Rear Window Seal Replacement and Rust Treatment - 1st Gen W126 1984 300SD

[Theseus]

Following are my experiences with removing and replacing the rubber seal on the rear window. This is a common problem with the W126. If water is collecting in the trunk, look under the glass for any rust spots. If there is water leaking from the corners of the glass, the seal is leaking. A second sign is a "foggy" appearance in the corners of the first-generation back glass. This is the result of the glass delaminating. You can use this as a companion to the FSM.

Gen 1 or Gen 2 backglasses may be used. I got one from a junkyard from a 1991 W126. BEFORE REMOVAL, DISCONNECT THE BACK GLASS HEATER STRIP!

With a utility knife, I removed my back glass from the inside. The glass should pop out easily. What I found was severe rust.

Fix the rusty channels as necessary. Mine was wirewheeled and treated with POR15 rust remover. I chose the composite approach. It was coated with POR15 and reinforced with fiberglass cloth and thickened POR15. Then, it was primed with POR15 primer, and smoothed with Bondo fiberglass strand filler. It was sanded down, bondoed with bondo gold filler, spot filler, primer, basecoat and then 2k clear urethane spray.

Under the window, I brushed on Loctite rust reformer. I then sprayed the underside with CRC Marine corrosion wax.

[Theseus]

(Cont.)

I opted for the URO rear window seal. It was made of new rubber as opposed to an older OEM NOW rubber. You can see that the seals are nearly identical. When purchasing a seal, BE SURE TO SPECIFY 1st GEN or 2nd GEN trim for the seal, THEY ARE DIFFERENT.

I opted for a more secure water tight method and put a bead of 3M non-hardening Auto Bedding and Glazing compound, 08509, in the nook where the window and seal interface. The reason I chose this is that the corners actually collect water. It it is going to be sitting there this whole time, it will find a way down inside. This is why I believe there are so many failures. The FSM says to insert a bead after the seal and window are installed. However, this won’t get it deep into the groove, doing it before installing the seal was what I chose.

The URO seal is a good quality seal (see comparison to the older OEM seal that was pulled out). Unfortunately, it is missing some sharp bends near the bottom corners. Fortunately, the seal will still install, you will just need to have some green tape and some clamps to help keep it in place until the trim is fully installed. Use the string method to install the seal and trim. I spoke with URO and they apparently will make updates to add those corners.

With the seal and trim installed, I put a bead of 3M 08509 into the groove in which the frame opening channels will be fit in. The 3M product is what professionals use for other car glass installations. I highly recommend it.

Take the window assembly, place it in the bottom channel. This is where you check to see if it is centered where you want it. Once it is, start the process of pulling the string and easing the back glass in. I did myself, but a helper helps. The glass suction cup tools are very useful if doing it yourself.

Finally, the seal on the outside should be tucked in. If it is, use a plastic crevice tool to fold it out. I used more 3M 08509 to fill in the space under the flap seal. There is no water getting in now!

Reattach heater and clean up the 3M 08509 with Goo gone.

Enjoy your rear window!

*BTW - Impalaman on youtube released a video the same time I was working on mine for this process on his 300SD. I recommend viewing it as well. Good luck!

https://www.youtube.com/watch?v=x2Yt1ifjPPE

[87tdwagen]

Nice Job!
Good call on using the 3M window glaze, it is very important on the W126, W124 and W201 cars in select places or entire windows. A lot of water entry issues between glass and seal as well as seal and body can be mitigated with the use of this often overlooked step.

Happy leak-free motoring

[Theseus]

Thanks! If the leak in the corner is the main defect for water leaks, there has to be a solution. I firmly believe the 3M window glazing is what will prevent leaks it in the future.

Forgot to add some photos of the finished window frame before installing the window.

[ykobayashi]

Nice writeup. Thanks. I did this years ago on my 300sd. Found rust. I actually had to rebuild missing material with new sheet metal because areas of my pinch weld were just gone. I recall using steel stick to rolling the badly destroyed areas in the corners. Then I pained with rust oleum brush on paint. Not optimal but it held.

Great detail that Gen 1&2 glass works. Does that include long wheelbase to short wheelbase too? Like 91 560SEL to 82 300SD?

