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77 320i rear defroster

4670 Views 3 Replies 2 Participants Last post by  manuelwindows
To make a long story short, I am the proud owner of a 77 320i that has some issues but no major ones as you can probably tell by the title of this post.

So I'll be back with some questions about lower ball joints and a tie rod but right now my question regards the rear defroster system.

First, when I bought the car, the defroster switch (actually a round button on the console) did not work and was stuck in the out position, which was stuck on the "on" position, with the light always lit up and the button was hot to the touch.

I got a working switch from a different 77 320i that was not broken and which you could push on and off and I installed that switch in the correct place in the console. When the switch is depressed, it is "off" and the backlight on the button turns off. Then, when you press on the button, the button protrudes out a little more and the light comes on. So the new button works. One way I also know that the switch is working is that when it is depressed (off), the button is not hot to the touch, but that when I turn it on, the button is hot to the touch. About 110 degrees Fahrenheit. I measured it with an infrared thermometer.

Problem is that replacing the switch with a working switch did not get the defroster wires on the rear back window working.

I haven't done any more electrical testing except just to replace the defroster fuse in the fusebox and I've also tried using a new relay in the fusebox and neither of those got the rear defroster wires to go hot either.

So, coming to my question, anybody else dealt with this and have an easier idea than pulling the wires for the whole system. I'm also not terribly concerned because I live in a climate that never really goes that much below freezing anyway so fortunately, not a big issue, but my wife complains about it so that makes it a problem for me even if it is not a problem for me, it has become a problem for me for that reason. So if you're wondering why I am so worried about this rear defroster when I'd rather be replacing the front tie rod assemblies, it's because this is the issue that my wife is asking me about on a daily basis.

No, I have not consulted the Bentley manual yet because I am hoping that someone here might just have dealt with this issue here and has a quickie idea and I'm not very good with wires so there's that too.



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Check for voltage at the window terminals. Supply to earth; across the element; element earth to body. If you get 12v across the element it is damaged. You might be able to fix it - I have soldered terminals back to the block on the glass - but if the strands are open circuit then you are stuffed.
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Will do exactly that tomorrow and report back and thanks!
I promised to report back and I don't have much but.

I am getting voltage coming to the connector at the rear window and I am picking up voltage on the wire strips for only about 1 1/2 inches horizontally away from vertical strip connected to the connector wire where the voltage is leading in. There's a break that seems to go exactly in a vertical direction down the wires about that distance from the vertical line.

I don't have an exact voltage reading but I'm thinking that the voltage is not all that important just to make these wires get a little warm. So now I am thinking about finding an older windshield and testing the strips for connectivity and replacing the rear windshield or possibly considering repairing the break in the parallel wires that run horizontally on the rear window but my guess is that route will be ugly. Otherwise, diagnostics and work on this have been delayed by rainy weather and the lack of a garage to do the diagnostics and some other concerns with the front suspension that are taking precedence for the moment. I'll be back.
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