Mercedes Vacuum Diagram

I developed the following digram for the 1975 R107 Mercedes-Benz 450SL so I would have a complete guide should anything go wrong after the car is put back together. This digram does not show the engine vacuum. 

LEAK DETECTION:

When diagnosing leaks in the Mercedes vacuum system, a vacuum pump with gauge is mandatory or should you have a smoke machine on hand, that would be the ultimate way to go. 

First, start by checking the vacuum tanks for leaks. One is located directly under the front fender on the passenger side above the wheel (LHD). This tank supplies vacuum to the central locking system and cruise control. The other tank is located behind a panel to the left of the emergency brake. This one hold vacuum for the climate control. 

Unscrew plastic panel and slide away to expose tank. To remove tank and panel, e-brake assembly must be removed.

Passenger side U.S. model

The vacuum tank for the climate control should not have any issue with deterioration other than the rubber grommet that seals the tank where the vacuum line enters. This part is still available through Mercedes.

The central locking vacuum tank however is susceptible to damage and deterioration as it is open to the elements just above the front wheel. Both the tank and grommet are still available from Mercedes.

A good test to perform to insure proper vacuum is maintained, is to pump up the tank down to 10 mHg. Make certain that your connections are leak free and your gauge is good. The tank should hold vacuum for at least 48 hours without any loss of vacuum. Conduct this test on both tanks. I personally would not recommend using the smoke machine to test the vacuum tanks. 

Finding leaks in the system is not complicated, but time consuming. Most often it is the rubber connectors that dry rot and begin to leak. If you have the original system, it is best to test each section of vacuum line by disconnecting, capping and testing one section at a time. The digram will show how the system is divided up throughout the car.

Vacuum leaks are also very prevalent at the diaphragm type locking modules. At 40+ years, the rubber diaphragms are quite crispy and will likely leak. These include the heater tap, fresh air flaps at cowl, passenger side door, fuel flap, and trunk. The passenger side door lock, and trunk lock on earlier 450SL's are no longer available from Mercedes.

Other points of leakage are the driver door lock master vacuum switch and the vacuum switch at the A/C switch. Both switches are identical in function but are designed differently. Both are available from Mercedes.

Vacuum for A/C Switch is still available through Mercedes-Benz

The final potential point of a vacuum leak is the Ford-Philco cruise control unit. The cruise control unit is most likely under the dash behind the glove box on a U.S. car.

It is also wise to change out any check valves. They are located under the hood at the fire wall driver side. Original check valves are black and green while the new ones are blue and black. These are unmetered.

Mercedes-Benz Mislabeled Parts

If you own an R107 (convertible) and need either a new fuel tank filler neck grommet (seal) or a fuel filler door vacuum lock, they have been improperly labeled by Mercedes.

107 Fuel Tank Filler Neck Grommet

If you order this part from any of the resellers or directly from Mercedes-Benz, for your R107, you will most likely receive part number 123 997 35 81. This is incorrect, as it is for a W123 and does not fit the R107. The correct part number is 107 997 24 81. This however does not end the confusion. This part number, although correct, is labeled in the Mercedes system to only fit the C107 (coupe) which is incorrect. It fits both the R107 and C107 cars. 

Yes, this seal is important so water does not enter the trunk during rain or washing. 

The photo shows an old R107/C107 seal and a new W123 seal. As you can see, very different design.

107 Fuel Filler Door Lock

Similar to the grommet, if you purchase the R107 fuel filler door vacuum lock for an R107, it will turn up an incorrect part. The correct part is 107 800 12 75 which once again shows it will only fit the C107 (coupe).

NOTE:
As with any purchase, make sure to check your individual chassis and part. Mercedes, like many manufacturers changed parts etc., throughout production.

If you are unfamiliar with the Mercedes part numbering system, the first three digits of the 10 digit number usually indicate the body/chassis production design, i.e., 123 000 00 00 is for a W123 (240, 300...) and 107 000 00 00 would indicate R107/C107 (350, 450 560...)

While Mercedes does supersede part numbers on occasion, it will still generally be superseded with in the same similar time period of production. For example a W116 might have a part number supersede to an R107 and visa versa. 

Mercedes W123 Brake Peddle Repair

With the introduction of automotive plastics in the 1970's, time is now and will continue to take its toll on these parts. 

A part that will more than likely fail if you own a W123, will be the plastic brake peddle bushing/retainer. Thankfully, Mercedes still has the parts to get the job done.

If you notice side-to-side movement in our brake peddle, it's time make the repair. By leaving this part unattended to can cause brake booster failure.

