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DIY: Do It Yourself
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Old 10-09-2008, 03:48 PM   #1
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DIY - Rain Sensor Reattachment Fix

I apologize if this has been posted already but I didn't find anything with my search. I bolded the instructions below because I noticed they would be easier to read if printed.

I purchased a 330Ci several months ago with what was apparently a non-OEM replacement windshield (it didn't have the BMW logo on it and the rain sensor "opening" appeared to have finger prints and bubbles in the adhesive).

I noticed from the time I acquired my car that the windshield wipers didn't work properly in the rain sensing mode and I attributed this to the poor adhesion interface between the rain sensor and windshield.

After browsing this site, the conventional school of thought appears to be that one should buy the OE windshield (with sensor or lens attached from the factory) or deal with a non-working rain sensor function.

After breaking open the plastic shroud around my rear view mirror, I took a look at the sensor and came up with a fix that seems to work just fine.

Some points to clarify first:
  • The purple rain sensor "lens" is adhered to the windshield glass.
  • A sensing element is attached to the purple lens using sliding clips. These clips can be detatched by pressing with a small instrument such as a jeweler's screwdriver. With my sensor detatched from the windshield I was able to release these clips using my fingers but this might be difficult given the angle and position of the sensor as installed.
  • The adhesive used to apply the purple "lens" to the windshield glass appears to be a fairly thick gelatinous tape or a very precisely applied liquid sealant.
  • The fundamental problem with most non-OE replacement windshields appears to be that the purple "lens" is reattached to the windshield imperfectly. I haven't had a windshield replaced myself but the replacement windshield that came with my car appears to have had the sensor/lens assembly reattached with the original adhesive "tape". Some of this "tape" appears to have been destroyed or left on the old windshield during the transfer.
  • The air "bubbles" and finger prints resulting from reuse of the damaged tape probably cause some sort of optical/refractive discontinutiy that confuses the underlying sensor.
  • I restored the optical quality of the windshield/lens interface by removing it from the old windshield, cleaning both surfaces thoroughly, and reattaching the purple lens element with clear silicon sealant.

After accomplishing the above fix, the sensing function of the windshield wipers appears to work perfectly. I've had a number of BMWs in the family and don't notice any difference between the operation of my wipers and those which have not had the sensor disturbed.

Here is the rain sensor installed as I received the car. The fingerprint and air pockets are clearly visible. It's not hard to understand how this could distort the signal received by the sensor laying underneath the adhesive and lens..

Here is the 2 piece plastic shroud attached behind your rear view mirror. I simply pulled it apart at the seam using my hands. There isn't much tricky about it.

After removing the plastic shroud, it may be best to detach the rain sensor from the purple lens (uless your lens is barely attached to the windshield as was the case with mine).
Whether you choose to detatch the sensor from the lens or remove the sensor and lens as one assembly, the process is not difficult.
For either process, you'll have to disconnect the sensor's electrical connector first. The connector is obvious and is located on the upper passenger side of the rain sensor.
Because it was tough to get a good photo of the installed lens/sensor assembly, I've included a picture of me holding the sensor to illustrate the proper orientation. Obviously, this sensor is installed farther up the windshield behind the rear view mirror.

If you wish to remove both pieces together, simply pull/pry the sensor away from the windshield. If this isn't happening easily, I'd suggest detatching the sensor from the purple lens first.
To detach the sensor from the lens, push both sliding lock mechanisms as noted in the image below.

Probably the most important step! Clean the purple lens and windshiled adhesive interfaces. Not only will this ensure proper bonding (probably the lesser concern), but it will promote a more transparent (bubble free) film of silicon between the lens and windshield.

Cleaning the windshield is fairly trivial but ensure that you use a good glass cleaner or alcohol to wipe any residue from the glass after all adhesive remnants are removed.

Clean the sensor using warm water, soap/goof-off, and patience. The adhesive film can mostly be removed by hand but some chemical or warm water will be needed for the last bit of residue.
No pics - sorry.

Apply the silicon adhesive to the purple lens element only. I figured it was better to let the silicon dry without the weight of the sensor attached. Although you might be ok to attach the lens and sensor together, it's easy to snap the sensor into place so I can't see why someone would bother taking the shortcut. In my case I was also about to take a nap between 12 hour night shifts and didn't want to wake up to a failed bonding job with silicon all over my dash.

Here is the silicon I used. I bought it at Advance Auto Parts for less than $4. It seemed to dry at least as clear as the original film/tape. I know some companies (GM) sell a "reattachment kit" for rain sensors but I figured the clear silicon would make an optical interface better than any imperfect application of clear tape. Given the akward angle and position at which the sensor is installed, I thought the silicon left more room for error.

