Battery Well Corrosion Nightmare
 

Man, this car is turning into a can of worms every time I turn around. I purchased some POR-15 today to get some tiny rust spots within the rear doors and one small spot next to the right exhaust header that I figured was caused by excessive heat peeling the paint away over time. I quickly remembered that there may be some corrosion around the battery tray area, so after removing the washer reservoir and the battery, I was literally sick to my stomach. On what is otherwise a very clean CA/AZ car, the acid from the batteries over the years had eaten holes through the tray and the space beneath the tray was full of corrosion and debris. Why why why would Mercedes put a bunch of wiring down there? It's like a rat's nest of old corroded wiring running all over the place. I'm assuming since I'm pretty sure everything works in my car that the wires aren't totally as messed up as they look. But still, what a mess. What do I do? POR-15 the crap out of everything? What about the nuts and bolts and small pieces like that? I guess I should replace that bushing underneath the plastic cover too while things are apart. I'm wondering if the small rust I am seeing next to the exhaust header is actually coming from the other side where the ****show is. Pics included...

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Mike, not that it's going to resolve your existing problem, but recent batteries I have purchased from our local MB dealer are sealed. So there is no issue with leaking battery fluids. I would be reluctant to cover any wiring with POR-15; however, I would take the time to clean some of the connections and wiring insulation. The wiring situation is not desirable, but it does not appear so far gone that a full replacement is required.

I doubt that heat from the exhaust header caused the other problem area.

That will probably clean up rather well. So many batteries are not vented away from that area resulting in corrosion. Use the vent elbow in the side of the battery connected to a tube to vent the gas/acid out of the well and away from heat or sparks.

Good luck!!!

The original battery had a vent tube which ran down the fender and allowed the acid fumes to escape. They were almost never replaced when the batteries were. You see the results. Use an AGM battery to avoid future problems.

What you have isn't uncommon and looks worse than it is.

First; Clean the area THOROUGHLY. Use lots of soapy water, a scrub brush and rinse well.

Second; Clean the area again, rinse and allow to dry.

Now you can inspect the extent of the damaged area. Don't just slap on some POR15. The odds are you will be merely covering over rust and creating future problems.

Wire brushes and a shop-vac are your friends. While you are cleaning this area vacuum out the hinge pocket area. You'll be dismayed at the amount of crap in those areas.

I don't see where the wiring is affected. The wiring has insulation and is shrouded. I see it is stained, but have you verified that it is corroded? Take apart some of the connectors and check the condition of the metal conductors.

Update
 

I finally made things as right as they're going to get. I bought POR-15 and did two coats on the rusty spot on top of the suspension hole, along the base of the false firewall, and purchased a better battery tray and brackets at the wrecker and coated them as well. Waited at least a week for all to fully cure and then re-installed. The wires looked like a total mess to begin with, but they cleaned up pretty good in the end. Let's just say that I am now super paranoid about parking underneath trees. Maybe I can figure out how to make a thin custom screen that goes between the edge of the fender and the hood hinge somehow.

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Good job. When you think about it, Mercedes had the good idea to mount the battery on a removeable plate. In case the plate rusts, you can just replace that plate. I had some other cars where the battery was mounted directly on the frame rail (like VW MkII) and when that rusted you had to cut and weld.

One trick if you ever find rusted parts like this: fill a tub with regular white vinegar and immerse the part in it for a couple days. When you take it out it will be completely stripped and all the layers of rust will flake off if you hose off the part. Make sure to grind/sand and paint the part directly after or coat it with something. I find that putting POR-15 directly over rust almost inevitably makes the rust come back after a while.