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#1
12-08-2006, 03:12 PM
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How to use vacuum gauge?
I bought a vacuum gauge and hooked it up to some lines I thought might be the problem with my vacuum leak.
The gauge built up vacuum and held it, it seemed. Some lines took more pumping to get up to vacuum. How do tell if you have a leak using this gauge. I'm not stupid, just haven't used a vacuum gauge before. The car won't shut off sometimes, won't start sometimes, and the locks don't hold vacuum overnight. EDIT: 1984 300TD Thanks for any info...
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'84 300TD 100 % Biodiesel Last edited by whunter; 12-08-2006 at 03:43 PM. Reason: year and model added, spelling. 
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#2
12-08-2006, 03:37 PM
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Quote:
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-justin 1987 300TD, 1987 300TD 2008 R32, 2000 Passat Wagon
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#3
12-08-2006, 03:49 PM
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Answer:
Quote:
Here is the help you need. Vacuum Diagrams W123 and W126 models W123 Vacuum InterLock Diagram Fast navigation Do It Yourself Links http://www.peachparts.com/shopforum/diy-links-parts-category/146034-fast-navigation-do-yourself-links.html#post1314064 Have a great day.
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ASE Master Mechanic asemastermechanic@juno.com Prototype R&D/testing: Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician. Noise Vibration and Harshness (NVH). Dynamometer. Heat exchanger durability. HV-A/C Climate Control. Vehicle build. Fleet Durability Technical Quality Auditor. Automotive Technical Writer 1985 300SD 1983 300D 1984 190D 2003 Volvo V70 2002 Honda Civic https://www.boldegoist.com/
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#5
12-08-2006, 06:24 PM
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Vacuum gauges
I have two vacuum gauges (eBay cheap) permanently installed in my '85.
One is in the engine compartment; it monitors the vacuum in the line that (among other things) goes to the auto tranny. This gauge will show you if the vacuum pump is working and if the leaks (that make the vacuum line approximate what the transmission expects to see from a gasser so M-B can use the same tranny with both types of cars) are working correctly. That is, the vacuum goes down when you press on the accelerator. That gauge runs at 10-15" Hg depending on the throttle position. If that vacuum isn't correct, the transmichigan won't shift correctly. The other vacuum gauge is back in the trunk, teed into the line to the egg-carton vacuum reservoir (in the 'roof' of the trunk compartment between the lid and the fuel tank). That gauge gets up to about 17" Hg and gradually goes down as the door locks are cycled until it finally runs out of vacuum. Watching that gauge day to day (not running the car) tells you if there is a vacuum leak in the door lock circuits. You could also put this gauge in the engine compartment but I didn't know which line to cut. The trunk was more obvious and I'm lazy. The accurate measurement of absolute vacuum isn't as important as a precise relative measurement -- comparison of day to day changes or changes when you do something to the car. See my thread "Vacuum gauge fun" for more info and pictures. Vacuum gauge fun
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 "Buster" in the '95 Our all-Diesel family 1996 E300D (W210) . .338,000 miles Wife's car 2005 E320 CDI . . 113,000 miles My car Santa Rosa population 176,762 (2022) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970
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#6
12-08-2006, 06:38 PM
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Quote:
You need to study those vacuum diagrams and more importantly, read an re-read the threads on troubleshooting vacuum problems.
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-justin 1987 300TD, 1987 300TD 2008 R32, 2000 Passat Wagon Last edited by justinperkins; 12-08-2006 at 06:39 PM. Reason: be more clear
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#7
12-08-2006, 11:37 PM
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If you have a Mityvac or equivalent.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92474 With this tool you want the red rubber line that is shown, or equivalent, and a small piece of the hard plastic lines that go through the Mercedes vacuum system. These 2 pieces will allow you to connect the vacuum gauge to the various parts. You can do 2 basic tests. 1. Test the output of the vacuum pump. I do this by disconnecting the large black hose from the vacuum pump to the brake booster. Disconnect it at the brake booster. Connect the Mityvac to one of the small nipple lines coming off the large black line. Disconnect the other small line and block it ( golf tees work well ) Turn on the car and place your thumb over the opening at the brake booster connection hole of the large black line. This will give you a true reading of the vacuum pump output ( or input  )the pump should pull about 23-25 inches of mg. 2. Now if you have a good vacuum pump then you check all the other components. How much you have to pump the Mityvac depends on the volume of air that the part holds ( the large tank in the trunk requires quite a bit of pumping. A door element 4-5 pumps and you know if it's OK) . In checking vacuum just get any amount of vacuum and see if the needle does not move. If it doesn't then it is a good element or line. As you check the AC/DOOR lock elements. This test will let you know what is, and is not holding vacuum. IE the diaphragm in the part is torn. Trying to check transmission vacuum is a different sort of trouble shooting. You will connect it and while driving check the operating vacuum and it's variations. Hope this helps. Dave
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1970 220D, owned 1980-1990 1980 240D, owned 1990-1992 1982 300TD, owned 1992-1993 1986 300SDL, owned 1993-2004 1999 E300, owned 1999-2003 1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD 1988 560SL, 100,000mi, owned since 1995 1965 Mustang Fastback Mileage Unknown(My sons) 1983 240D, 176,000mi (My daughers) owned since 2004 2007 Honda Accord EX-L I4 auto, the new daily driver 1985 300D 264,000mi Son's new daily driver.(sold) 2008 Hyundai Tiberon. Daughters new car
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