minute job. Google that or post back here if you need instructions.

I have read that installing a used rain sensor requires reprogramming by BMW. Can anyone confirm if this is true?

Replacing a module - definitely true

Replacing a sensor - not so much. Usually just requires clearing any trouble codes if any were even generated.

Not sure, but I don't think you need to reprogram for a same-unit swap out. I seem to have stumbled across others on-line doing the swap-out for failed modules w/o problems. You DO need a reprogramming if removing the rain sensor altogether to get normal interval wiping back. You probably also need to reprogram if adding a rain sensor to a vehicle that was not originally equipped with one. Basically, whatever module is controlling the wipers needs to know if it's there or not.

As a compooter enginear that's tinkered with interfacing to the BMW's electronics, I doubt that they'd need to plug it in to DIS to re-code a swapout. The module is stamped with a firmware rev, so that comes with the module. I am speculating though. I know reprogramming is needed for the IKE (instrument cluster) and LCM (light control module), and DME replacement, but that's cuz those devices also back-up the mileage and store the VIN.

Let me know how it goes. :laugh