The saga of my Mercedes 300D's malfunctioning transmission

Why do I need a sub-head? The title is pretty self-explanatory. This is ridiculous.

When I got my Mercedes 300D, I knew that it had a fairly rough-shifting transmission, but I wasn't really focusing on it, because I was too busy focusing on the squealing fan belt. But once we (me and my dad) fixed the fan belt, the transmission got worse and worse as the weather got colder. Once the temperatures dropped below about 40 degrees, the car became close to undriveable.

The first thing we thought about was the vacuum system. Basically, a lot of stuff on the W123-generation Mercedes--including the automatic transmission--is controlled by a complex vacuum system, which controls a lot of things that would be controlled electronically in modern cars, and is also extremely failure prone. So, we checked the vacuum lines to see if we could find a problem, and, uh, we did. One of the vacuum lines was not connected to anything, and was just hanging around uselessly in the engine bay. So we reconnected it, and while it didn't totally fix the problem, it did make things a lot better.

But, seeing as there are still several paragraphs left in the story, you can probably guess that more things went wrong. The problem was that the weather got even colder, and the Mercedes once again became close to undriveable (and this is in Maryland--it's a good thing we don't live in Minnesota or Vermont). I thought that maybe the vacuum lines might be the issue--maybe there was a leak in one of them, or something. So we decided to check the vacuum system to see if that was the issue.

What we did was, we unhooked one of the vacuum lines, and then attached another line, which went through the firewall and into the cabin, and was attached to a dial which measured vacuum pressure or something. The idea was, we would be able to see the vacuum level in real time as we were driving along, so we would be able to know if it was acting normally or not.

We hooked up the vacuum gauge and started driving, and we eventually discovered that, unfortunately, the vacuum system was behaving normally. I say "unfortunately" because that meant that the transmission problem must be mechanical, and it would be much more difficult to solve.

Eventually, through some internet research, we decided that the problem was likely the transmission clutch packs, which would mean a transmission replacement. Needless to say, this is slightly beyond our capabilities, so we did things a little differently. I'm still hoping to get the Mercedes into my Automotive Technology class next year--they have a full garage with lots of typical garage things in it, so hopefully I'll be able to do a manual swap. If the Earth is attacked by aliens, or whatever crazy thing is due to happen this year, then I guess I'll have to save up enough money for a human mechanic to replace the transmission--assuming the aliens decide to keep us around as pets rather than kill us. Or something.

Until then, we decided to use a transmission additive--basically magic, highly toxic ketchup that temporarily fixes transmissions--to keep the car going. We got a bottle of the stuff from an auto parts store, and then went to work adding it to the transmission fluid.

When we did so, we discovered two things--the transmission fluid looked burnt and was probably in need of replacement (which we didn't do then for some reason, but hopefully soon), and there was way too much of it in there.

See the first groove on the dipstick? The fluid should have been up to a little less than there, but instead it was up almost to where the bottom edge of the paper towel is in this picture.

So, thinking we had found the issue, we decided not to put in the additive, and we drained out some of the transmission fluid. And, uh, it did not go well.

It was about 60-70 degrees out when we were doing this, and yet once we had drained out some of the transmission, the car went from "pretty much undriveable" to "100% undriveable." During the (very brief) test drive, when we were going slightly downhill, the transmission was slipping like crazy and refusing to downshift. And when we were going slightly uphill, the transmission was slipping like crazy, and refusing to downshift, and the car physically would not go over 20 miles per hour (at least not without risking severe engine/transmission damage). Clearly, the transmission fluid was overfilled for a reason.

So we decided to add in the transmission additive. It was extremely viscous, and some of it dripped down onto the engine (you can still smell it burning off whenever you drive the car). Once we had done that, we (very nervously) tried to drive the car again, and it worked well--it wasn't perfect, but it was tolerable. We decided that the problem had been (mostly) solved, and that the car could be driven normally until next fall.

And then the next morning came, and I started dreaming about taking a jackhammer to the transmission.

I'm pretty sure most of the improvement we saw with the transmission was due to the higher temperatures, because when I drove it in the morning, when it was colder, the problems came right back. Thankfully, the temperatures will only be getting higher, so I think the car should still be OK until next fall. But the car would be even more OK if Mercedes had made it with a manual from the factory rather than the world's worst automatic transmission, because having an indestructible engine isn't that helpful if you also have a complex, fragile and difficult-to-repair transmission.

German engineering.

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Comments (5)

  • I below muy parents had one of those and said the exact same thing about the auto gearbox

      1 month ago
  • Ugh...transmission problems. 😑

      1 month ago