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Old 09-27-2004, 01:40 PM
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Story on headlight wipers

Figured I would add to the data base of knowledge here at the shop. Contrary to common conscensus, the headlight wiper assemblies are repairable, all be it, not really cost effective. However with my scotch background, I hate spending what I consider to be excessive amounts of money when it is not required. Here's the story.

Having been forced with a defective passenger side headlight wiper assembly on my 95S420 (w140), I was remiss to spend $300 CAD on a simple device. Research on the web indicated that there was only a small number of repairs posible on an "acting up" headlight wiper. Contrary to the evidence, I decided to experiment. Results, depending on the problem, they are fixable.

The problem I was having could best be classified as a lazy wiper motor. When the system was started for the first time, the motor would move a stroke (maybe) and then stall. Subsequently turning off the system and starting again would have the wiper crawl 1/2 a centimeter and stall again. Leaving the system off for a few hours would let the arm move about 4" and then stall. Very frustrating.

Well I took the assembly appart and started investigating. The funny thing was that on the bench, the damn thing would work correctly. Really became frustrated. So observations:

1) There is no diode. The elecrical drawings are wrong! The total break out of this motor assembly is the motor gear network, a ciruit board with (2) jumper wires only on it (what it does is still beyond me), (2) cherry type db2 spdt microswitches and (1) PTC. That PTC is found to be the culprit.

Leasones learned. The PTC is a thermal resister who's true purpose in life is to restrict current flow (PTC stands for positive thermal coefficient). If you take your unit apart, you will see it as a 1cm x 1cm x 1mm thick wafer sandwiched between two copper contacts. Composed of granular carbon in a plastic (exact material name escapes at this time) substrate, it acts as a fuse for the circuit. As the current increases, the whole PTC expands, causing the resistance to increase inside the unit. Obviously, if resistance increases then the voltage drops across the PTC, causing more heat, causing more expansion, causing more resistance causing more eventually the voltage potential across the PTC reduces the 12v feed to a small enough value that the motor will not run. Very effective fuse. The beauty of this is that as long as the power is supplied, the circuit is passing only a small amount of power, therefore motor doesn't have enough torque to turn or burn out. Turning off the car allows the PTC to cool, re-establishing the connection only to repeat the process. If your like me I'm now thinking about pulling the whole PTC and let the motor paragraph. This is novell in that as the motor working gear ages, mechanical efficiecies are lost causing more resistance in the electric motor which ultimately exceeds the threshold current allowance of the PTC. So mercedes is not wrong in saying replace the assembly, but, it bothers me to replace an assembly when the only thing wrong is the "johnson" 12v dc motor aging.

So in summary, as the car ages and the motors are worked the internal resistance of the electric motor will increase over time. Effectively the added current loads now exceed the threshold current of the PTC and ping. It stops dead (until you allow it to cool in which case it will continue for a short period until the PTC heats up again).

2.) The other thing to note in the incorrect electrical schematics, is that the PTC supplies power to both the headlight washer motor and the wiper motor. Interesting in that my measurements show the headlight wiper motor to need 1 amp (maybe 2 under start) but the washer requires 4 amps running up to 6 when starting. Most peturbing.

What I did was order a new PTC in various sizes and inserted the replacement between the black wire feed. Don't forget to short out the original one (I removed it from the circuit). Raychem makes the PTC's in various current sizes ranging form 3 amp threshold to 14 amp threshold and there less than $2 CAD each.

Just a note about speeds. The passenger side wiper motor controls via one of the db2 microswitches when the wash water splashes the headlight. If your motor in the passenger side is getting lazy, there is nothing you can do to sycronize the washing of the driver side headlight. What I did was short out the switch so that whenever you turn on the washer it gives a constant stream of wash water. It will cost me more in washer but thats like 1.19 a gallon, I'll carry an extra in the trunk.

If you also have a fused / relay floating around you could also feed the washer circuit through the relay and achieve about the same result.

A furthur note about all the forum's comments on having the washer arm park in the wrong position. The obvious answer is that the nut has loosened on the wiper arm effectively allowing the blade to swing indescrimentally. Not the only solution though. The position control of the wiper arm and the wash water is determined my an intermal 8 position cam. Only one cam position controls the park position. If this has shifted, the washer will park in the wrong position. An easy test to see if this is the case is to press the activation button for just a short time. If it requires 1 to seconds to get the wipers to go on their own then the cam has probably jumped a notch.

Additionally, the swinging arm is designed to jam against stops inside the motor housing itself (That way it won't just coast by the stop when you release the button and never stop). This is a real problem for the motor to overcome these stops and really sends a spike of current when it trys to get past it. I attempted to grind the stops a little and now my unit won't park. My solution was to retard the cam one notch so it parks now in the vertical position (versus the horizontal), so if anyone has a unit there not using...I could sure use some replacement parts.

Cheers and Good luck everyone.

Tower (Ron)

PS sorry for the long thread.

1995 E320 -171k-km
1995 S420 -333k-km
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Old 09-27-2004, 10:29 PM
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Sounds like a couple of my misadventures.
Thanks for the post.
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Old 03-26-2005, 09:51 AM
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w140 headlight wiper

great info on the PTC. I too went through the wiper motor ordeal. After the dealer said I needed a new motor $95 to diagnose, $300 motor and $300 to install...I laughed and went home. Only to eventually come back and buy the motor from the dealer. I installed the thing and guess what. Didn't work. I studied the wiring diagrams and decided that it was likely a ground. But, in search of the ground problem, I decided to replace the washer motor since in the wiring diagram the washer motor is a critical element in the circuit and a short in the washer can ruin the works. So, new headlight washer pump and viola! Everything works!

