C280 Heated rear window switch won't go out

[MarkBc280]

Hi there everyone i am recently a new and very proud owner of a silver w202 C280 sport. (pictures to follow) once i pick her up on Saturday!

one slight niggle that is bothering me is the heated rear window switch light does not seem to want to go off. if you press the button quickly it goes off eventually then comes on again and stays on when ignition is turned off! quite annoying and wondering if this could lead to a flat battery?!

i have search the forum and cannot see any reference to this particular issue (only suggestions for when heated window does not work and suggesting cleaning or swapping out the relay) i must admit i could not tell in warm afternoon if the screen was heating up - and i guess during the excitement of buying the car and looking for the more important pitfalls i did not spend too much time dwelling on it.

I am thinking it could possibly be the relay or the switch and if it's the switch hoping it's not the control unit behind the console (as i believe it's a complete unit?, is this right?

just wondering if anyone else has experienced this issue? and would be very grateful for any suggestions or pointers?

beside this niggle i will be very happy to be finally behind the wheel of one of these and looking forward to restoring to former glory and keeping it going for hopefully a further 20 years.

Cheers everyone

Mark

 

[Dec]

What year is it Mark?

Dec

 

[MarkBc280]

Hi Dec, sorry i should have said it's an R Plate - says date first used/registered 01/01/1998 so must have been made later 97'

Cheers

Mark

 

[MarkBc280]

sorry should have put more info as i guess a car could end up on R-plate and been made a lot earlier. It's a facelift W202 can send a pic once ive set up a URL

thanks

Mark

 

[Dec]

Put a cup of boiling water under the back window to steam it up and let the switch stay on with the key out of ignition to see if the window de-fogs.
Yours like below picture?

Dec

 

[MarkBc280]

Hi Dec, yes that's the same layout as mine - ok i will try this saturday, good idea! thank you at least i'll know if it's working. I can just envisage a flat battery, or having to remove the fuse every time the car is not being used if that light stays on. Do you think a faulty relay could be causing the light to stay on? hoping i don't have to remove the panel and replace a climate unit for one bad switch or connection. but i guess that's cars for you

 

[Dec]

Test with the hot water when the car is cold, it will steam up better with cold glass.
When working properly, the switch light stays on, it should then switch off automatically after 15 minutes.

Sounds like a faulty heater control unit, probably easier to replace the unit from ebay than to attempt a repair.

The fuse for the heater, I think, is #4 in the boot, because it’s 40amp, worth cleaning/reseating it just in case it’s causing the problem.

Dec

 

[MarkBc280]

Thanks for all your help Dec, I will try what you have suggested and post up what happens, fingers crossed it's the fuse, I didn't think about the fuse in the rear box, was checking the fuse in the front fuse box which I think regulates the indicators too, but that's very useful will let you know how it goes. Good man, thanks again!

 

[Dec]

It could be something else too, a relay maybe… don’t know where that would be…it would be interesting to see if anything else, electrical, is not working on that car.

Dec

 

[MarkBc280]

Hi Dec, got the car home today and did the following.

Checked everything electrical - all works - heated window light still on and won't switch off....so i
boiled the kettle took a flask of hot water and sat it on the parcel shelf ...windows steamed up... heated window did not demist.

checked the fuse in the rear fuse box 40amp, fuse fine and seated correctly.

next step I guess is to look/check the relays... except not sure which relay it is in the front fuse box or if there is a relay for it in the rear?

unscrewed the front fuse box, flipped the lid and have a few yellow relays a blue one and small black one - not sure which one and the relay diagrams online I think are for the pre facelift c280?

any further pointers would be appreciated. failing the relays I guess it could be heater control panel which would be shame as all other switches and climate controls work fine as does everything else. looks like I may have to bite the bullet and get an auto electrician to look at it. Does anyone know any good/proven MB electrical/specialists in the Reading area, I can see a few local advertised, but wondering if anyone came recommended?

thanks

Mark

 

[Dec]

B is washer pump jets
C&D are wipers
A F G E i dont know what the are for.

