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Old 11-15-2009, 11:12 AM
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Vacuum 101? - Beginner needs help

I have a 77 450SL that I suspect has some vacuum problems (heater etc). I've searched the threads in this forum as well as looking at the MB Maintence CD and Haynes Manuals for info on how to trouble shoot vacumm systems. There is lots of info but it seems like you have to pretty knowledgeable on where the parts are located and how to do the tests with tools like Mighty Vac (which I have but ony used for brake bleeding).

What I am looking for is some very basic instruction on the vehicle's vacuum system - what and where are the main vacuum components, how does the vacuum system work, What are the indications of a vacuum problem for various systems, how do you use a Mighty Vac for troubleshooting, etc.

I'd appreciate if anyone could point me to some articles or threads that would be helpful for a beginner in troubleshooting the vacuum system.



77 450SL
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Old 11-15-2009, 01:03 PM
450slcguy's Avatar
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Originally Posted by 77450slowner View Post
What I am looking for is some very basic instruction on the vehicle's vacuum system - what and where are the main vacuum components, how does the vacuum system work, What are the indications of a vacuum problem for various systems, how do you use a Mighty Vac for troubleshooting, etc.
There are three specific vacuum systems on the car. Engine, Climate control, and locks. Each is a seperate system. The climate control system is the most difficult by far. It's quite a complicated and neck/back breaker to work with under the dash. The door, trunk, and gas cover locks are less involved. The CD deals with these in seperate chapters, each with their own diagrams, diagnostics, and components.

The first thing you need to do is print out and study a copy of the engine and climate control vacuum diagrams from the CD. There are no magical pieces of information or advice that will enlighten you better than the factory CD. The CD has every engine and climate control component location and function explained and documented, you just need to disect and study it.

As for the climate system, many of us have spent countless hours studying the various components and workings of it. To my knowledge, it is likely the most complicated climate control system ever engineered. The most common problems are leaking vacuum flap actuators and leaking vacumm fittings. The CD has complete diagnostic procedures, component locations, vacuum and electrical diagrams.

I'd start with the engine chapters first, as that is the simplest and most important. Once the car is running well, the rest are secondary in importance.

After you have reviewed the CD thoroughly, pop the hood and remove the air cleaner. Match up a few key components, say start with the Warm-up regulator, then start tracing the colored plastic vacuum lines against the vacuum diagrams. Just like a road map, follow the flow. Keep referring back to the CD and the diagram. This pertains to both engine and climate control systems.

After you have done all that, go to your local parts store and buy about 5-10 feet of rubber vacuum hose. You'll also need to buy several 3 and 4 way rubber vacuum splitters(referred to as distributors on the CD). You can get these online or the dealer. Also, be carefull with the plastic tubing, sometimes they get very brittle and will snap if bent. Probably a good idea to obtain several feet of plastic tubing at the same place you get the rubber splitters(distributors), sold by the foot. Don't worry about the colors to much, just buy a foot or two of several different sizes(mm's). Everything can be had for about $30-$40 dollars.

When you have all the supplies, start tracing all the plastic colored tubes in the engine compartment, same for climate control. They'll all be terminated with rubber vacuum hose connected to the various components. Pull off the old rubber and cut the new rubber hose to the same length as the old one and replace them. Do one hose at a time. The 3-and 4 way vacuum splitters can just be swapped out with new ones. Just make sure everything is copied the same way it came off and be sure they matche the vacuum diagram.

Replace every piece of rubber vacuum hose you can locate, even if it looks good. Rubber vacuum line is very cheap and it takes only a few moments to cut and replace a piece.

Go online with Mighty-Vac and read the operating instructions, their quite simple.

For now don't get all hung up on trying to understand the specifics on how all the various systems work, that will come with time, research, and need to know. Repalce most of the 30+ year old vacuum rubber. That alone will solve many of the most common problems these cars have.
Question Authority before it Questions you.

Last edited by 450slcguy; 11-15-2009 at 04:55 PM.
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Old 11-15-2009, 11:25 PM
nobby's Avatar
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Join Date: Aug 2005
Location: Barrie, Ontario
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Some info here.
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