Heated rear window problem

Fred2B

New Member
Joined
Apr 6, 2009
Messages
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Location
Greater Manchester area
Your Mercedes
MB C180K, Ford Puma 1.7 VCT, Rover 1.4, Talbot camper
I have just reglued the tab onto the heated rear window of my newly aquired W203 C180K. Only the top half of the window clears when I switch it on. There is 12volt in the heater element but the bottom half still does not clear. Should I be testing for some resistance in the element? at the moment there appears to be no resistance at all. Could this point to the large black resistor type thing being faulty. It seems to be a common unit serving the top and bottom elements so i can't see how it can be unless it splits inside
Any advice greatly appreciated.
Fred
 

wireman

Senior Member
Joined
Sep 20, 2005
Messages
2,632
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Location
lancashire
Your Mercedes
nice 201 2.5D 1993 & very nice 129 SL500 1994
Most rear screen defogger elements are printed onto the glass and as such are not realy repairable.

Each line across the glass is connected to the vehicles 12v supply by the broad stripes of conductive paint at each end of the elements. A more conductive thickness of paint is applied to the end strips so as to keep all the individual element stips supplied equaly without excessivly warming the end strips.

Very small breaks in the coating result in any strip so affected not working, all of which are supposed to be supplied with 12v.

Given that the total current will be somewhere around 10-20A each intact strip will draw something around 1A.

It is possible in some cases to apply conductive paint to the broken strips as a restorative process but the vaguaries of electrical conductivity/conectivity which will result may give a poor connection which could lead to localised overheating of the glass which responds by shattering. Ive done that.

A replacement glass is by far the best cure.
 


www.W140.co.uk&www.r129.co
Specialist in parts for W140 and R129 Mercedes-Benz models.
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