Rear Subframe Question W124 Sedan - PeachParts Mercedes-Benz Forum

jake12tech · #1 07-18-2017, 12:42 AM

I was thinking of repairing the rear subframe mount rust on my W124. You could drive it as-is for awhile without worrying, but I'm bored. Would I have to drop the subframe completely or could I get in there to replace the metal where it mounts into? It's a small box section that I could easily build and mig in.

Mxfrank · #2 07-18-2017, 04:01 AM

I have a lot of experience with Jaguar, where even a change of brake pads requires the rear subframe to come out. For that reason, I think that any subframe work is best done by dropping it out of the car. It's really an easy job...support the body on solid stands, undo the driveshaft and parking brake cables. Remove the LCA bolts from the shocks and sway bar links. Remove the springs. You may have to remove the muffler. Place a jack under the differential and take the weight off the mounts. Then undo the four bolts that hold the frame to the body and drop it out. It sounds like a lot, and the first time through it may seem harder. But it's an hour and a half''s work to get it out, and after that, anything you need to do is easy, from replacing the center to replacing mounts to doing all the control arms. Otherwise, everything is an on-your-back struggle.

BusterBoyBenz · #3 07-18-2017, 06:42 AM

Quote:

Originally Posted by Mxfrank

I have a lot of experience with Jaguar, where even a change of brake pads requires the rear subframe to come out. For that reason, I think that any subframe work is best done by dropping it out of the car. It's really an easy job...support the body on solid stands, undo the driveshaft and parking brake cables. Remove the LCA bolts from the shocks and sway bar links. Remove the springs. You may have to remove the muffler. Place a jack under the differential and take the weight off the mounts. Then undo the four bolts that hold the frame to the body and drop it out. It sounds like a lot, and the first time through it may seem harder. But it's an hour and a half''s work to get it out, and after that, anything you need to do is easy, from replacing the center to replacing mounts to doing all the control arms. Otherwise, everything is an on-your-back struggle.

additionally dismount and hang the calipers and get behind the rear seat, unplug the differential ABS sensor wire connector and remove the screw holding the clamp securing the wire in place, push the sensor pigtail/connector/rubber chassis grommet down and out under the car so it's free and clear. Push the fuel filler drain hose up and out of the hole in the sub-frame through which it passes and is friction fit. My experience is that disconnecting the ABS sensor is far easier and better than attempting to remove that sensor from the hole in the diff where it resides, often times it's bound in very tight and very likely some rust corrosion. if by chance for some reason that sensor has to be removed from its place it uses a very thin O-Ring that is not easily sourced except from MB and it has to be used or it will always leak. If the sensor is stuck hard it can be easily broken trying to wrestle it out.

Care should be taken reinstalling the four sub-frame mount bolts, they are very fine thread and easy to get cross threaded when trying to start them, the worst part is they will cross thread pretty easily and you won't really notice until you've got a couple revolutions and it starts to really bind up. I always run a tap up into the threads in the frame and clean off all the old dried blue threadlock on the bolts before trying. Get all four bolts started with a jack under the diff holding the entire sub-frame in place then pull each bolt one at a time, apply new blue threadlock then they can be drawn up into place. You don't want to cross thread those it's very difficult to straighten out the female threads up in the frame.

depending on the condition of the car be prepared to fight the sub-frame mounts off the frame, on these 124 cars there is a metal insert in the rubber mount that is aluminum and it can get really really seized and corroded into place. Then you have a very hard time because you can only pull or lever the sub-frame away/down and it will only stretch the rubber of the mount rather than apply all your force to the stuck parts. I and others on the forum found that using an air hammer judiciously against the aluminum mount sleeve from below can work pretty effectively to finally break the bond between the mount and the mounting part of the frame. Sounds bad and it can, be but it sometimes isn't and it come apart pretty easily but you want to be aware and prepared in the event in case your doing the job someplace where you might run into time or space constraints. With the car rear off the ground you can loosen each sub-frame mount bolt in turn and see if each of them comes apart easily before you step off into the big full job as well

compress ignite · #4 07-18-2017, 09:00 PM

Locator

Thanks ! Guys

vstech · #5 07-18-2017, 10:57 PM

there are several threads on here about dropping the subframe on these cars.

I made one in 12/13 on a wagon (same procedure, bigger vehicle...)

bohdibenz did a full tear out, rust repair, etc... on her 87.

look it over before you tear into it.

if you have a lot of rust, getting the parking brake cable out is going to be a pain, but well worth it.

have fun!

Mxfrank · #6 07-19-2017, 07:43 AM

If you're talking about a 124, the cables can be removed with the axle by releasing the compensator at the center of the car. That will leave them hanging from the rear brake splash shields. Only complication is if the cables need to be replaced, then it will take some persuasion with a drift or punch to separate the ferrule from the shield.