For almost a decade immersed in the world of Mercedes diesels, a vision has consistently occupied my thoughts: a 617 turbodiesel engine paired with a manual transmission. The reality is, Mercedes never produced a manual 300SD, and manual transmission five-cylinder diesel models were never officially imported to the US. Locating one stateside is akin to searching for a mythical creature. While transmission swaps from the more common four-cylinder 240D are possible, I’ve heard cautionary tales of driveline vibrations caused by flywheel imbalances with the larger engine. This deterred me from undertaking such a project, fearing the potential to compromise a perfectly good car due to improper execution or missing components. However, fate intervened last week, as if Rudolph Diesel himself was smiling down, and this car appeared for sale online.
It was a complete US-specification 300D, remarkably equipped with a Euro-market 5-cylinder 300D manual transmission setup that had been successfully swapped in previously. It even boasted several years of enjoyable manual gear changes. This vehicle presented itself as the ideal donor for my 1984 300SD project – a ready-made manual conversion setup that I could adapt for my larger chassis car with minimal modifications.
Now, you might question the ethics of dismantling a seemingly decent China Blue W123 Mercedes to extract its prized manual transmission for my 1984 300SD. Admittedly, based on the online description, I initially hesitated myself. However, experience has taught me a fundamental truth about sellers of vintage European cars: descriptions are often embellished, whether through omission, ignorance, or outright misrepresentation. In this instance, it was a combination of all three.
The car was advertised as rust-free. The stark reality was significant rust damage in prominent areas, including door bottoms, lower fenders, and along the rear window. Beyond the driver’s side damage disclosed in the listing, the hood exhibited rippling, as if it had been subjected to some rather enthusiastic weight.
Mechanically, it was equally challenged. The turbocharger failed to engage, the battery wasn’t charging, window regulators were defunct, the climate control system was beyond repair – the list of issues went on. However, crucially, the transmission, the component of paramount interest to me, shifted flawlessly. Even within the context of the dilapidated car, the manual gearbox felt exceptional. It underscored the baffling decision by Mercedes executives to limit manual transmissions in the US market. Therefore, any reservations about repurposing the donor car vanished, and work commenced immediately on the swap.
Let me introduce the recipient of this manual transmission transplant: my 1984 Mercedes-Benz 300SD. I acquired it nearly two years ago from a dealer friend who discovered it at a wholesale auction in Dallas. Remarkably, it was a trade-in from the original owners when they opted for a Mini-Cooper. The trunk contained complete documentation spanning three decades, from the original purchase agreement to every oil change and repair receipt. Its condition is remarkably original, both inside and out, making it the finest Mercedes diesel I have ever owned. Further details and photos from recent years can be found in this thread. Here’s a photo taken at a Cars and Coffee event last week, positioned next to my S600.
Today, my mechanic began the project by carefully removing all the necessary components from the 300D donor car. It’s already missing its front end and some other parts, earmarked for another project.
He also started work on the 300SD, removing the automatic brake pedal assembly and adapting the mounting to accommodate the manual pedals.
The only component I’ve ordered so far is a speedometer from a 1989 300E. I needed a cable-driven speedometer that can be adapted to fit within the all-electronic 300SD instrument cluster. The 1986-89 300E models share the same speedometer face size and a similar rear differential ratio, ensuring accurate speed readings. Once everything is installed, the driveshaft will be measured and modified to the correct length by a local specialist shop.
My long-held Mercedes diesel aspiration is finally becoming a tangible reality.
