The Mercedes-Benz 300SD (1981-1985) is one of several vehicles built on Mercedes' W126 platform, which it shares with many other models. It utilizes a three speed automatic transmission coupled to an inline five cylinder turbo diesel. Like other Mercedes diesels, this is a stratified-charge engine capable of running on practically any fuel that is not too volatile. The vehicle was offered with either leather or vinyl interior and just about any appointment you can think of besides a cup holder. With a sunroof, leather interior and power seats, the car cost $33,000 in 1981 - or about $70,000 2003 dollars. The 300SD is well-known to get around 30 miles per gallon during freeway driving, due to its highly efficient powerplant.

THE W126

The W126 chassis (body) is notable for being the first Mercedes to use high-strength steel throughout its construction. As such, a 300SD weighs only about 3500 pounds, relatively little for a full-size Mercedes. (For comparison, the W116 300SD, its immediate predecessor, weighs 1,815 kg — 4,000 lb.) This resulted in an extremely rigid vehicle. The W126 utilizes double wishbone front suspension, and semi-trailing arm rear suspension, as well as four wheel disc brakes. A near-coil suspension (as opposed to coil over) provides both good ride and easy maintenance. The rear wheel drive and independent rear suspension make this car handle very well, though it is still very comfortable over bumps. Turning radius is excellent, as is road feel and handling during hard cornering.

Other W126 vehicles

This writeup is not really about the W126 platform, but about the 300SD. The vehicles which share the platform are:

Without going into detail, SD models are turbo diesels and SE models are gasoline-powered; 1986 and later models (Typically with an "L" on the end) are long-wheelbase vehicles, and are slightly longer than earlier models, but still carry the W126 designation.


The 300SD is powered by a somewhat anemic OM617.952 five cylinder turbo diesel, with 123-125 HP and 170 ft-lb. The turbocharger sits on the right side of the motor and the air cleaner is mounted directly above it. The injectors are prudently placed on the other side of the engine and fuel is delivered to them through brass tubing. In fact, nearly anything under the hood that has a good reason to be made of brass is, including the caps on the air conditioning service ports.

The engine is particularly high-revving for a diesel, with a redline of 4700 RPM. In ordinary driving it will often rise as high as 4500 RPM. Once the RPMs rise to about 2500 (idle is 800) the turbo typically spools up and full output is produced; about 120 horsepower and 170 lb-ft of torque, both measured at the flywheel. Low-end performance is frankly not very good in modern terms, but is just fine for a three liter from the 1980s. Highway pickup is excellent and more than adequate, save for going up steep hills which tends to be a challenge for this car. In general practice, you will not be doing any passing uphill, even slightly uphill.

The engine is equipped with some nice features, like a hand pump for priming the fuel injection system (some early fuel-injected vehicles must be primed using a machine if you run out of fuel) and an emergency stop lever attached to the fuel system in case the car is dieseling and will not stop itself. The case castings have been well-smoothed and are easy to keep clean. There is a block heater to allow starting in extremely cold temperatures. This is probably the heaviest-cast three liter engine you will ever see and its durability reflects this, but so does its weight.


The W126 body has considerably forward-looking styling, and with the installation of the right wheels (for example the AMG Mercedes wheels) the car looks like it could have been made today except for the lights. European cars got a metal/glass composite headlight/driving light made by Bosch while US cars got a two-piece headlight/driving light combo with a rectangular headlight. The rectangular light is itself an update from the round headlights set in a louvered plastic panel in the immediately preceding generation of mercedes. Original and aftermarket euro-style headlights are available for prices between $200 and $600 per headlight. They take H4 headlight lamps, and H3 lamps for the driving lights, which are each the least expensive example of a lamp of their type. It is also possible to purchase aftermarket clear tail and corner lamp housings for about $25 and $200 per pair, respectively. When coupled with new wheels (14 inch and smaller alloy disc wheels went out of style in the late eighties) the look is definitely modern.

The vehicle was offered with a wide range of paint schemes over its lifetime. They are always painted two-tone with the lower plastic cladding painted the second color, and the most common colors are white/grey, tan/grey, and tan/brown. There is also a blue/gray available from early on, and as the years progressed, silver/gunmetal became a popular choice, as did other metallics including a striking pale gold color.

300SDs were sold with a lightweight 14" aluminum-alloy disc wheel and 195/65-14 tires. This combination allows for the sidewall to soak up a lot of impact and allows for a smoother ride provided tire quality is sufficient. Most newer vehicles elect to perform vibration damping in the suspension system, and utilize a wide tread, short sidewall, and a much larger wheel (typically 17 to 18" in full-size Mercedes.) 18 and 19" AMG wheels are available as a direct bolt-on replacement for this vehicle.


The 300SD wouldn't be an S-class if it weren't for a highly detailed and designed interior. Gauges are easy to read and fairly complete, with the notable omission of a boost gauge. Oil pressure and coolant temperature are represented as gauges, not lights, and there is a tachometer present. An audibly ticking clock (when the car is not running) also sits inside the cluster. Below this are warning lights for the parking brake, glow plugs, brake wear, charging system, and high-beams. The dash is smooth overall, with a small vent for the climate control system's temperature sensor. Typical of german vehicles, the steering column does not tilt, and you must move the seat around it. The dash is covered with plastic even in vehicles which have leather interior as it better handles excessive direct sunlight, and it is appointed with one of several choices of (real) wood trim. The wood is used throughout the vehicle on all four doors, across the dash, down the center of the dash and back along the center console.

