Non Door Vacuum Leak? - PeachParts Mercedes-Benz Forum

Lambrettaman · #1 01-20-2013, 06:38 PM

My newly resurrected 81 300TD has a pretty good vacuum leak. I before it attempted suicide by throwing its timing chain, It developed a vacuum leak. The door locks all move in unison, and so I think it is on the transmission side. The the pump was just rebuilt and it will hold vacuum for while.

Where do I start tracing? I just spent an hour searching and reading vacuum posts but didn't find my answer yet.

Thanks.

kerry · #2 01-20-2013, 06:52 PM

Much more likely to be in the climate control. Disconnect the green line leaving the engine compartment and see if the problem goes away.

ROLLGUY · #3 01-20-2013, 06:59 PM

A large vacuum leak can also be caused by the brake booster. If you don't have one, buy a vacuum tester (Mityvac), as it will be your best friend.

Phillytwotank · #4 01-20-2013, 07:10 PM

Year?

kerry · #5 01-20-2013, 07:18 PM

81

HammArch · #6 02-19-2013, 05:37 PM

I am in the middle of tracing a vacuum leak on my '83 #00 CD, and I first segregated the engine side of the firewall from the cabin side by disconnecting the yellow line to passenger door, fuel door and trunk. I plugged the Climate Control line, and then the nighty vac to test each of these lines. This demonstrated that the leak was in the cabin. I checked all of the line in the engine compartment and they all held vacuum as specified. I am now in the cabin testing each lock line and tomorrow will check the vacuum tank in the trunk.

if needed I can write up my process of elimination and post it tomorrow.

charmalu · #7 02-19-2013, 09:55 PM

Here is the W123 vacuum Interlock Diagram.

http://www.peachparts.com/shopforum/diesel-discussion/160169-w123-vacuum-interlock-diagram.html

Charlie

Lambrettaman · #8 02-22-2013, 04:10 PM

Quote:

Originally Posted by HammArch

if needed I can write up my process of elimination and post it tomorrow.

I would appreciate that!

Phillytwotank · #9 02-22-2013, 04:33 PM

Basically.. You're gonna want to use golf tees to block off and isolate parts of the system untill you find the leak. Gonna have to have a hand pump vacuum pump with gauge like a mityvac so you can suck it down and monitor the vac pressure.

Just go through system at a time and check for leaks.
Locks
Engine shut off
Climate control
Tranny
Vacuum resivoirs.. Will take longer to "pump down" but will hold vac all the same.
And... What else? I'm not familiar with all the systems of an '81 TD.

mikeyfev1 · #10 02-22-2013, 10:54 PM

OK, so I'm reading this thread and I remember what I was wondering when I went through my car testing for vacuum leaks.

How long do these different components need to hold vacuum?

Some door locks would hold 15psi (or is it -15 psi?) for longer than others. Is 15 (or -15psi) where I should pump the component down to? How long should the component hold vacuum steady? Is a 1 lb loss per 2 secs OK? What loss rate is or isn't OK?

For that matter do the non-driver doors even have to hold vacuum for any significant time at all? Don't they just react to the application of vacuum on one side or the other (lock/unlock)? In fact, isn't some loss even required? If I held -15psi constant on the lock line and then applied -15psi on the unlock side is the unlock side supposed to overcome the still present -15psi of the lock side? Or is it assumed that once the vacuum has been applied on one side that it is slowly lost so that vacuum applied to the other side can function?

Phillytwotank · #11 02-23-2013, 09:10 AM

Should hold vacuum forever (theoretically).

Start testing the vacuum "circuits" by testing the pump. Pull everything from the main line including the brake booster. Run the engine and meassure the vac on the main line alone (with the other holes plugged of corse).

Add the brake booster. Should still hold steady vac. A leak at the booster would be a major leak and would probably stop everything from working.

-15 psi I think I a bit low but whatever it is that you read there at the mainline is all your pump can do. Can't remember the actual spec. but it is around -20 psi I think.

After that you'll just go from circuit to circuit like I said before.

Locks for example... Pump it down to -20, turn the key to lock and everything should lock. The vac pressure will decrease as all of the actuators do work and you may need to give it a few more pumps to get everything locked but after it locks then it should hold vac again and be ready to unlock if you turn the key to unlock. The three way valve In the drivers door controls wheather the other door locks receive lock or unlock suction. If the locking circuit is not holding pressure then you'll have to to some real work to find the leak. Removing door panels, tracing the vac lines under carpets, isolating each actuator untill you Fing the leaky culprit.

Refer to a vacuum diagram of your specific year/model for more complete info. Some have vacuum circuits that others do not but all 123s have
Door locks
Vac engine shut off
Vac resivoirs
Climate control

I hope this adds to the general knowledge on the subject.

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