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Discussion Starter #1
Hello All,
I've been visiting your forum for a few weeks now, and must first give a hearty thank you for sharing your vast knowledge of these fine automobiles.

So, very briefly, does anyone here know of or remember an issue with "FY" brand glass having defective defrost elements? Or of any "hidden" electrical connections for rear defrost beyond the 2 visible copper tabs on a W126 sedan? I'm showing power at the window connections, but no continuity across the glass.

Not so briefly, I bought my 1983 300SD about 5 years ago from the 2nd or 3rd owner and it currently has 245,000 miles on it. When I first got it, I was very pleased with the efficiency of the electric rear defrost. The rear window appeared to have very thin wires, closely spaced, running horizontally through the glass, as opposed to the thick conductive decals stuck on the inside of the glass as most other vehicles I've driven. I mention this because I read a post on another forum that stated that original "Factory" glass on these cars had a vertical defrosting element, and I never looked closely enough at the glass to ascertain whether or not it had the MB logo. In any event, defrost worked great until somebody thought it would be super fun to kick in not only my rear glass, but my windshield as well.

So, off to the Auto Glass repair shop, carefully chosen because of its proximity to a movie theater. Had front and rear glass replaced, and was pleased to see the familiar thin wires laminated within. A couple weeks later, finally had occasion to use the defrost and...nothing. Took car back to Glass shop, owner sprayed some foam on the glass and agreed defrost wasn't working, so he replaced it. This one didn't work either, but he was unwilling to replace again, as "it's virtually impossible to get two defective windows, it must be something in your car". Anyway, life happens, and I've got two mirrors.

Fast forward a few years, and now that I've got some time to fix a bunch of little things in my car, I figured I'd run down the defrost problem. With the defrost switch on, and the probes of my 12vDC circuit tester on the little copper tabs on either side of the window, I'm showing power. With the switch off, and the probes of my Continuity tester on those same tabs, I'm showing no continuity across the glass (though with the nature of heating elements, I'm not sure I would...would I?). Other telling symptom: with the engine off, but the head lights and dome light on, activating the defrost system (switch lights up, I hear the relay click, and showing power at the window) does not dim the head lights whatsoever.

Any thoughts (and please, I'm already kicking myself for not addressing this immediately with the Glass Shop and insurance company)?

Thanks so much!

PS This is my first post, and I'm having difficulty previewing it. My signature should say something to the effect of:
Gusto Benz
1983 300SD
Sacramento, CA
 

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I am not yet a wiring guru with regards to these cars, yet (but I am, by necessity, slowly becoming one...)

Were you getting 12v on both sides of the glass with the wires attached to the rear window, or detached? If attached, that would tell me that the grounded side isn't. And that there is continuity through the wires in the glass. If detached, then it would seem that something is screwy with the wiring.

I don't know what a continuity checker would show...I would expect a VOM or a DVM to show some small resistance in the wires, to generate heat and melt the ice on the cold mornings that are coming...

Why can't it just still be summer? :(

EDIT...
It just occured to me...if you are getting 12v from side to side when the car's wiring is attached to the glass, then there is indeed no continuity across the glass. (It was late last night when I wrote the original post).
 

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Best bet is to go to a JY and get a used rear glass out of a junker. I have gotten two and both worked. The one I got in Rose now, came out of a C4C car in the aadlen yard, back when they were crushing the car. Put it in a month ago and the defrost is like new. Some yards have glass sales and I got the rear glass for S'No Angel for $17.00 and change. I just cut it out of a Gen II 420 SEL.

Then go to a Safelite auto glass store and have them install it, buy a new MB seal.
Safelite charged me $100.00 labor to swap out a bad rear SEC glass and swap in the one I had. I did get the car ready, so all they had to do was justr emove and replace the glass.
 

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If you have no connectivity from tab to tab on the window itself (where the wires connect) then it is your window that is defective. You can stop there, time for another piece of glass. Bring your tester to the JY to make sure you have connectivity on the window you are pulling.
 

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Discussion Starter #5
Wow! Thanks for the kind and prompt responses.

Of course, was really hoping for "Oh yeah, this was a big problem for FY a couple years ago. As a responsible corporation, they've admitted fault and will replace your defective glass at no cost. Just take your car to any licensed Auto Glass shop and they'll handle it." ;) (assuming this icon means "dripping with sarcasm").

So, a couple further questions before I head to the Junk Yard:

1. JoeBob55 mentioned testing with the car wires attached and detached. The car's wires are actually soldered to the tabs. Could this soldering have damaged the element? And I agree, if the wires were detached and the window was showing 12 volts across the tabs, something would definitely be screwy! :)

2. Which models specifically have rear glass that is interchangeable with a 1983 300SD? None of the parts suppliers I've visited online sell glass, and though the seals seem to be the same across a wide variety, I wouldn't be surprised if MB engineers changed the glass specs while retaining the seal specs.

3. A few folks have complained of "dead spots" in their defroster elements. Most of these seem to be de-lamination issues, but I would like to avoid inadvertently picking up a dying window. So, if I took an Ohm Meter to the JY, would a good window show a specific resistance, while a failing one would deviate from that? Is there a specific known resistance for a good window, and what would that be?

4. Chadahar mentioned he just "cut it out of a 420". Is that as simple as it sounds, i.e. just take a utility blade and cut around the outside of the seal and pop out the window, chrome included (being careful of the tabs and wiring, of course)? And does "did the work to get the car ready" mean pulling the rear seat forward and removing the hat shelf? I read another very detailed post on that so will anticipate doing that when I'm ready to have the glass swapped. Don't recall the original Glass Shop doing that, perhaps in their haste that's what damaged the windows.

In any event, thank you all so much for advice. Have a great Thanksgiving!
-Gusto Benz

ps Liammarsh, I appreciated the conciseness of your post as well. Did you have to modify anything on your car to run it on Biodiesel?
 

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When I referred to getting 12v with the wires detached, I was assuming using the DVM with the - lead (black) grounded, and checking voltage at the wires. I was envisioning a situation where you were getting 12v going into each side of the glass...a not very likely scenario (unless both our cars had the same previous owner...I've been finding lots of screwy wiring in my 300SD), but one which would cause the defroster not to work.
 

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Here is exactly what I did to remove the rear glass from the 420SEL,
1. remove the C pillar trim. remove one screw from each side, electrical connection(power) on right rear and ground from left rear.
2. removed the chrome trim strips from the window seal as I did not need them, and discarded.
3. I cut around the edge of the glass using a razor knife or equivalent a ( you have to find the edge inside the rubber). a friend came by and offered a wire( from one of the car's many wire looms. we fished the wire under the edge of the glass and I went inside and he stayed outside and we attached handles to the wire, like a Garrot, and pulled the wire around the whole inside edge of the glass and freed it from the seal.
4. remove the glass carefully and you are done.
 
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