Dec

or Bay... http://www.ebay.co.uk/sch/i.html?_f....TRS0&_nkw=w202+heater+control+panel&_sacat=0

 

[MarkBc280]

Thanks Dec, much appreciated will check it out and let you know how it goes

 

[MarkBc280]

So spent a bit of time with the car today and found the following,
Rear headrests drop down from the switch, but don't go back up, is this right? And could this be connected? as they seem to run off the same 15 amp fuse. Also in the rear fuse box fuse number 7 (5amp) was blown, put a a new one in and that one blew instantly. On the fuse layout it's the fuse for the alarm siren. A neighbour who used to have a clk said they had a problems with their alarm siren causing lots of electrical issues and once it was disconnected all was good. Now I guess I should focus on the siren. At least I know there is a problem in the circuit somewhere. Just a bit nervous of attempting this myself so trip to a specialist I reckon.

 

[Dec]

The rear headset drops down only… engine may need to be running…. the must be raised by hand, so no fault there.

The alarm siren is located behind the big Yellow arrow as illustrated in picture below, unplug it, and it wont blow fuse 7 any more. It remains to be seen if the alarm fault is related to the rear window heater fault.

-Jack up front passenger side
-Remove/pull back front part of wheel arch liner.
-Unplug siren

The alarm siren can be replaced for around £100 from main dealer, don’t replace with a used unit, buy new only.

You can also do without the alarm if you aren’t fussy about security, the car locking and immobiliser (immobiliser is the key fob) will work fine without the alarm siren. You won’t have any theft insurance if the alarm isn’t working.

Dec

 

[MarkBc280]

Thanks for this Dec, the info provided is really appreciated. Seems the siren runs off the same circuit as the heated rear window. Hopefully disconnecting the siren will sort the heated window light but will see. Tried to find the heated window connectors today (to either unplug or check if bad/or loose,but started running out of time and didn't want to pull the car to pieces. Do you know if the heated rear window connectors are easily accessible?
Cheers Mark,

 

[Codger49]

I would recommend leaving the heated rear window connectors well alone until you have replaced (or removed) the siren.

You shouldn't need it for the next few months anyway.

 

[Dec]

My understanding is that the back window heater elements are separate from the top 3 lines in the window glass, the top 3 lines are the antenna for the car radio and the also receive the RF (radio frequency) signal from the remote fob when locking/unlocking, however, the top 3 lines are connected to the heater element lines so not entirely sure how it all works.

The heater element circuit is separate from the alarm, but that’s not to say that the faulty alarm isn’t causing the heater switch fault… that remains to be seen when you unplug the siren.

As said above, I would leave the heater connections in the rear window alone for now, it is still likely to be the heater control panel that’s at fault, that’s just my guess.

Under certain sunlight or under the LED light of a smart phone flash… or if you take a picture, you can see the connections of the top 3 lines, the heater element connections aren’t visible so not sure where the are but you can examine the lines at the sides for any damage, with a smart phone light .

The left 2 pictures below, as seen with a smart phone, the right picture is with daylight only.

Dec

 

[MarkBc280]

Ok, thanks both for the sound advice, will see what happens when the siren is disconnected and will let you know how it pans out.

 

[MarkBc280]

So an update, ... I got Chris from Benz tech to take a look (top guy) , he unplugged the siren, to sort that problem,then plugged the car into diagnostics, no issues came up other than the HRW switch was showing as being stuck between on/off. So Chris advised it was likely to be the heater control panel, (good shout Dec), so ordered one off eBay, turned up today and I fitted it, .... HRW light .... Still on!, (everything else works on new control panel,So it's back to the drawing board. I am hoping it's a relay, or maybe bad connection to the rear winda. Either way determined to find out. Just hoping it's not anything too bad!
Will keep you posted. Mark

 

[Dec]

I believe the HRW switch, when operated , is controlled by a timer which should shut off the power to the rear window after 15 minutes, so… that would be computer controlled, it might be worth unplugging the car battery for ½ an hour to see if anything resets itself.

BE CAREFUL NOT TO SLAM THE BOOT SHUT while the car battery is disconnected as you will have trouble opening the boot IF the boot lock is seized and you can’t be operated with a key blade in the boot lock.

Dec