A quick search of this forum brought up no information on the purpose of the vacuum reservoir in my 1987 300D Turbo (W124, OM603). The reservoir is located with the coolant overflow bottle in the right wheel well. [Yes, the overflow hose is disconnected. The picture was taken a couple of years ago when I opened up the wheel well to replace the hose, which was rotten.]





The vacuum diagram shows the reservoir is connected to the 'blue flying saucer' that, with the VCV, controls vacuum to the transmission. In the following diagram, the reservoir is #9 while the blue flying saucer is #123.





Unlike W123 cars, engine vacuum is not used for door locks. I can only imagine that the reservoir helps keep the transmission vacuum steady. Maybe the transmission modulator requires more vacuum for a second or two than the pump can supply. Anyone have a better guess?

Jeremy

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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

I'm trying to figure out why it's all the way on the right. The black hose doubles reservoir volume.

Sixto
87 300D

Plug it off and see if the tranny shifts differently. For both hot and cold engine scenarios. Slam it back on and do some A/B comparos in each scenario. Document your findings here.

Be the first to post what the "double bubble" does with respect to the blue UFO.

Gotta like these technical terms we toss around here.

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Cheers!
Scott McPhee

1987 300D

Most probably the reservoir exists to damp otherwise normal fluctuations in vacuum levels. Lots of vehicles have reservoirs even though they have no need for "residual" vacuum, as would be the case with door locks.

Yup, it's there to keep the vacuum to the automatic climate control even, so the doors always do what they should and can switch quickly when the vacuum supply is lower, EG idle.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

That picture also shows the coolant overflow tank. I want to take mine out and clean it... no telling what is in there or how gross it is :S

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

I have a similar question, on my car there is a brick red vacuum line branching off main vacuum from behind the brake booster (HVAC/headrest control lines) - to the right of the car under the wiper cowl to behind the battery and disappearing into the right fender. Is this the same thing?

Mine was pretty cruddy. I posted a thread here, in fact you made a comment on it.

The overflow reservoir was not easy to clean, IIRC, there is only one small opening into it, plus a vent. Hard to get a brush into. I used hot water and various nasty chemicals, I think.

__________________

"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

Since I have vacuum reservoirs on the brain this morning, I thought I'd revive this old thread and make an observation.

In most W124 vacuum circuits, there is only ONE vacuum line going to the vacuum reservoir. http://www.w124performance.com/service/w124CD1/Program/Engine/602_603/07.1-0500.pdf While I've never seen the insides of a reservoir, my guess, from the outward appearance, is that there are two chambers....and that when the first chamber gets depleted it draws on the second chamber.

But, for years 90-91, there are TWO lines that go to the reservoir: one from the BFS area and one from the passenger side. Assuming that the reservoir design is exactly the same, I suppose that would allowing for buffering going in both directions.

So, it seems to me that, when you eliminate the vacuum rat's nest on the passenger side of the engine in connection with the wastegate swap that many of us have done, you unwittingly create a vacuum leak unless you plug or eliminate the reservoir part of the circuit.

I did the wastegate swap almost two years ago but never noticed this until today. I can only surmise that I've had a "leak" all this time without any ill effects (perhaps because of how the internals of the reservoir are designed). In any event, I've now eliminated the reservoir to eliminate the possibility of leaks from that part of the system.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I know this is a very old thread, but hopefully one of you is still around. I purchased my first W124 this summer and love the car. I do appear to have a vacuum leak and issues with the vacuum controlled doors on the climate control. I am particularly interested in the wastegate swap mentioned in one of the last posts. It appears this was used to eliminate the vacuum reservoir in the passenger fender. How can I tell if this swap was completed on my car? I have no reservoir in the passenger fender and no vacuum lines in that area. Only the place the reservoir should be mounted.

Find the wastegate actuator. What's connected to it: a vacuum line or a pressure line? If it's the former, then the swap has not been done; if the latter, then it has been done.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

What model year 300D do you have?

Sixto
98 E320 198K miles

The reservoir acts like a capacitor, or maybe a battery is a better comparison, which will tend to keep the vacuum more stable during operation.

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1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

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