Not so fast. NOW, the passenger side wiper will stall out after three or four presses of the washer stalk (cycles). Washer fluid still comes out but the passenger side wiper stalls at 12 o'clock position. Letting it cool down and recycling (ignition and headlights) gets it back to normal.

My question is this: with a new motor could the PTC be bad or would there be any reason to suspect an overvoltage / overamperage through the PTC...but I thought this was "thermal" resistor. Any ideas?

I may be glad I bought the motor at the dealer as I am about to test the MB 12mo warranty on factory parts! HA.

Any thoughts are welcome.
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Old 03-28-2005, 06:39 AM
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If I remember the wiring diagaram, only the passenger side controls the washer fluid flow for both of the headlight washers. I have never dissected the driver side but I would almost bet it works on the same setup. The motor stalling but able to restart after cooling off (allowing the PTC to shrink back to its original position) are the classic setups.

Regarding your question. The reality is that the motor assembly begins to wear, friction increases and puts more demand for electrical power. Substituting a larger PTC allows more current (ie power) to the motor to push should it need it. The original PTC is probably still capable of meeting its design loading, I really don't know if its the PTC that craps or something elese.

Its a bit of a risk but you may want to remove the PTC from the driver side washer and maybe mount a larger one (slightly more amp capacity to it) in hopes of getting the system to work externally to the system. Unless you place a new on within the motor housing, you will have to install (2) new ones externally to the power wires of the wiper motor. (2) because one wire activates from the stalk switch and the other is the dedicated power that constantly flows to the wiper motors. A fair warning though, increasing the PTC value increases the current flowing through the system which may effect some of the upstream components (ie, convience relay I believe). Watch for that.

1995 E320 -171k-km
1995 S420 -333k-km

Last edited by tower; 03-28-2005 at 06:46 AM.
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Old 03-29-2005, 02:14 PM
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pesky wipers

thanks for the info. seems odd that a new motor would only have the guts to run for a few (button push) cycles.

I am going to ask the MB dealer parts guys if they've seen many returns. Everywhere you look there are lazy wipers on W140s so I've got to believe that some of them have new motors that are inop.

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Old 11-02-2007, 07:29 PM
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really old thread but a question

Why not replace the ptc's with and external fusable link?

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Old 03-02-2008, 10:12 PM
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Why not disable the wipers and wiper washer motor? Do you REALLY need to wipe your headlights?

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Old 03-02-2008, 10:35 PM
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PTC fuses do not have an infinite number of cycles in them - in fact, they are rather limited and people who design them in should know this. I'd consider that the wipers may see very high currents if you clear ice or snow off of them, for example - which you may see where you live. You may have seen a lot more cycles than someone in, say, Florida.
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Old 03-02-2008, 11:13 PM
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Originally Posted by tower View Post
."...................... However with my scotch background, I hate spending what I consider to be excessive amounts of money when it is not required.
Tower (Ron)
Dear Sir...I have to take issue with you.
Being of Scottish ancestry with a family crest and tartans...two of them btw,
I have to protest at your use of the term 'Scotch'.

This term is used for Whiskey and a wooden wedge, neither of which you would be.

"Contrary to what Webster 1913 says, the Scots (being the people of Scotland), do not like being referred to as scotch. They prefer the appellation Scots.[/B]"

I also refer you to:

Being able to trace my own family back to 785 CE with the help of 'Burke's Peerage, Baronetage & Knightage', I wish to make it clear that being a 'Scot' or being 'Scottish' is correct, but being 'Scotch' is inaccurate.
A small thing, I know.

I do hope you take this information in the manner I offer it Sir.

Yours respectfully,

I will now try to repair my own 300e head lamp wiper/s ...or is that, 'head light' wipers ! Oh gord, here we go again.
Head lite, head lamp or head light.
Main beam, Hi beam or brights...or is that 'brites'?

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Old 03-03-2008, 09:34 AM
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The statute of limitations on that infraction is 3 years and had therefore expired.
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Old 03-03-2008, 09:47 AM
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I always thought head light wipers were silly. If they don't work, does it interfere with anything else?

The new Bezs don't have these anymore, do they? When did they stop?
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Old 03-03-2008, 10:01 AM
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Originally Posted by Dee8go View Post
I always thought head light wipers were silly. If they don't work, does it interfere with anything else?

The new Bezs don't have these anymore, do they? When did they stop?
I seem to recall a car with h/l wipers that rotated like a helicopter blade/s on the 7inch round domed glass..
or did they occilate back and forward....anyway, anyone remember?

Then there where on second picture down.
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Last edited by dkveuro; 03-03-2008 at 10:15 AM.
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Old 03-21-2009, 12:18 AM
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Does anyone know how to disable the headlight wipers and the washers?
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Old 03-21-2009, 12:46 AM
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Pull the wiring plugs to the motors and pump. You can buy a headlight washer pump delete plug for your washer bottle and remove all the hoses along with the wipers.

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