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A few of the grid lines on my hatch defrost grid weren't working. Of course, I couldn't remember which few. Since the hatch was off the car for re-sealing (a separate post will eventually appear about that process), I made some 16AWG alligator clips, connected a battery to the terminal tabs at each side using those alligator clips, and probed for voltages.
My window tint had failed years ago, so I removed it long ago. This process won't work if your window is tinted since the tint is a film that completely covers the defrost grid.
The defroster draws 12A of current (according to the cheap multimeter I was using), so take care to avoid thin gauge wires and accidental fires.
Defrost Grid Overview
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I bought Permatex 09117 "Rear Window Defogger Repair". It contains a tiny tiny vile of conductive paint and quite a number of other things that you may or may not need, including a pouch of conductive adhesive for re-adhering loose defroster tabs.
Permatex 09117
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Conductive Paint Close Up
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I marked the breaks with a little arrow. There were three - two within the grid and one where a grid line meets the bus along the side. I marked other questionable-looking areas with a little dot. It looks like at some point in the last 23 years of owning this car I hauled something that rubbed against the glass.
Repair Spots Marked and Masked
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Repair Spots Close Up
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The breaks (with the little arrows, above) are hard to identify visually. I also looked for worn looking spots (with the little dots, above). Those worn spots still conducted. My point is, you can't go by looks alone to identify breaks. You have to be methodical with a multimeter.
The Permatex kit includes a masking sticker, but regular masking tape works just fine.
Spots Repaired Overview
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The conductive paint is more orange than the golden defrost lines. I decided each repair should be 10-20mm (3/8" to 3/4" give or take) so the break is fully covered.
Breaks, Close Up
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Breaks, Close Up, Mask Half Removed
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The above two photos show the same area. You can see the color of the paint.
Overview, Mask Removed
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The above photo is a more zoomed-out view of the repairs previously identified by little dots.
After the paint had time to cure (the instructions say 24 hours - use your judgement), I connected a battery again and probed for voltages. The conductive paint works great!
danmartinic wrote: Tue Nov 08, 2022 4:36 pm
I vowed never to use the electric rear defrost on the Porsche.. and thankfully, I can't: the wires are broken at the hatch supports lol
Well, then you aren't using the electric heated side mirrors either. In the Pacific NW that's a must have. The main large gauge wire connected to my driver's side strut is broken on the 951. No idea how that happened, and I'm not sure how I'm going to fix it. I do have a spare connector with similar gauge wire so I guess I could splice it, but would prefer that it be hidden behind the B-pillar trim, so a bit of a job, I guess.
danmartinic wrote: Mon Dec 05, 2022 6:28 pm
The side view mirrors are heated ??
Yep
From my "What's Inside: Door Mirrors" post. You can see the two terminals on the back side of the mirror glass support, and the corresponding conductor wires coming from the mirror housing. (The green oval applies to the "Door Mirrors" post, so pay it no mind here.)
I assume activating the rear defrost turns on the side mirror heating?
If the rear defrost is not connected at the hatch strut, will the mirror heaters still work? Is there any potential problem turning the switch on in this situation?
danmartinic wrote: Tue Dec 06, 2022 10:33 am
I assume activating the rear defrost turns on the side mirror heating?
That's how I understand it. Honestly, I didn't know the side mirrors were heated until I took them apart this past summer. And I've owned the car for over 20 years now.
Disconnecting either power lead at the hatch struts (one side is power, one side is ground) will prevent the rear defroster from working. I suspect that has no impact on the side mirrors.
I've used my rear defroster extensively for the entire time I've owned the car. Temperatures well below freezing in Wisconsin. Throughout the Rocky Mountains in heavy snow. Hotel parking lots in Wyoming.
I suppose it is possible for stresses in the glass combined with differential heating could lead to a crack or outright failure, but I've never worried about it. Recently while re-sealing the hatch glass, I used a high temp heat gun aimed directly at the glass to loosen the remaining factory adhesive. I was smart about not overheating any one area. Still, the heat gun applied much more heat than the defrost grid does.
Just fixed mine.
A couple of things I've read/discovered:
1) the mirrors are heated... so are the windshield washer nozzles!
2) installing the rear hatch struts incorrectly (there is a left and a right, a top and a bottom) will cause the fuse to blow
3) installing the incorrect struts will cause the defroster to not work at all
4) Clark's Garage to the rescue again: https://www.clarks-garage.com/shop-manual/elect-05.htm
