w123 Rear window defogger/defroster - PeachParts Mercedes-Benz Forum

Shern · #1 02-01-2019, 01:30 PM

I'm having trouble getting this thing running...

The fuse is good and the relay works. Light switches on and I confirmed that I am
getting 12V to that first wire, screwed into the pillar (driver's side) behind the trim above the back seat. I pulled the trim on the passenger side and found a similar wire though the screw I guess had vibrated itself out over the years. I'm guessing this is a ground? I installed a new screw and reconnected that wire to the pillar.

Still nothing happening when I flip the switch.

I have no idea what I'm doing back there and couldn't find any diagrams online.
What's the next step after confirming 12 volts are making it to the driver's side rear?

Clemson88 · #2 02-01-2019, 02:15 PM

I'd clean the 'ground wire,' well then test for continuity between the two wires. That will test the wiring in the glass. It's possible the wire has vibrated loose inside the rubber. That's a suck issue to have. I suppose if that's the case it can be re-soldered but you might want check around and see what your getting into.

Shern · #3 02-01-2019, 02:55 PM

Quote:

Originally Posted by Clemson88

I'd clean the 'ground wire,' well then test for continuity between the two wires. That will test the wiring in the glass. It's possible the wire has vibrated loose inside the rubber. That's a suck issue to have. I suppose if that's the case it can be re-soldered but you might want check around and see what your getting into.

Is the only thing that provides continuity between the two terminals the copper wires in the glass? Seems unlikely they'd all be broken, but I suppose it's possible?

vwnate1 · #4 02-01-2019, 08:15 PM

That's how it's tested : by measuring the resistance between the two screws .

If it's too low some of the wires are most likely open circuit . there used to be repair kits, you'd ever so carefully lay down tape on each side of the wire then apply a liquid compound, after everything ws dry you carefully remove the tape and look at the now FUGLY mess and it would work but slowly, very slowly .

Of all my old M-B's, only my 1982 240D's rear window defogger works and it takes forever to make any difference .

Diesel911 · #5 03-09-2019, 11:57 AM

Quote:

Originally Posted by Clemson88

I'd clean the 'ground wire,' well then test for continuity between the two wires. That will test the wiring in the glass. It's possible the wire has vibrated loose inside the rubber. That's a suck issue to have. I suppose if that's the case it can be re-soldered but you might want check around and see what your getting into.

On my 84 300D the grid is external on the inside of the rear window glass.

That means the grid can be scratched and create an open circuit.

Shern · #6 03-08-2019, 04:12 PM

Finally got around to removing the rear sails (?) in order to expose the +/- screws. I'm getting an open circuit here...

Unless I'm missing something mercifully obvious, the implication is that the tab has uncoupled itself from the glass. Is there a way to test for this?
I poked around the rear window seal to see if I could get a better look at the tab, but am reluctant to fully commit to that route not knowing what it might entail.

vwnate1 · #7 03-08-2019, 08:47 PM

So, you've tested the resistance ? .

If the window's circuit is open, that's that .

Try to find the repair kit, last thing I knew it was still available in England .

Shern · #8 03-08-2019, 08:58 PM

Yep, tested resistance. Got none.

This repair kit is proprietary? Somehow different from the stuff at autozone?
The tab having come loose behind the windscreen seal is the only thing I can think of.
It seems unlikely that every single element would be broken.

vwnate1 · #9 03-08-2019, 10:21 PM

I don't know if proprietary, if Autozone has it, buy it and give it a whirl .

I was amazed the other day when it was rainy & foggy and as a total lark I turned on the rear defogger on my '84 300TD graymarket, ! it works ! .

Wow .

Let us know how thw kit works out .

Usually the wires get thin from years of cleaning the rear window and go open ~ your having the tab come off/loose is an outlier .

Shern · #10 03-09-2019, 11:46 AM

Thanks Nate.

Wish I could find a picture. The wiring diagram in the manual isn't very helpful.
Does anyone know what's involved with removing a foot or so of the rear seal?
I suppose it's also possible the + terminal on the driver's side isn't connected to the widow?

Diesel911 · #11 03-09-2019, 12:17 PM

To check to see if the tab is attached apply one lead of the Ohmmeter to the tab wire and then probe on the grid with the other lead.

Find a magnifying glass and take a good look at the grid lines.

I can't recall exactly (I have a thread on it) but when I removed my rear window I found a lot of busted glass when I removed the rear shelf and apparently my rear window was a junk yard replacement and it had no wires attached to it.
The wires were still in the car. It was some trouble but I managed to solder the wires to the wide side parts of the grid and with the Ohmmeter got conduction.
I installed the Window expecting it to work and it did not. (see the attached picture) What I found is that the copper grid on the inside of the window had tiny corded spots all over it that opened the circuit (won't conduct).

In the pic the corded areas are the darker brown area.

If I had know that when the window was out I could have taped off one grid at a time and restored the conduction with those little bottles of copper conductive paint that used to be about $8 each. Or at least restored enough of the grid for it to be useful.
Once the window was back on the car I felt there was not enough room to do the taping and painting so I did not bother.

Diesel911 · #12 03-09-2019, 12:53 PM

It took me a while to find this old thread of mine: Can The Rear Defogger Grid Wires Be Re-soldered back on (123)?

Unfortunately the photos are NLA.

Someone posted this site a long time ago.
Search= Window defroster repair kit
Window defroster defogger repair grids
http://www.frostfighter.com/

Diesel911 · #13 03-09-2019, 03:56 PM

Generic: https://www.instructables.com/id/How-to-Repair-a-Rear-Window-Defroster/

https://www.frostfighter.com/defroster-repair-fix-it-guide.htm

1983 300CD · #14 03-09-2019, 05:35 PM

Start with a wider test of continuity and narrow it down. Put a tester lead on the far left and far right window tabs. If it shows continuity end to end then the window itself is not suspect. If no continuity then start from one of the tabs and lay the other lead along the first available thread on the glass and work outward to zero in on the break. To preserve the threads avoid using the sharp tip of the tester.

Shern · #15 03-09-2019, 05:43 PM

Thanks guys.

What ohm setting should I be using?
There is a significant amount of corrosion on the surface of the grid. So far, I’ve been unable to get a reading (1) at 20k. It’s been a while since I’ve had to polish my moms silverware, but I’m assuming there’s something I can apply to remove it?