Last year, the lease on my wife’s car was ending, and I saw it as a great opportunity to get a new (to me) daily driver while she borrowed my 2017 VW Golf R during her final stretch of school. Instead of choosing a sensible car, I convinced myself that a $5,000, 30-year-old German convertible would be a perfectly reliable mode of transportation. That might have been true—if I had bought the right car. Instead, I “saved” $10,000 by picking one in the perfect color (its color scheme actually inspired this website), but it came with no maintenance history and obvious must-fix issues. My naive optimism led me to believe that the premium for a lower-mileage car with service records wasn’t worth the five-figure difference.
Despite its quirks, I have no regrets about buying this car, and I’m determined to preserve it for decades to come. The R129 SL represents a golden era of Mercedes-Benz, when engineering and craftsmanship were at the forefront. With an original MSRP of $90,000—equivalent to nearly $200,000 today—it stood as the brand’s flagship model. Even now, it offers impressive performance, timeless design, and a driving experience that feels genuinely elevated.
In this post, I’m going to document where I’m currently at in “restoring” this car and what I plan to do in the future.
What have I Fixed?
When I bought the car there were a few big ticket items that I knew needed addressing, and some more minor maintenance stuff that I did preventatively. I spent about 3 months last year from February until early May doing these tasks in the evenings and on the weekend. I’m going to do a follow-up post discussing the costs of everything that I’ve done.
The Engine Wiring Harnesses — In the early nineties Mercedes switched the jacketing material on some of their wiring harnesses to one that over time will plasticize and crumble. The common story for the reasoning for this material change is that they wanted something “bio-degradable,” but after working in the automotive industry for the past decade I’m skeptical of this story. I think it’s much more likely that the material change was driven by other factors and they were ignorant to the long term durability issues this change made. None of this changes the fact that two of the main engine wiring harnesses on this vehicle were crumbling and needed to be fixed. The more complex of the two called the “upper harness” can be purchased and the installation only takes an afternoon’s worth of work. The less complex of the two called the “lower harness” is not available anymore so I had to remake this harness from scratch. Luckily this lower harness is only a few connectors, but unluckily some of those connectors are very difficult to find and expensive. I remade this harness to as close to “motorsports” standards as I could using Raychem DR-25 heat shrink as the wire covering and MilSpec /32 wires.
The Convertible Top Hydraulics — The R129 was the first convertible vehicle on sale with a fully automatic roof system. This system has twelve hydraulic cylinders and one distribution block that contain seals that will eventually leak if not properly addressed. Last winter I removed all of these components from the vehicle and sent them to Top Hydraulics in Oregon to be rebuilt. While theoretically I could have bought the seals and rebuilt them myself, if I were to damage a single cylinder in that process it would have cost more than the entire cost of getting them rebuilt professionally.
Everything Else — Here is a non-exhaustive list of the other maintenance tasks I did last winter (many of them preventative)
- Rubber fuel line replacement
- New Tires
- Fluid Changes - oil, power steering, transmission, brake fluid, and coolant
- Spark Plugs
- Accessory Belt
- Valve Cover Gasket
- Filters - engine oil, transmission oil, fuel, engine air, and cabin air
- Front Suspension - control arms, tie rods, idler arm, and strut mounts
- Front turn signal connector - this was a tough one to find, couldn’t figure out a fast blink for weeks
- All bulbs replaced
- PCV hoses
- Steering damper
- Hydraulic trunk shocks
- Head Unit Refurbished - I sent the head unit to Becker Auto Sounds in New Jersey to get bluetooth added to the stock head unit
- Battery
- Hood pad - every vintage mercedes needs this done
- Water pump
- Thermostat
- Fan Clutch
What is still Broken?
Because there is nothing more expensive than a used German car there is still a laundry list of things that I should address before I do any cosmetic or performance upgrades on the car. I did spend last summer driving the car as my main transportation method which exposed some issues that still need to be fixed.
- Transmission Issues — Last November I removed the transmission from the vehicle because the car stopped shifting into reverse when the car was warmed up. This is not an uncommon problem with Mercedes transmissions of this vintage, but unfortunately it does require the removal of the transmission to fix. This issue is what ultimately ended my “summer of fun” and resulted in my wife leasing an Equinox EV so that I could return to daily driving my Volkswagen. I got bit by the “while you’re in there” bug and did a full rebuild of the transmission over Memorial Day weekend. This work has taken way longer than I initially planned, but finishing my master’s degree and planning a wedding put a lot of other projects on hold. I’m hoping to have this work done by the end of labor day weekend and the car drivable again.
- Convertible Top Window — Last winter while I was rebuilding the hydraulic cylinders for the convertible roof mechanism in my cold garage I caused some fatigue cracks in the plastic rear window of the canvas roof. You can just replace the plastic windows, but its not that much more money and effort to just replace the whole 30 year old top. I’m hoping to do this work next year (or at least pay someone else to). For the time being I can install the hard top roof on the vehicle, which many people prefer the look of anyway.
- Air Conditioning Problems — Currently the air conditioning doesn’t work in this car and the blower motor makes an awful noise sometimes when I start the fans. For all I know the refrigerant slowly leaked out over the past 30 years and simply needs to be refilled, but the worst case scenario is that the evaporator needs replacement which necessitates the removal of the dash. The blower motor noise is a common issue on these cars and is probably caused by the plastic mount breaking, a zip-tie and a couple hours of work should be all it takes to fix this issue.
- Audio System Issues — The door speakers on the car are on their way to being blown out and finding drop-in replacement speakers has been a tougher thing to do than I thought. If you read the forums, many people say that Bose speakers of this era have an impedance of 1 Ohm and that any off-the-shelf speaker will not be properly balanced. Bose does not sell any OEM parts to replace this so the only “correct” thing to do is to replace both the speakers and the amplifier. Because I’m very much not an audiophile I’m leaning towards buying some off the shelf low impedance speakers that are the right size and crossing my fingers that they sound okay.
- Rear Suspension Refresh — While there are no noises or harshness coming from the rear suspension currently, the components look original and are due for a replacement. Luckily, I have already done this to the front. Controversially, I’m not planning on replacing the dampers in this vehicle. They have no signs of a leak and still have plenty of gas pressure. I’m afraid that any dampers available today will have different and worse valving.
What is the Dream?
- Manual Transmission Swap — The four-speed automatic that came stock in this car matches the “character” of the vehicle when it was new. It is perfect for a retired gentleman in the sundown years of his life to get to church and the golf course. I am very much not in this demographic so I really want to row my own gears in this car. This isn’t a simple project and it requires combining components of at least three different cars combined with custom components to get working. I had my friend 3D scan the automatic transmission in the car as well as the transmission tunnel to help with this project. I think this will all be very much worth it and also increase value of the vehicle.
- Full Interior Restoration — Thirty years of use is not kind to leather.
- 6.0 “AMG” Engine Rebuild — More power baby! Renntech in Florida offers this, but I think this will cost multiples of the purchase price of the vehicle.