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Pierre Hedary

Understanding the 1985 Mercedes-Benz diesel vacuum system

The other day, as I was working on a 1985 300CD, I began to wonder if an in-depth article about the Mercedes-Benz 1985-only diesel vacuum system had ever appeared in The Star. After looking through my library, I found very little information about the  Mercedes-Benz 1985 W123 and W126 series automobiles. I am sure there are skeptics wondering “why the heck would anyone care about the vacuum system on an old ‘85 300 diesel." 
For those of you who can’t relate, here is why this matters: Most aspiring W123 and W126 diesel owners want a 1985 model year example. They had more power, shifted better, ran better, and they are the last year of production (whatever that counts for as an advantage). There's only one drawback: these vehicles had a more complex vacuum system. This system is so poorly understood that I have only seen a few that were hooked up correctly. While we don’t have the space to go over every single line connection, we’ll cover all of the major components in the system. 
Major changes for 1985
The vacuum system on the W123 turbodiesel was re-engineered for 1985. Gone were the little plastic valves with delicate flaps on the valve cover. The transmission now used a device called a pressure converter to create seamless gear changes. The exhaust gas recirculation (EGR) valve was now electronically controlled, and to help improve turbo lag and emissions an air flap was incorporated into the turbo. These were controlled by an EGR relay in the right fender, accessible from the interior, as well as a new over-voltage relay, mounted inside the dash to the right of the glovebox insert. Almost every item on ‘85 model year cars was redesigned or repurposed, with the vacuum pump and brake booster/vacuum distribution line being the exception.   
Transmission pressure converter
The new pressure converter was mounted on the left inner fender, taking the place of the somewhat wear-prone valve cover vacuum control system. The pressure converter used turbo boost (taken from a line attached to the ALDA – German acronym for altitude compensator – on the top of the injection pump), EGR input and atmospheric pressure to modulate vacuum going to the transmission. It is a simple, durable part with no adjustments (that I know of) and with the right extra parts it can be adapted to earlier models. I have seen these leak from their casing, so if you have an ‘85 with strange vacuum issues, this is worth checking.
If you look closely at the converter, you will see the words “TRA” and “VAC.” If you are chasing an issue, the “VAC” port should have about 9-12 inches of vacuum with the engine running. The “TRA” port should hold vacuum perfectly, as it goes straight to the modulator on the transmission. 
EGR Valve and turbo air flap
Some of us still need to have an EGR valve to pass emissions. This valve used to be controlled by a simple thermo-vacuum switch but this is no longer the case. Instead, a temperature switch on the cylinder head tells the EGR relay to open the respective vacuum control valve on the inner left fender, and vacuum is sent to the EGR. This system works with the turbo air flap, because the air flap was originally designed to help draw exhaust gases into the intake. The turbo air flap is a simple diaphragm on the side of the turbo (see image 4, page 80) that helps alter the air current inside the short distance between the turbo and the air pipe. Even without the EGR working, it is applied under acceleration by a rack position sensor on the injection pump to help create better bottom end torque. 
What you should note here is the EGR failure mode. Low power, black smoke, awful fuel economy, stumbling or stalling off the line and rough starting are all signs the EGR may be stuck open or may be getting a signal to stay open when it is not supposed to. For testing, you can simply disconnect and plug the EGR vacuum line. 
Failure of the turbo air flap control is more subtle, but like an EGR leak, it can produce a subtle vacuum drain, especially  when under throttle. 
Injection pump vacuum control valve
Remember the little white vent valve on the back of the injection pump? Well, it is still there, but instead of dumping excess vacuum for the transmission circuit, it now dumps excess vacuum for the pressure converter, EGR and turbo air flap control. The vacuum control valve on the ‘85 can still be linked to shifting abnormalities, but not as obviously as it was on older W123, 126 and 116 models
Interior vacuum controls
On the W126 and 123 the interior vacuum controls are mostly untouched. The 126 still uses a connection in between the two firewalls for the climate control (red and green) and engine shut off (brown and blue).
The 123 still uses the same layout: Base yellow for locks, with a gray and yellow for the reservoir, green for the climate control and brown for the shut-off control. Coupes also have a blue line for the seat-back lock. The complication on the W123 is that the interior vacuum systems are also tied to the EGR and pressure converter systems, making finding a leak anywhere in the system a time-consuming process. 
Troubleshooting for leaks
I felt a good addition to this article would be a leak testing guide. Low vacuum can cause hard shifting, unresponsive vacuum locks and climate controls and slow shut off. 
First, you will need the 1985 Introduction to Service manual to get a good explanation and diagram of the vacuum system. Second, you will need a vacuum gauge and some golf tees. 
The best way to troubleshoot a vacuum system is by working backwards until you find a strong vacuum supply, and then branching out until you find the vacuum drain. 
Here's an example: Let's say a 1985 300D has a rough shift, with banging on upshifts and downshifts. This is certainly a vacuum issue, so I would start by plugging my gauge into the “VAC” port on the pressure converter with the engine running. If the gauge reads less than nine inches of vacuum (let’s say it reads 2-3 inches), then we do indeed have a vacuum supply issue. Now I must work backwards. I must go to the point of origin for the “VAC” line, which is the vacuum booster line. 
If I hook up my gauge there, I get a reading of eighteen inches. Not bad, but still a bit low. At this point, I clean out the little orifices where the rubber lines connect to the booster line, as they can get blocked with crud from the EGR system. A retest shows 19 inches at the orifice on the main booster line. An improvement, but not by much. So now I must block off individual vacuum lines, one by one, while looking for a vacuum drain. 
Then I move my gauge back to the “VAC” port, since I know the vacuum pump is working. I get to the EGR line, remove it and block it off, and my gauge shoots up. Here is a smoking gun. The gauge on the “VAC” port produces a reading of 10 inches. This is better, and now I can isolate the issue to the EGR circuit! Working backwards to the EGR vacuum controller on the left inner fender, I see that one of the connectors is damaged. While repairing it does not yield much of a result, I notice the inside is dirty. 
This tells me something is contaminating the system, namely EGR soot. I reconnect the EGR line at its origin, and plug it at the EGR valve itself. Presto – vacuum levels at “VAC” port are good now, with 10-11 inches as long as the EGR vacuum line at the EGR valve itself is plugged. 
I used this example to demonstrate that most vacuum issues on 1985 model year cars are EGR issues, with the EGR valve either getting vacuum when it should not, or with the diaphragm in the valve rupturing. Any worn vacuum connectors should be replaced with the correct part from Mercedes (and not generic silicone vacuum tubing!). All vacuum issues are diagnosable with a diagram and a gauge, and by plugging off different ports until the vacuum drain is found.
A unique system
For all its complexity, the 1985 vacuum system is a thing of wonder. It took the Mercedes-Benz W123 and W126 diesels from firm and rigid shifting to flexible and smooth shifting, with torquey acceleration uninterrupted by gear changes. So, if you do not own one of these cars yet and are wondering about buying one, yes, it is definitely worth the extra trouble.