Before I get into it, I'm not sure how great these rain sensors are to begin with, but mine wasn't working at all when I got the car.
While it was connected, it thought it was raining all the time because the wipers just kept going.
So I did some research and found out that rain sensors work based on reflected near-infrared light that's generated by two IR LEDs.
I pulled out my sensor from the windshield and looked at the two LEDs through my cell phone camera. (to make sure the sensor is on, I placed the key in the first position that allowed me to turn on the wipers (position 2).
On the camera I saw that one LED was dim and the other one almost dead.
I removed them from the sensor board and noticed they were flat at the top. I looked online for the closest match.
Found Optek OP266W and OP165W which are near-IR, and are flat at the top. They're smaller than the original LEDs in the MB rain sensor. Original: 4.75mm diameter, while OP266W is 3.05mm. I used OP266W. Costs less than a dollar at mouser dot com or digikey dot com.
Long story even longer, I replaced the two LEDs paying attention to polarity, and once I started the car, I turned the wiper switch from 0 to I.
It wiped a few times, then it started wiping intermittently.
I switched it to II, then to III, then back to 0 and then back to I. (I read somewhere that these steps allow it to recalibrate itself)
I switched it back to I. Wiped twice and then stopped. I turned it back to 0.
Luckily last night it rained, so I was able to test it.
It's working. It's not fabulous, in the sense that it doesn't wipe *exactly* when I want it to, but it does "read" the difference between light and heavy rain and adjusts the wiping interval accordingly.
No matter how hard I looked I couldn't find an exact match for the original LEDs but so far I'm happy with the ones I found.
If you want to attempt this repair on your own and need some more direction, give me a shout.
Have a great day,
Tudor

nice work!

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'77 240D, 504H, OM617.952, etc.

Howdy, I have one that doesn't work at all. I will look at what you mentioned in your post but I was wondering if you had any pics of the sensors. I was a little worried about the polarity portion but I can work that out once I figure out which is pos and neg. Any help would be great! Thanks for your time.
99 SL500

Very cool! I was not even aware that rain sensors were an option on this car. I too would appreciate a picture.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

What do you mean by "doesn't work at all"?

Mine had the following symptoms:

- with the sensor connected, the wipers would work continuously on position I, just like in position II.

- With the sensor unplugged, the wiper would work intermittently on position I (a slight improvement over having the defective sensor plugged in)


I'll try to make some time later today to remove the sensor and take some pictures.

Basically if you want to see if it works, you can do this:

- remove plastic cover from behind rear view mirror. Sensor is underneath, attached to the window.
- you'll see two plastic pieces that slide outward, one on each side of the sensor. You can pry them out with a flat screwdriver. You don't have to remove them -they'll slide out a few millimetres.
- the sensor should come out easily now.
- while it's still connected, turn the key to position I (wipers should work in this position without turning on the engine or switching ignition on)
- aim a cell phone camera at the sensor. You should see two IR LEDs emitting a faint light. I'll try to take a picture of that as well.
- If the light is bright enough (compare it against an IR remote), then your sensor has a different problem.
- while you're at it, you might want to try and disconnect the sensor (there's a three-pin plug that you can easily remove) and see if your wipers behave differently (as I described above).
- The two IR LEDs (if deemed defective) are easily replaceable if you have some soldering skills; I'll help with more instructions if needed.

Tudor

Great job, looking forward to the pictures. Now I have to find out if my car has the option.

__________________

'98 E300 DT
'87 190D 2.5 turbo RIP
'92 S500 Sold
'03 3.2TL-S A-Spec

My '99 has the option and it works well. Ocasionally it runs continuously till I clean the glass over the sensor, simply give it a good wipe to get road grunge off.

__________________
1999 Mercedes E300TD daily driver sold at 238K miles 106K miles were mine, rust worm got it :-(
2006 Mercedes CDI new daily driver! 56,000 miles May 2016 now 85,625 Apr 2018 and Apr 2019 101,000 miles Apr 2020 109,875. March 2024 135,250

Nice detective work, Tudor.

Pics would make this even more useful.

Either way, the details make it good for English-speakers, and the sources for the leds is great.

Two thumbs up!

THe sensor is located right behind the rear-view mirror:
https://www.dropbox.com/s/boq28rp469ry56q/E300D_Rain_Sensor_Before_Removal.jpg
Turn the mirror toward the passenger side and pull out the large round plastic cover.
The sensor has two tabs on each side (right and left). You should be able to pull them out. They're not quite visible in all picture, but some of them should give you an idea:
Once the two plastic tabs are out (they don't come out completely; I think they slide about 3 - 4mm, not more)

http://www.dropbox.com/s/pcskksfpsj43tg5/E300D_Rain_Sensor_Before_Removed_tabs_Out.jpg

https://www.dropbox.com/s/wcoyko7txp40tg2/E300D_Rain_Sensor_Removed_Tabs_Out_01.jpg

Once the tabs are out, you should be able to detach the sensor from the windshield. It'll look like part of it remained glued to the glass. Leave it there.

Turn the key to the first position where the wipers can be turned on. Aim camera at the sensor. You should see this:

https://www.dropbox.com/s/r7rgp6vjb27dw6z/E300D_Rain_Sensor_On.jpg

Turn it off: you should see this:
https://www.dropbox.com/s/jorbg4ag4s53ujc/E300D_Rain_Sensor_Connected_Off.jpg

If your LEDs are bad (which after 14 - 15 years they might be), you'll see a very dim light or no light at all coming out from the IR LEDs. Mine were VERY dim, so the only way I could see them work was by switching the ignition on and off.

Ok, I'll stop here with the write-up because I didn't get a chance to disassemble the sensor again. I'll do it tomorrow and follow up with more pics. Sorry about that.

Basically it's straight forward from here: you need to remove the top plastic cover, unsolder the LEDs and solder the new ones back in place. I gave you the part number in the original post.
If needed, I'll take it further apart and post more pictures.

Tudor