For almost a decade immersed in the world of Mercedes diesels, the vision of a manual transmission paired with the robust 617 turbodiesel engine has been a constant aspiration. The reality is, Mercedes never produced a manual 300SD, nor did they introduce any manual transmission five-cylinder diesel models to the American market. This scarcity makes finding a manual 300SD stateside akin to searching for a mythical creature.
While transmission swaps from the more common four-cylinder 240D models are possible, tales of driveline vibrations due to flywheel imbalance with the larger engine have always given me pause. The fear of compromising a perfectly good car with a poorly executed swap or missing components kept me from ever starting such a project. However, fate intervened last week, as if Rudolph Diesel himself was smiling down, and a car perfectly suited for my needs appeared for sale online.
It was a complete US-specification 300D, but crucially, it had already undergone a successful Euro-market five-cylinder 300D manual transmission conversion. With years of reliable manual driving under its belt, this car presented itself as the ideal donor. It was a turnkey setup that I could essentially reverse engineer and transplant into my own 1984 300SD, requiring only minor adjustments to fit the larger chassis.
Initially, the thought of dismantling this seemingly nice China Blue W123 for parts gave me pause. The listing suggested a well-maintained vehicle, making me question the ethics of such a drastic move. However, experience has taught me a crucial lesson about older European car sales: descriptions are often embellished, whether through omission, ignorance, or outright falsehood. This case proved to be a trifecta of all three.
The car was advertised as rust-free. The harsh reality was far different. Significant rust was present in easily visible areas: the lower door panels, the bottoms of the fenders, and along the rear window frame. Adding insult to injury, beyond the damage highlighted on the driver’s side, the hood was noticeably warped, as if it had been used as a trampoline by overly enthusiastic teenagers.
Mechanically, the donor car was also a mixed bag. The turbocharger failed to spool up, the battery wasn’t charging, window regulators were defunct, the climate control system was a disaster, and the list of issues went on. Yet, amidst this mechanical chaos, the one element that mattered most to me – the manual transmission – performed flawlessly. Even in the dilapidated state of the car, the gear changes felt fantastic. It reinforced my long-held belief that Mercedes executives severely underestimated the demand for manual transmissions in the US market. Any reservations about sacrificing the donor car vanished, and work commenced immediately on the swap.
Let me introduce the recipient of this manual transmission transplant: my 1984 300SD. I acquired this car 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 for a Mini-Cooper. Inside the trunk was a treasure trove of documentation spanning three decades, from the original purchase agreement to every oil change and repair record. The car is in exceptional original condition, both inside and out, and is undoubtedly the finest Mercedes diesel I have ever owned. More details about its acquisition and condition can be found in this forum thread. Here’s a picture of it from a local Cars and Coffee event, parked next to my S600.
Today, my mechanic began the project by carefully extracting all the necessary components from the 300D donor car. As you can see, the donor car is already being stripped down, with the front end and other parts removed for another project, highlighting its less-than-perfect condition.
Simultaneously, work began on the 300SD. The automatic brake pedal assembly was removed, and modifications started to adapt the mounting for the manual pedals.
The only part I’ve had to specifically order so far is a speedometer from a 1989 300E. A cable-driven speedometer is required to interface with the manual transmission and adapt to the all-electronic instrument cluster of the 300SD. The 1986-89 300E models are ideal as they share a similar speedometer face size and rear differential ratio, ensuring accurate speed readings. Once all the components are installed, the driveshaft will be measured and professionally modified to the correct length by a local specialist shop.
My long-held Mercedes diesel manual transmission dream is finally becoming a reality. The anticipation of rowing through the gears in my 300sd Mercedes is immense, and I look forward to sharing the next stages of this exciting project.
