How to fix leaky Mercedes tail lights, and everything else they broke


How to fix leaky Mercedes tail lights, and everything else they broke

I purchased a ML series SUV (2008 ML320 CDI W164), and there were weird things happening. Sometimes one headlight would blink when exiting the car. The weight sensor for the passenger would sporadically start to alert while driving. The rear hatch wouldn’t open. But most importantly, the battery was slowly draining when sitting. Turns out it was all due to a leaking rear tail light.

Leaking tail lights are common in this generation of ML and GL series. The factory was installing these seals incorrectly. They were putting the adhesive side towards the light, rather than the body. Thus rain and road water would drip past the light and into the body of the vehicle.

How I got to this point was a bit of an adventure. First, I hooked up the Mercedes-Benz STAR computer to the car, and the rear SAM (Signal acquisition module). It had all types of faults. I opened the hatch of the car, and took off the right bay cover. I pulled the electrical connectors out of the SAM, and removed it from the vehicle. I opened up the plastic cover. There was corrosion all over it:


Signal Acquisition Module with white corrosion from road moisture

I used Deoxit D5 to clean up the board. It dissolved it with a satisfying hissing and bubbling:


I began looking into where this corrosion was coming from. Istumbled upon various forum posts that talked about these malfunctioning tail light seals. So I decided to remove the tail light to take a better look myself.

There are 4x 8mm nuts that hold the light to the car, and a electrical clip that’s connected to the light.

As suspected, see the adhesive incorrectly facing the light instead of the body:


Old seals shown incorrectly installed from factory with adhesive towards the light

I scraped the old adhesive off of the light, and cleaned it up. I had to scrape old foam that was stuck to the body of the car.

There doesn’t appear to be any decent ways to fix this without new seals. You can see the new seals installed below; they are MB parts 1648261691 and 1648261591. You’ll find the circular seal on the right is sold as part of these part numbers. It’s highly recommended in this case to purchase OEM seals from the dealer or a MB parts reseller.


New seals correctly attached with adhesive towards the body, not the light

I re-installed the clean SAM and I was still having battery drain. I continued digging. I knew the hatch wasn’t working so I figured I’d try to fix that, not realizing that it was related to the battery drain issue. I removed the spare tire, and the carpet underneath that to look at the hydraulic pump that powers the hatch. This is what I found:


Rusted hydraulic pump for the reach hatch

The electrical connector was holding on by one wire; the rest had rusted and snapped off. I started researching a new pump. Apparently, these pumps are fairly compatible with other model versions. The exact version replacement was $250 at the time, but I found a very similar one for around $100 on eBay.

I reconnected the hydraulic and electric lines. I had to prime the pump to remove air from the lines. If you end of doing this too be sure to look up the differences between the type of pump; vertical or horizontal.

Now the hatch was working again. I tested the battery drain by hooking up my multimeter between the car and the negative terminal. Specifically I was analyzing the current draw. I’d cycle the vehicle key to on (not turning the starter), and then turning it off. Eventually I noticed that the amps would normalize after about 5 minutes. This value was around the same as another ML320 I have.

So corrosion on the SAM caused all sorts of weird problems. The battery drain was due to a wire shorting caused by corrosion on the hydraulic hatch pump. However, all of these issues were due to a leaky tail light because the factory stuck the seal on backwards.

Thanks for reading


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