[JHZR2]

Originally Posted by Theseus

Thanks! If the leak in the corner is the main defect for water leaks, there has to be a solution. I firmly believe the 3M window glazing is what will prevent leaks it in the future.

Forgot to add some photos of the finished window frame before installing the window.

Nice job. How did you blend the paint in the channel to the body? Is there a paint line somewhere thst is obvious or hidden? Not poking your work on that at all, I’m thinking about the need to do this on one of mine at some point.

Curious if you did the phosphate conversion before POR-15, and why (maybe you did) you didn’t grind down to brighter metal first.

Thanks!

[URO Parts Support]

Originally Posted by Theseus

I spoke with URO and they apparently will make updates to add those corners.

Hi Theseus, nice install! Yes indeed, we're going to add molded corners to Rear Window Seal 126 678 0120 / 1266780120 on our next production run, which should happen in about six to nine months.

[Theseus]

Originally Posted by ykobayashi

Great detail that Gen 1&2 glass works. Does that include long wheelbase to short wheelbase too? Like 91 560SEL to 82 300SD?

Would apply for both. The rear windshield on my 84 SWB is from an 89 LWB. Any backglass can be used, but you must match the correct rubber seal to the corresponding trim, according to the service manual.

[Theseus]

Originally Posted by JHZR2

Nice job. How did you blend the paint in the channel to the body? Is there a paint line somewhere thst is obvious or hidden? Not poking your work on that at all, I’m thinking about the need to do this on one of mine at some point.

Curious if you did the phosphate conversion before POR-15, and why (maybe you did) you didn’t grind down to brighter metal first.

Thanks!

The key is to paint according to sections to make it blend better. I would wet scuff the surrounding area first with fine grit sandpaper. The trunk area had defined lines, so those were masked. The area near the roof and sides were not masked. Spray primer on the channel. Then the base coat, I feathered it out. Then clear. Ultimately after drying it will be wet sanded down. ChrisFix on youtube had a good tutorial on fixing rusted body panels.

I did the POR15 metal prep on the top according to instructions. I then put POR15 over that to protect it before the next steps of building up the channel. Loctite rust converter was done underneath.

[BillGrissom]

I had to replace the rear window in my 1985 300D after a tree branch fell on it, plus dented the top of left quarter-panel. Wifey was happy the branch blew over her minivan to fall on my car. The window channel at bottom rear was rusted thru at the top lip, but enough metal left in the bottom of the channel to seal. Seems to be a common problem and indicates a bad design. This car has lived its life in California, most in a garage, so I imagine much worse in Germany.

I wire-brushed the surface rust and treated with rust converter. I think most converters and metal-etching spray paints use phosphoric acid. You can buy it in gallons at Home Depot as "Concrete Etch and Metal Prep". A better rust removal is "Evapo-O-Rust" which is a different chemical, but you need to keep it wet for a day (soaked rags, covered). I put the rubber on the car body opening first, using a sealant the bottom, then worked the window into the OE rubber using the rope trick, then pushed in the chrome lock-strips. My rubber was still fine and pliable. I might have used a butyl rubber sealant, which has worked best for me in sealing camping trailer windows and such. I don't find what 3M 08509 is, but might be butyl.

Modern car windows are sealed much better, using a urethane rubber sealant. Indeed, I found that for some classic cars, there is now special glass which can be bonded similarly directly to the metal opening, but not cheap (>$1000 per windshield). If you have a nagging leak at a windshield, one possibility is to pull back the rubber and clean as best you can, then use Permatex "flowable silicone RTV" - bubble packs at auto parts. I get a leak during rain onto the passenger floor in my 1984 300D. I suspect the wiper penetrations and gaskets on the cowl cover since I had that off on my 1985 and saw it has degraded seals underneath.

[greazer2b]

Originally Posted by URO Parts Support

Hi Theseus, nice install! Yes indeed, we're going to add molded corners to Rear Window Seal 126 678 0120 / 1266780120 on our next production run, which should happen in about six to nine months.

I have 2 85sd that I need to pull the rear glass and replace the seal. I wanted to know have done what’s stated above?
Thx greazer2b