Help - Single Stage Rear Defrost Wiring & Testing

[tom1394racing]

I am getting ready to install the glass on my '70E restoration project. The car came with the clear glass/single stage rear defrost standard package. The wiring for the heating elements seems pretty simple.

The photos show my defroster harness connected to the rear windshield looking at the inside of the glass. The harness enters from the bottom drivers side (RHS in the photo). The red/white wire runs up the seal to the drivers side terminal and the ground wire runs along the bottom of the seal and up the passenger side to the second terminal. Can someone confirm that this is correct?

How does one test the window to make sure it functions correctly? I measured the resistance along the left and right terminals and got around 0.45 Mohms. If I connect it to my 12 volt DC/3 amp power supply should I be able to feel some heat on the glass?

Any help would be appreciated.

[gled49]

In the cars we would take a test light lay it down so point of test light couldn't damage painted on lines. Ground the test light and start testing each segment from the + side. As you move across each line the light might become dimmer. The segment lines can corrode and do very often, when your test light goes out, the segment is damaged. Looking very closely you will see the break. I've never found any repair conductive paint to work. Hope this helps Gordon.

[tom1394racing]

Like this.....

https://m.youtube.com/watch?v=R0ZKoK_x6XM

Thanks

[tom1394racing]

Well...I am stumped.

I have about a half dozen of the single stage defrost units (9 elements). I checked each one of them. All showed very high resistance in the Mega ohm range (open circuit) across the heating strips. I used the test light method with my 12V DC power supply and could only get the light to go on for the first inch or so of each grid. I also checked the current draw and found zero current draw. How could every heating element in each one of these windshields be damaged? Not likely I would think.

I also checked several of the two stage defrost windshields (19 elements) that I have in my stash. I tested each one of these the same way. The resistance across the elements was around 3-5 ohms. The test light method worked as expected along the length of most all the grids and the current draw was in the milli amp range.

So how could it be that every element in all of the single stage units be damaged and the two stage units be working fine. I must be missing something.

HELP

[tom1394racing]

Looks like I am not alone in this dilemma

http://www.early911sregistry.org/for...t=Rear+defrost

[gled49]

Again I'll ask, even though the painted wires look like they are there, they are really corroded and non conductive. Look closely with a magnifying glass, you should see where the material has flaked off. Hope I'm wrong Gordon