STEP 1
Remove lower dash covers

STEP 2
Unplug wiring as needed and lower cruise control module. You can let the cruise control unit hang to the side

STEP 3
Remove brake master cylinder and booster

STEP 4
Unplug brake light switch. This is a two part plug, but can be unplugged as one.

STEP 5
Remove brake peddle holder assembly. To do this, wiggle assembly back and forth while pulling toward back of car. It will take a bit of work to release the assembly from the dash brace and rubber buffer. Don't lose the rubber buffer, and check to make sure it is in good shape. You will want to make sure you have this rubber part as it will cause vibration and other noises if you do not.

STEP 6
Remove return spring from peddle and remove brake peddle from assembly.

STEP 7
Gently tap bushing into brake peddle with rubber or plastic mallet until fully seated.

STEP 8
Reinstall brake peddle to assembly.

STEP 9
Do not order the bushing until you remove the entire unit first. It appears some of the cars needed shims. If all is not very tight it will cause excess movement in the peddle and cause the new bushing to fail. 

STEP 10
Return assembly as it was removed.

STEP 11
Return booster, master cylinder and fluid reservoir

STEP 12
Refill system with approved brake fluid and bleed braking system. Make sure there is no air in the lines. You can make this process much easier and a one man operation if you purchase Speed Bleeder brake bleeders. Excellent product and excellent customer service.

W123 • Fuel Pressure Relief Valve

Final check to insure a smooth idle... The Fuel Pressure Relief Valve. It's simple, quick and just might make all the difference in the world. 

It's hard to believe just how much bad information gets on the web. What's worse is how gullible we often are to buy into it without question or further research. 

On occasion I scour the web to see what bad information I can find on ways people suggest fixing particular automotive problems. After just a few minutes of reading or watching a Youtube video, it becomes evident most people never read the workshop manual. For 99% of any problem that will arise on your car, there is a corresponding fix from the manufacturer. No guessing required what so ever.

Unfortunately so many great older cars are being butchered from bad advice, laziness, or the assumption the manufacturer had no clue what they were doing from the start. Indeed there are manufacturers who built very unreliable products that should have never been put on a car, but as it pertains to Mercedes, there are few components that were engineered that did not perform near flawlessly.

With that in mind, if your diesel is still running rough with all the previously mentioned items checked or replaced, there is one last component to getting your W123 diesel to run like it did from day one. The Fuel Pressure Relief Valve.

STEP 1
Locate Fuel Pressure Relief Valve highlighted in green.

STEP 2
Remove valve. Fuel will run out so have a rag under the valve and/or a drip pan under the car. Note from photo wish bolt to loosen. 

Once the valve has been removed, it will need to be separated and checked. To separate, either hold with two wrenches or carefully place in a vice and loosen the top "bolt" cap. WARNING: Only loosen! Separate valve by hand and over a pan or rag. A spring and ball bearing are inside. 

Once separated, the is what you will see.

Mercedes-Benz Fuel Pressure Relief Valve from a 1981 240D

STEP 3
Make sure everything is thoroughly cleaned after taking this apart. It should already be very clean since fuel is constantly flowing through this valve. It can however attract sludge depending on how the car has been maintained. 

Once clean, measure the spring using a micrometer. When I measured the spring that was removed from my valve, it measured 19.76mm. According to the Mercedes-Benz worship manual it can measure up to 27mm max. Based on the max of 27mm, I gently stretched the spring to 25mm exactly, and reinstalled.

This simple procedure made the most drastic difference of anything I could have done. While the car was running much better with all the other adjustments and replacement parts, this "put the icing on the cake, as the video in my last post demonstrated.

Mercedes must have known this could or would become an issue since it is specifically mentioned in the manual. Regardless, it's worth the check to see if your spring has compressed over the many years in service. I'm sure it has, and could use a stretch.

I would not recommend starting off with 27mm however. Good luck.

W123 Mercedes 240D Idle Test - Video

A well running W123 diesel should be very smooth. Over the years I have been told "they all rattle and vibrate, they always have, even new." 

I have had a hard time believing Mercedes-Benz would or could ever sell a car that vibrated as much as I have been told is "normal." Mercedes was up against high-end luxury cars in the United States and had to meet a fairly high standard of luxury and comfort regardless of whether or not it was a diesel.

With that in mind, I set out on a journey to see just what was necessary to make my 1981 240D run like Mercedes designed it to. With many checks and a number of maintenance items replaced, this car is once again running excellently and I am pretty convinced it is running close to factory if not at factory spec. 

It should be noted that the process of "tuning" should begin with a compression test. If that fails, no need to go further until that is rectified. 

