Beagle, Well I wont to be the first to admit that I had never considered that. That burned up car that I got, I have an idea the vacuum system was not maintained by other things I found.

I just inspected a "spare" diaphragm pump I had and I discovered It has a round, 12mm hole (with the piece right there) right above where the head of the center bolt for the diaphragm comes up. Theres a round indentation in the casting for that bolt head and it looked like it went a little to far. On the other side is the chamber with the two checks, the holes right between them Like I said, it was a spare. Thanks for the good info.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

If that is the case, it would take that much pressure differential to prevent the spring from returning the piston, would it not? Even with near-perfect vacuum, that would require a piston surface area in excess of 8 square inches. With a more realistic pressure differential, the required piston size would increase by around 50%.

I bet the spring when collapsed has a hold mechanism of some sort.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

If the piston is held forward by the pressure differential as claimed, why would there be a need for a "mechanism of some sort?"

all this talk here lately of vacuum pumps exploding has me scared. where is it? i bought the service manual cd from ebay and just went looking through it hoping it would lead me to it, but it is either named something else, or they didn't scan that section and put it on the cd.

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85 300SD 165k

Hi Beagle, is the Mercedes special tool assembly plate 115 589 14 21 00 absolutely required to rebuild the vac pump or can it be improvised?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

If you have strong hands and a helper, you can do without it all together.

I'm curious as to how the spring is held compressed also. Can it be tested by pulling a vacuum on the pump when it is off the engine? If so, can someone with a spare pump experiment with that? I've only worked on the diaphragm pump and that spring seemed quite strong. I wouldn't have expected that vacuum could suck down that spring.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Thanks for the links Beagle - is this an indication that you'll perhaps pop back and see us from time to time?

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I don't think that it is; the physics are just not there. The piston is too small and the spring is too strong. From a durability standpoint, it seems to me that having the roller glued to the cam would be preferable to having it skip around in a "hit and miss" fashion. And I suspect that is precisely why the spring is as strong as it is.

So you're saying Beagle's claim is incorrect? The mechanism is always in contact with the cam so the pump is never really at rest?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

How much does a brand new vacuum pump cost? I'm sure it's way cheaper to just replace the bearings inside.

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Never be ashamed to ask for help.

1985 300D
1987 300SDL
1970 Jaguar E-Type OTS
2002 Dodge Intrepid (Traded In)
2011 Ford Crown Victoria

I am stlll puzzled by the claim that vacuum parks the piston forward. Wouldn't this require that the piston be fitted tightly enough in the cylinder that the vacuum would not bleed down over time. Is that possible? I would think that vacuum would slowly bleed off around the piston in a relatively short period of time.
Count me as a skeptic on this claim at the moment. But I've never had one apart so my opinion should be taken with a grain of salt. Would a weakened spring in the pump cause a hammering against the cam?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The physics to support the claim are just not there. Given the small area of the piston, there is no way that atmospheric pressure is going to offset the force of the return spring. And from an engineering standpoint, not having the roller in constant contact with the cam would be highly undesirable.

So what is the consensus??? Is the pump a "continuous duty" pump? or Does it "duty cycle", based on vacuum demand. I have had one apart once, but never paid attention to this aspect. It looked to me that it might be a continuous duty pump, so vacuum demand would not make make it "work harder." Please correct my ignorance if needed, as I am not claiming to be an expert in this area or how it works.

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Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi