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 heat....to 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 burn...next 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.

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