DAMSEL IN DISTRESS! Turn headlights ON,all goes WRONG!
 

Newbie to a '85 Euro 500 SEC - everything seems to be great until night falls and I turn the headlights ON, within minutes - windshield wipers are going full speed & can't be stopped unless lights are turned OFF.
Also the wipers will engage if the horn is honked
(whether the lights are on or not).
Checked the fuses, all is fine; relays look good too,
but I am unsure of which relay controls these functions -
sorry, no manual to check & I've thoroughly searched all previous threads for info - please help so I can drive at night!

Sounds like you have quite a problem there. I am no expert, but it sounds like an electrical short somewhere, but if it were a short, I would think a fuse would blow. Is the proper amp fuse in all of the circuits?

When you turn the lights on (and the wipers blare), are the lights bright, or are they dim? Also- how long have you drove with the lights on and the wipers going? If for awhile and nothing happened, it may not be a short, or else something severe may have happened. I am also thinking perhaps a bad switch. The wiper is on the same circuit (fuse) as the horn- at least on the USA S-Class 81-85 models. It should have a red (16 amp) fuse in that slot. Strange that the headlights have an effect on them, since they are on a totally different circuit. Has your car ever had any modifications or wiring that was tampered with?

Here is a "map" of the fuses on a 1985 500SEL (from another MB owner), which should be the same as your car - I am sure that you have this chart in your fuse box, but here it is just incase.
http://www.forums.mbnz.org/gallery/pics/12_1766296_130200684600PM.JPG

I would not recommend driving the car until this is resolved.

I am sure someone here with much more knowledge will chime in- there are a lot of knowledgeable folks here. I just wanted to drop in and let you know someone will soon respond, so do not despair.

Have the charging system checked..... WHILE the problem is there!

I would think the battery or the alternator, low voltage issues will create some rather strange behaviors including unpredictable wipers.

Check ground connections, particularly the dash behind the instrument cluster.

I agree with jbaj007 with the added advice to check the contacts and wiring in the steering column.

Sounds to me like there is a short or bad connection between the horn contacts, wiper switch, headlight high beam switch.

Something is messy in there - thats where they are all in close contact.

Cheers

Electrical
 

1984 300d Turbo 137k
Went To Start Car No Lights Or Battery Power, Pull Ground On
Battery, Clean Battery Post And Cable With Battery Tool From Auto Zone Inside Is A Wire Brush, Install Back On, Lights Ok
And Started Up Diy 1/4 Hr.

I thank everyone for their replies!

I have checked all fuses - all good & of proper amperage
Battery is 6 mos. old & voltage is good

Question regarding advice on checking ground connections behind dash & contacts/wiring in steering column - is this something do-able by a
"basic", but enthusiastic, amatuer? I'm an avide DIY and never worked on cars until this one, but I loved it & have fixed many things so far thanks to this website! (BEST SITE EVER!)

I'm up to learning & tackling the job, but I also need to know what should be left up to the pros!
Thanks again guys!
Hollmark

Most certainly is, particularly if you understand what you are doing with a multimeter.

Search for instrument cluster removal (described well in other threads) and steering wheel removal (again well described). The only caveat I would put on steering wheel removal is if you have an airbag.

I would also suggest if you are an avid DIYer you get yourself a set of the $30 service CDs from MBUSA. They will help a lot and where they don't the forum is here to help (and sometimes just make sense of the service instructions!).

Good luck, I hope you enjoy your car.

Wild Wipers
 

I'd have to second the voice of experience in this post:

"M.B.DOC
Moderator Join Date: Mar 1999
Location: Snellville, Ga USA
Posts: 5,397


Have the charging system checked..... WHILE the problem is there!
__________________
MERCEDES BENZ MASTER GUILD TECHNICIAN (6 TIMES)
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. Shopforeman ..."

I had almost the identical problem on my '86 190e which disappeared after changing the voltage regulator on the back of the alternator.

Happy trails...:)

Follow Steve's advise. It's almost certainly the voltage regulator... an easy DIU fix.

Hopefully you has not tore into the steering column or instrument panel yet, since this seems like to be the problem. :uhoh:

Probably much easier to repair.

Starter
 

I had a 126 300SD and when I changed my starter I started to have almost the same problem. It ended up being the heat shield around the starter got rusted and was making a short with the new starter and made my lights and wiper go on and off. Check that out.
Robert

I will 3rd the weak charging system.

When the SD was loosing its first regulator, my wipers would go and the high beams would come on and it was just...nuts...

Defintely just check the voltage on the battery with the headlamps on :)

problem resolved?
 

problem resolved?

enquiring minds want to know. :D