Smear some silicon on the purple "lens" and press the lens onto the windshield. Ensure the orientation of the molded lens matches that of the properly oriented rain sensor. I applied a piece of tape over the lens while it dried. The tape is useless for applying meaningful pressure to the lens but might have saved my dash if the silicon bonding failed before drying (I laid a towel over my dash/console too).

Note: The rain sensor shown below is (obviously) reversed but pay attention to your purple lens element to ensure that it is properly bonded to the windshield. In the event that you screw this up, you won't have enough cable slack to reattach you rain sensor to the improperly oriented lens. Fortunately, the silicon could be removed failry easily and this process repeated if you screw it up.

One tip:
In other applications the conventional wisdom is to apply minimum glue/adhesive necessary to ensure proper bonding. I began my lens installation by applying a modest amount of slicon, spreading it, and sticking the sensor to the windshield. After doing so, I had 2 small air bubbles in the adhesive. I pulled the lens element off and applied some extra silicon and the result was fine (with a slight amount of pressure needed to remove all but the trace bubble I mentioned below).
You don't need to use any significant fraction of the tube for this job but it won't hurt anything to apply more silicon than is "needed" to attach the lens to the windshield. You'll avoid air bubbles this way and it's not difficult to remove the silicon that is forced out from between the windshield/lens mating interface. For reference, the plastic shroud behind the rear view mirror easily concealed the excess silicon visible in my photos below. I saw no need to remove it.

Here is the lens reattached to the windshield with silicon adhesive. This shot is in direct sunlight and was taken before the silicon dried. After drying, the silicon had one VERY small visible air bubble that was probably 0.5mm or less in diameter. Because this bubble looked insignificant in size and was located away from the visible path of the sensing elements beneat the lens I didn't bother to remove and reapply the lens.

I apologize for the photo and writeup quality of this post. I did it in a hurry but I think it makes the point. I could have easily just said "use clear silicon" and I think most people could have managed this process just fine. I was up for 20hr when doing this and didn't have a photographer available. Handling the silicon and using a camera inside the car was a bit tricky. I would have taken my time with better (and more) photos if I were to do this again.

I work in the aero industry and I'm aware of a number of silicones that might work fine in this application. However, given the common availability of this Permatex 66B and the cheap price I'd recommend just sticking with it. My wipers work great now - your mileage may vary. For less than $10 in materials it's worth a shot if you bought a car with non-OE glass and an inoperative rain sensor function.
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WadeDwyer is offline   Reply With Quote
Old 10-09-2008, 04:22 PM   #2
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Great job. Mine has a couple of bubbles but it seems to work like it's supposed to.
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Old 10-09-2008, 11:03 PM   #3
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Just what I am looking for! Thanks for the write up and pix. I'll give this a go this weekend, and post my results.

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Old 10-09-2008, 11:58 PM   #4
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Oh boy, mine never worked since I bought the car used (also non oem windshield). That was 5 years ago! Will try this asap!
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Old 10-12-2008, 12:02 PM   #5
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I bought the silicone and was all ready to do the fix. When I opened the housing to expose the sensor, I saw the sensor was hanging down, not touching the windshield. That explains why it didn't work.

I had my windshield replaced (aftermarket) +1 year ago, and it never worked right. I brought it back to the glass shop 4 times, and they never got the rain sensor right.

Anyway, I clipped the sensor back in place, and tested by spraying a light mist of glass cleaner on the windshield, and the wipers turned on.

I'll take a closer look, but maybe they broke the clips, and the sensor worked itself loose while driving. If thats the case, I'll use some of the silicione I just bought to hold it in place.

Failing that, I'll try the OP's solution.

Thanks again for the post. Your DIY inspired me to open the assembly after 1 year!
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Old 10-15-2008, 02:55 PM   #6
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This is great! Mine stopped working after I had glass replaced.
I tried taking the small glass piece off the windshield, but I managed to crack it all now. Any idea where to buy those replacements? Dealers don't seem to have a clue. Even the glass shops aren't sure. I hope they don't just come with the windshield.
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Old 04-11-2009, 09:57 PM   #7
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Great write-up! Thanks for showing me what's in there, trying to retrofit auto-sensing wipers into a non-premium car. Just fyi, the sensor works by refraction--when the windshield is dry, emitted light (not sure what freq) transmits through the glass, then "bounces" off the outer glass layer when it hits the glass/air boundary, back to the 4 sensors. When there's water on the windshield, the angle of refraction is much less, so most light transmits through the boundary into the water droplets, and doesn't go to the sensors, so it's wipey-time. Anyway, great fix! I wouldn't have guess silicone would have worked, but it obviously did!
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Old 04-18-2010, 08:12 PM   #8
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Hi. Thanks for this DIY. Also can I use a Super Bonder (glue) or some Glue for rear

Hi. Thanks for this DIY. Also can I use a Super Bonder (glue) or some Glue for rear view mirrors? And if I need cover all the part with glue.

