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If you already own a Mercedes W123, 240D it will be no secret that vacuum is used everywhere. Despite the fact that this car, in my opinion, is one of the greatest cars ever built, both in a timeless look and mechanical longevity, they do indeed have their challenges and shortcomings.

One of the biggest issues with the older diesel models, were the yards of hard plastic vacuum line and the dozens of rubber connecters from front to back and side to side and a vacuum pump. While the products used have truly held up amazingly well over the nearly four decades, it becomes a nightmare when so many devices fail to operate and some times very suddenly.

There will be no secret when you have a part or parts of your system begin to leak. A number of items will fail to function when vacuum is lost or is in short supply. They are as follows: your brakes, central locking system, not being able to shut the motor off by way of the key, cruise control does not hold, and the transmission doesn’t know when to shift. Mercedes did plan for such catastrophe however on several fronts by allowing the driver to manually lock all the doors and trunk, but not the gas flap, and shut down the motor by the metal “tab” labeled “STOP” to the side of the valve cover.

When vacuum is low or absent as just mentioned, you will likely face very erratic shifting. For example. On the W123, the transmission is regulated by a vacuum module that dictates when the transmission is to shift based on load. Should you notice that all of your shifts are delayed, especially 1st to 2nd and the trans. wants to slam into gear when it does shift, you are most likely facing either a failed transmission vacuum module or a broken vacuum line or failing/failed vacuum pump.

Now it’s time to test a few systems for vacuum and two great tools to help are a hand brake bleeder and a Fuel-O-Vac meter.

First step is to make sure you are getting the proper vacuum from the vacuum pump. Disconnect the power brake booster line from the vacuum pump line. Start the car and connect your Fuel-O-Vac with the appropriate rubber fitting to the vacuum pump line, making a tight seal.

When you are certain you have a good seal between the gauge and line, increase your idle speed slightly by way of the “STOP” lever and note what the vacuum is reading. If you are getting a steady 18 to 25hg, the pump is performing as it should. You can also feel a good bit of suction on your finger if you cover the line. Should the vacuum pump return a bouncing needle on the gauge or low vacuum you will need to either rebuild your pump or purchase a new unit. At this point you can shut down the engine.

If the pump checks out, you will next begin going through each vacuum line that feeds through the firewall and to the transmission. Here is where your hand brake bleeder will come in quite handy. First examine all of your lines for dry rot and cracks. If none are detectable, begin a methodical process of removing one line at a time and testing that line for a leak.

Once you can assure a good seal has been made, begin to vacuum down that line. A few pumps should do it. IMPORTANT: Do not over pump as there are delicate diaphragms at the end of those lines that can only take so much vacuum. If you are able to record a vacuum on the gauge, watch it carefully to see that it holds. If the vacuum begins to bleed down, you ether have a bad vacuum line, or an improper connection on your hand held pump. Recheck all connections and do the procedure again. Should you continue to get the same results you either have a cracked line, rubber connector or module (valve). If that is the case, you will need to trace that line, locate all the potential connectors on that line and replace the entire line and connectors. If your system is still original, you will notice the various colors of vacuum tubing. I don’t believe you can get the “OEM” tube any longer as I understand it from the Mercedes Classic Center and my local MB dealer. It truly doesn’t matter however, just be sure you get the correct size tubing for the specific application, both I.D. and O.D.

The parts image shows a number of the components that can go bad in our 30+ year old cars (not to scale). If any of the parts leak vacuum, your entire system will be effected as the system is not able to store up vacuum for later use in the door locks. If the leak is too bad, it will effect your brakes, transmission and shutting of the engine.

Parts to check: Vacuum pump, vacuum tank, main line from pump to booster, 3/2 way valve, all check valves, all vacuum modulators, door, trunk, fuel locking mechanisms, all rubber connectors, VCV valve, transmission modulator, fuel shut-off valve, and the ignition switch vacuum lines or vacuum unit itself.

Most of the vacuum parts are all plastic, and while good quality, they have been exposed to decades of heat and chemicals and have become brittle and have either broken or are on there way to breaking. Even the slightest leak in your vacuum system will most likely cause immediate problems.

My 3/2 way valve for example, while not cracked, was stuck open and the plastic tab that the cam slides against to open the valve was missing. This was either leaving the transmission with full vacuum at all times or none altogether, because it was clogged with dirt. As the photo indicates, the 3/2 way valve, is opened when the throttle is depressed and subsequently sends vacuum to the VCV (vacuum control valve). This valve is designed to “bleed” off any excess vacuum based on load the transmission vacuum modulator does not require at that time. The VCV is in turn linked to and regulated by the throttle linkage. If this part is not functioning properly you will have transmission shift issues and you will need to replace or adjust it.

The engine image below, shows a general overview of a 1981 240D vacuum line path. This car is equipped with A/C adding a few extra lines. The image above shows the connection points at which you should check your lines for failures.

This is a general over view of a 1981 Mercedes-Benz 240D with A/C. Always check your owners manual before you determine how your particular model is to be set up. The 300 series with five cylinders and the turbo models have a different vacuum line configuration so do not assume they are all the same. Unfortunately too many random repairs have been made to these cars over the years and can be set up completely wrong. Even taking pictures before you start could prove useless if you are simply photographing the wrong set up.

In a later article I will discuss some of the specifics of the vacuum system as it relates to specific components such as heat, A/C, and door locks etc. Similar vacuum issues will be discussed with regard to the 450SL in the not so distant future.

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Written by :
West Kenyon

West Kenyon is a Multi-Media Design Consultant and automotive enthusiast. Cars in his collection include a 1956 Austin FX3-D, better known as a London Taxi, a 1965 Rolls-Royce Phantom V, and a 1975 450SL and 1981 240D by none other than Mercedes-Benz. West said, “owning one or a number of vintage cars is often an expensive proposition", which is why he does the majority of mechanical and interior work/restoration himself. West added, “not only does it save a significant amount of money to do the work yourself, it also provides full opportunity to know the job is being done with excellence and allows for an unprecedented appreciation and knowledge of the car.” For FCPEuro, West will focus on providing in-depth information on both the Mercedes-Benz R107, 450SL, D-Jetronic version and the W123, 240D. Glücklich Autofahren! (Happy Motoring)


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