The front power seats have four adjustments, including seat height front and rear, seat tilt, and front to back adjustment. They were offered with a heater, but all driver's seats have a pneumatic lumbar support inflated by the use of a squeeze bulb to the right of the seat along the center console. The driver's seat also has a fold-down armrest which locks into position. The control for the seat is located on the door and consists of two levers representing the seat and the seat back. The back control actuates the seat back tilt, while the seat control actually engages three separate switch elements to control the other three functions. Both front and rear seats are extremely comfortable with a great deal of spring, and there is a huge amount of headroom in all four major seating positions. A fifth passenger can ride (with slightly less headroom) in the center position in the rear. If they are not present, an arm rest may be folded down for comfort. Typically speaking, five average adults can fit into this car at once with reasonable comfort.

Automatic climate control allows the user to set a temperature (in degrees centigrade, or by the blue-white-orange scale) and never touch the system again except to engage the defrost. The system will operate the vent, fresh/recirculated air door, air conditioning, and heater valve as appropriate to maintain your desired temperature. The climate control system has ducts that open to the sides and in the center of the dash, as well as under the front seats (for foot-room ventilation for rear passengers) and in the back of the center console. A vent in the top of the dash draws in air from inside the vehicle and the system reacts accordingly. In most situations the car draws in 20% outside air to avoid the atmosphere becoming stale.

Three ashtrays, one in the center of the dash where it meets the center console at the bottom and two in the rear doors, each have their own light and lighter socket. Switches for power windows (when installed) are in the center console, and lit so they can be located at night. Windows use worm gear motor drives, while door locks use vacuum solenoids. Locking the driver's door can only be done from outside with the key, or from the inside when the car is already closed. Either way, it locks all the other locks on the vehicle. Pulling on inner door handles automatically opens the corresponding door lock. Each door has a reflector on it as well as a courtesy light. A front dome light can be controlled from a switch located on the light, and will go out a few seconds after both front doors are closed. A rear dome light has a switch on the center console and opens when a rear door is opened. There are reading lights at either side of the back seat.


Shortly after purchasing a 1981 300SD I proceeded to drive it for a bit less than two hours on the freeway through some relatively twisty hills. A week later I had also driven it on some very twisty up-and-downhill roads, eight lane highways, and a broad assortment of city streets. The car performed excellently in all of these situations, though as noted previously it lacks power for steep hills. Braking force is adequate for all conditions and while the suspension is less than nimble, it is far more agile than American cars sold to the same market. At highway speeds (and above) the vehicle is exceptionally stable. At lower speeds, it can be difficult to maintain high enough RPMs for efficient cornering, which in a rear wheel drive car requires some application of power. This leads to a necessarily sedate driving style which drivers of more powerful vehicles might find frustrating.

The interior controls are logical in general, although the climate control buttons could be clearer. Seat belt latches are sometimes difficult to engage but are very positive, as is typical in german cars. The vehicle is generally comfortable to sit in for long periods though persons with long legs may find that the sharp and unyielding center console puts a crease in their knee. The four stock speakers are excellent as far as such things go, and handle the output from a 40W/channel stereo admirably. There is a control in the center console for rear speaker volume which should be disconnected and patched around for use with aftermarket stereo systems.

A well-maintained vehicle can get fuel mileage as good as 30 MPG on the freeway, but in normal driving you will not likely see these numbers. My vehicle, which could probably use some engine rebuild work, recently got 22 MPG on a 300 mile trip that involved numerous hills and a fair amount of city driving - in San Francisco, which means more hills, and a lot of stop-and-go. Driving up steep hills means that you will either be going slowly or burning a lot of fuel. This is in part due to the gear ratios. German vehicles are expected to make high top speeds (presumably for driving on the autobahn) but the vehicle is extremely underpowered, so first and second gears are very low but the third gear is quite tall. Up any kind of significant grade at all, you have two choices; drive about 55 MPH in second gear, or around 75 MPH in third. In between, the RPMs are too high for second, and you cannot maintain enough power to keep the taller gearing of third going. This is because while gasoline engines make somewhat variable power depending on fuel delivered, diesels deliver fuel based on RPM and the pedal controls maximum fuel delivery.

I eventually parted company with my 1981 and got a 1992 300SD. Little did I know that my first car was old and tired. This newer machine (which has all the hallmarks of possessing a rebuilt engine, but the history is unknown to me) scoots easily up hills that the last vehicle bogged down on, in spite of having similar total mileage. The window motors and switches have about the same grade of problems, and the sunroof is still sticky even after I cleaned and greased it, as was my first. I still often wish for more power, but in general it is sufficient.

300SDs can now be had for less than $5,000, while their blue book price is over $8,000. Great mileage is somewhat tempered by the current outrageous prices for diesel fuel (which takes about 60% as much energy to produce as gasoline, and is sometimes a byproduct of the process of converting crude to gasoline) but the car is very well-designed in every way. For example, the battery, brake master cylinder, and windshield wiper fluid reservoirs are separated from the rest of the engine compartment by a wall that keeps them from excessive heat, and another partition is beyond this to separate the engine compartment from the cowl. The entire car proceeds in this manner, from the first aid kit compartment to the holes in the rocker panels that correspond to a peg on the jack. If Mercedes is guilty of doing anything wrong to this car it would be overengineering, but the number of 500,000 mile SDs out there suggests that they did everything right.


  1. Mercedes-Benz W116. (2010, May 15). In Wikipedia, The Free Encyclopedia. Retrieved 13:20, June 8, 2010, from
  2. Mercedes-Benz OM617 engine. (2010, June 7). In Wikipedia, The Free Encyclopedia. Retrieved 13:25, June 8, 2010, from

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