Best regards

Last edited by Solidjake; 01-04-2014 at 06:23 PM.
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Old 04-18-2010, 08:29 PM   #9
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let me first say that i am an autoglass technician by trade. with that said, that is very odd that u got the car with the rain sensor looking like that, because pretty much all aftermarket windshields come with the plastic sensor piece already on the glass. whether its OEM bmw, high quality aftermarket, or chinese aftermarket, they all come with the purple piece already mounted. i have seen i think one time where it didnt come on the glass, and i refused to use that one because i didnt have a new sensor pad, and if u try to take the old one off and reuse it, its not going to work (as u already know lol). it was just a hack job done, they should have either waited and got the right windshield, OR a new sensor pad. its cool that u were able to figure out a way to fix it on your own though
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Old 04-20-2010, 09:41 PM   #10
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this is great - mine never worked when i bought my car, going to try this soon. thanks
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Old 01-08-2011, 05:13 PM   #11
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thank you for the DIY, worked great!
I've been driving around without automatic wipers for years now thinking I need to replace the whole windshield to fix. After stumbling onto this DIY, took me 30 min to fix and wipers work perfect.

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Old 01-08-2011, 06:15 PM   #12
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I am a heavy user of silicone. I am surprised it worked in this application. Clear silicon is never really clear and would block out more light than whatever the original adhesive was. If I were to do it, I would probably use glass glue. But this would be a permanent bond and there would be no room for error.
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Old 01-08-2011, 11:04 PM   #13
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Thanks for this...I'll have to bookmark it. My car came with an aftermarket windshield and the sensor purple area looks just like this, although the sensor works fine I always wondered about this and why it looks like crap.

Granted my windshield had about a foot and a half long crack a few inches along the bottom, so I'll just wait to replace it one of these days.

Great do not look bad at all.
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Old 01-20-2011, 12:40 AM   #14
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I haven't used this web site in about a year. Having said that, I'm glad that at least a few people found my information helpful.

Thank you to all who have said thanks!
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Old 03-31-2011, 12:02 AM   #15
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I am going to try RLS Refurbishing Kit available from here: (also available on ebay, search for "Rain and Light Sensor Refurbishing Kit for BMW". Did anybody tried it to glue a prism?
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Old 04-02-2011, 05:28 PM   #16
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Any more info on how to remove the purple piece from the screen? It looks pretty fragile.
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Old 04-02-2011, 07:06 PM   #17
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Well, even if you break it - not a big deal. You can buy a new prism off ebay:

The main thing is not to break your windshield.
Basically, you just pull it off the windshield.
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Old 06-24-2011, 08:19 PM   #18
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I read the reinstall thread. So before i did it, I plugged the rain sensor in,held it against the windshield on a clear spot and it worked perfectly. But after I installed it, it didn't work. There was some RTV silicone that mushed in around the edges of the purple glass, but I don't think it was enough to interefere with it plus it is clear. I think it may have something to do with the other little red dot part of the sensor on the bottom right corner that also got some silicone over it. Anybody have thoughts on this?

Also, I broke the purple lens taking it off again.
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Old 03-27-2012, 12:02 AM   #19
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Nice Write-Up - If you do any google search for this type of issue, your post based on 'attach bmw rain sensor' comes up at least as high as number three. Anyway, followed your suggestions with the exception of attaching the prism to the windshield with the silicone first, then attaching the sensor to the prism. I just attached the prism to the sensor in the first place, put some silicone only on the prism and attached it right to the windshield.

The only issue with doing this method is the windshield has a nice little cutaway for the prism to line it up and when you attach the prism and sensor together, you can't get lined up perfect. But, the silicone allows you to adjust it until it lines up. Give it 24 hours to cure and you're good to go.

Thanks again Wadwyer!
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Old 04-07-2012, 01:47 PM   #20
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Anybody happen to know the part number for the purple lens. I broke it in half when trying to get it off the windshield. much appreciated it thanks
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