Piston Slap: Modulating the R107’s transmission leak?

David writes:

I have a 1983 Mercedes 380SL with 70K miles that on rare occasions blows a tremendous amount of white smoke. It’s really like a smoke screen. It only does this a couple of times a summer. All other times it runs fine. The car is stored winters. I have changed the transmission modulator twice and it continues. I have noticed it does lose automatic transmission fluid. Twice this past summer it was on the lowest point on the fill scale on the dipstick. Three different mechanics have looked at it and failed to solve the issue.

I have not noticed loss of coolant. Could it somehow be it’s burning automatic transmission fluid?

Sajeev answers:

White smoke is usually a sign of an engine burning coolant, but if the smoke never comes out of the tailpipes and you are losing automatic transmission fluid (ATF)? Sounds like you might have an internal leak in the gearbox (like a bad front seal), or a bad vacuum modulator, or an obvious leak in the transmission cooler plumbing or the pan gasket. The last bit is easy to spot by a local mechanic, and you already replaced the modulator. Twice. Which isn’t a great sign: What if they used a poor quality brand modulator, or even worse, used the wrong one? Because when the diaphragm leaks (or is the wrong one), it could leak ATF. And in high vacuum situations the diaphragm will suck the leaked ATF into the intake manifold, which as my Mercedes-guru friend said, will make it “look like Uncle Buck’s car.”

Even worse, these modulators come in three fun colors, but which one is right for your 380 SL?

Perhaps you are sure you got the right modulator, but it’s still worth a look to see which one was installed and confirm it’s correct. The Mercedes-Benz 722.3 gearbox may or may not mandate a certain color: With our luck modulators could be different between engines, countries, and if it originally came with California emissions.

Best bet is to visit your local Mercedes-Benz dealership’s parts department, give them the VIN, and see what they say. If the dealership can’t help, perhaps a call to the Mercedes-Benz Classic Center is in order. Someone will know the right modulator and ensure you get a high quality part. But what if the modulator checks out?

To be honest, I am running out of ideas. Difficult-to-diagnose issues like these are why most shops yank out the transmission and send it to a rebuilding specialist to do the hard work. And it is hard work, as transmissions are complicated and require special training and tools to rebuild. There’s a slim chance that the trans is leaking from the front pump under heavy engine loads, slinging ATF onto the SL’s exhaust crossover tube and creating the smoke show you’re experiencing. But I seriously doubt it.

I am very optimistic that the vacuum modulator is the problem here. And if I am right, do me (and the car) a solid and ensure your 380SL has the double roller timing chain conversion. You won’t regret that bit of future proofing!

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