The following test is usually performed on Rolls-Royce, but I thought it would be a great demonstration for a diesel, and the results truly surprised me. Prior to the work I performed, this glass would have bounced right off the valve cover.

W123 Diesel • Rough Idle

If you have come this far, you either own a W123 diesel or are interested in acquiring one. In either case, there is no need to settle for a rough running diesel Mercedes.

If you have a W123 diesel, 4 or 5 cylinder, there is no doubt they vibrate and are certainly louder than modern diesels, but that is no reason not to make sure your Mercedes is not running as smoothly as possible. 

When I acquired my 1981 240D, and not being familiar with just how these diesels were to perform, I was more forgiving than I am now. Regardless, the following are a few very important items to look at and know about.

Let's work from back to front to find out why it might be running rough.

• Is the fuel tank filter clean or new? If you don't know it's time to find out and most likely replace it.

• Are the rubber connecting lines from the fuel tank filter and along the way up to the front of the car new? If not they are most likely brittle and can easily be drawing air in to the system. Even if they are not leaking they can be allowing air in.

• The two fuel filters, primary and secondary, in the engine bay also need to be changed at specified intervals. 

• Check the plastic fuel lines that are probably no longer clear but dark brown. These too can become brittle over time and begin to allow air into the system.

• The primer plunger must also be checked. If you still have the all metal unit it is most likely leaking and should be changed for the the newer Bosch unit. If you have the newer plunger, depress it 6 or so times and see if it leaks around the base of the plunger. If it does, you are drawing air in to the system from there. 

• Another important factor are the injectors. How long has it been since they were rebuilt or replaced? Again, if you don't know it is time to pull those and have them tested, and if need be rebuilt. Also make sure they are not leaking. If they are, air will be introduced into the system at that point as well. 

• Check the air filter!

• Make sure all the hard injector lines are tight at the pump and injectors. Do not over tighten!

Image: 1981 Mercedes-Benz 240D

Once all these items have been checked and or replaced, you should notice a major difference in performance and smoothness of operation. I also recommend running a lubricant/cetane booster in every tank. With the growing distribution of low sulfur and ultra low sulfur  (LSD/ULSD) diesels, the older pumps are not sufficiently lubricated and require additives to reduce excessive wear. 

I would recommend Red Line 85 Plus Diesel Fuel Additive. This also keeps the engine running much smoother and appears to provide a touch more pep. Every little bit helps. If you are not running additives in every tank to help keep water and sludge from clogging up the system, your entire system might be in need of a good cleaning as well. A very good cleaner I use every 6 months is Liqui Moly Diesel Purge.

It should be noted, that there are many other factors that can cause rough idle such as poor compression, worn timing chain and injection pump timing.

W123 Brake Booster Failure

As our cars age, components will begin to fail and fail unexpectedly. 

While driving my 1981 240D several weeks back, I lost the assistance of the power brake system. Having never experienced losing power brake assist, I wasn't quite sure of just what had happened while attempting to slow the car from 65 mph while exiting the highway.

I did know one thing for sure, it took a lot of leg muscle and putting the car into neutral to come to a comfortable stop. Once at the traffic light, I noticed a loud hissing sound coming from under the driver side dashboard area. The hissing had to have been quite loud considering it was clearly heard over the clatter of the diesel engine.

Immediately pulling off onto a side street seemed like the best idea to find out just what might have gone wrong. The brake fluid reservoir was full and no leaks were visible at the wheels, which left an issue with the master cylinder, calipers, which seemed unlikely, or the brake booster. It wasn't until I put  my head in the footwell and pressed the brake peddle with my hand, did I realize it was definitely booster and vacuum related. That was confirmed yet again, when I arrived at my destination and could not turn off the car using the key. Thank goodness for the stop button Mercedes provided under the hood to burn your fingers on.

The following day I removed the booster and shipped it to Power Brake Exchange in California for inspection and a rebuild. They have rebuilt another booster for me with excellent success and great customer service. 

Several days later, Power Brake Exchange informed me that the "poppet valve" had torn, allowing vacuum to escape and causing the lack of brake assist. Who knew?

The following diagram from ATE shows where the poppet valve is located and it's function. This diagram also shows the general layout of the W123 booster whether your car has a Girlling or ATE system. 

The only purpose for this blog was to provide a bit of advanced warning that this part will probably begin to fail more frequently in more W123's as they age. The good side however, it can be fixed, and you can stop the car provided you give it some good leg. As I mentioned, shifting to neutral also made things much easier. 

Until next time - Drive safely!