VACUUM PUMP FAILURE! Are you neglecting yours?? - PeachParts Mercedes-Benz Forum

vstech · #1 06-20-2012, 09:04 PM

Quote:

Originally Posted by qwerty

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.

qwerty · #2 06-20-2012, 09:40 PM

Quote:

Originally Posted by vstech

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

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

funola · #3 06-20-2012, 10:46 PM

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?

qwerty · #4 06-20-2012, 11:13 PM

Quote:

Originally Posted by funola

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?

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

mach0415 · #5 06-21-2012, 10:58 PM

Quote:

Originally Posted by qwerty

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

I would think that since it takes this to do that, given the surface area of the diaphragm, it would take a hell of a lot of negative atmospheric pressure to hold it off the cam during times of less "demand." I am thinking about single and dual diaphragm brake boosters and the surface area of the diaphragms in them to produce power assist from an already 5:1 lever in the brake pedal. Given the size spring in the pump, the diaphragm should be bigger to hold that puppy off.

whunter · #6 06-23-2012, 08:09 PM

Quote:

Originally Posted by funola

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?

Not required, and/or you can make your own if desirable.

.

whunter · #7 06-23-2012, 08:08 PM

Quote:

Originally Posted by vstech

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

There is none.

.

vstech · #8 06-23-2012, 08:10 PM

Quote:

Originally Posted by whunter

There is none.

.

yeah... I think we are realizing this. I wonder what Beagle was thinking about this system?

I do think, that if the pump is constantly flowing air due to a leak, damage could ensue.

whunter · #9 06-23-2012, 08:53 PM

Quote:

Originally Posted by vstech

yeah... I think we are realizing this. I wonder what Beagle was thinking about this system?

I do think, that if the pump is constantly flowing air due to a leak, damage could ensue.

No complaint, my best guess; he was trying to figure it out.

The prime issue is particulate intrusion effecting valve sealing, and scoring the piston cylinder.

The OM617 piston pump vents the excess air - particulate into the crankcase, not desirable in mass quantity..

This is a prime reason why I constantly recommend capping the central locking and climate control, until new owners are prepared for the diagnosis - repair.

Climate Control:

I leave it for others to add links on central locking diagnosis - repair, as there are thousands of threads on the topic.

.

Beagle · #10 06-23-2012, 09:23 PM

Quote:

Considering how few vacuum pumps will pull 25" Hg, the claim is probably as irrelevant as it is dubious.

There seems to be a general misunderstanding of what I meant by the pump “parking itself”. The dynamics of it are really very basic school stuff. My own analysis comes from eight years as a Development Engineer in Perkins Engines R & D Test Shop and over 40 years in the F/I business, not out of the FSM or Google.

The facts are:

1) The piston is 70mmØ. That is 6in².
2) The stroke of the piston is 10mm.
3) The strength of the spring determines the level of vacuum up to what is practically achievable.

An absolute vacuum (hypothetical) would exert a forward pull of about 90lbs on the piston rod. This determines the max. strength for the spring, 90lbs to compress 10mm(length of stroke), and achieve equilibrium (i.e. park it) at a hypothetical 29.92”hg. A stronger spring would achieve nothing, just overload the mechanism and bearings and cause the pump to stroke vainly and continuously. It would be idiocy to design a pump to run continuously that is only required to work for around 5% of the time the engine is running.

A pump can be set to park at any chosen level of vacuum by adjustment of the spring strength. For example - a spring set to 45lbs to compress 10mm would park at about 15”hg. The pump starts pumping at full stroke and as the vacuum builds, the stroke progressively shortens until the vacuum pull from the piston equals 45lbs (+/-15’’hg) and equilibrium prevents the spring from pulling the piston back. There it will park and be held fully forward until vacuum falls below 15"hg allowing the spring to lower the rocker gently into the cam again. To park at 25" the spring strength is calculated to balance the vacuum "pull" at that level. It’s not rocket science!

If you drain the vacuum system, start the engine and listen with a stethoscope to the pump you should clearly hear the thump- thump of the pump on full stroke change to a tick-tick over 30 sec or so as the rocker lifts out of the cam and the roller just nicks the cam crests.

My own car still topped out at 25” recently and I am at over 2000ft here. I had changed the plate valves and bearings twice since I bought the car.

Quote:

It takes 57kg to compress the rocker to a full stroke...

This is not correct and a guess I made off the top of my head just to illustrate a point. 35kg would have been closer to the mark – Apologies.

Well that’s it folks – I’m outa here. Once again - Adios Amigos!

mach0415 · #11 06-23-2012, 09:47 PM

Quote:

Originally Posted by Beagle

There seems to be a general misunderstanding of what I meant by the pump “parking itself”. The dynamics of it are really very basic school stuff. My own analysis comes from eight years as a Development Engineer in Perkins Engines R & D Test Shop and over 40 years in the F/I business, not out of the FSM or Google.

The facts are:

1) The piston is 70mmØ. That is 6in².
2) The stroke of the piston is 10mm.
3) The strength of the spring determines the level of vacuum up to what is practically achievable.

An absolute vacuum (hypothetical) would exert a forward pull of about 90lbs on the piston rod. This determines the max. strength for the spring, 90lbs to compress 10mm(length of stroke), and achieve equilibrium (i.e. park it) at a hypothetical 29.92”hg. A stronger spring would achieve nothing, just overload the mechanism and bearings and cause the pump to stroke vainly and continuously. It would be idiocy to design a pump to run continuously that is only required to work for around 5% of the time the engine is running.

A pump can be set to park at any chosen level of vacuum by adjustment of the spring strength. For example - a spring set to 45lbs to compress 10mm would park at about 15”hg. The pump starts pumping at full stroke and as the vacuum builds, the stroke progressively shortens until the vacuum pull from the piston equals 45lbs (+/-15’’hg) and equilibrium prevents the spring from pulling the piston back. There it will park and be held fully forward until vacuum falls below 15"hg allowing the spring to lower the rocker gently into the cam again. To park at 25" the spring strength is calculated to balance the vacuum "pull" at that level. It’s not rocket science!

If you drain the vacuum system, start the engine and listen with a stethoscope to the pump you should clearly hear the thump- thump of the pump on full stroke change to a tick-tick over 30 sec or so as the rocker lifts out of the cam and the roller just nicks the cam crests.

My own car still topped out at 25” recently and I am at over 2000ft here. I had changed the plate valves and bearings twice since I bought the car.

This is not correct and a guess I made off the top of my head just to illustrate a point. 35kg would have been closer to the mark – Apologies.

Well that’s it folks – I’m outa here. Once again - Adios Amigos!

Thanks Beagle. That explains a lot in my mind. I see what you mean and how it works more clearly now.

whunter · #12 06-24-2012, 08:24 PM

Quote:

Originally Posted by Beagle

There seems to be a general misunderstanding of what I meant by the pump “parking itself”. The dynamics of it are really very basic school stuff. My own analysis comes from eight years as a Development Engineer in Perkins Engines R & D Test Shop and over 40 years in the F/I business, not out of the FSM or Google.

The facts are:

1) The piston is 70mmØ. That is 6in².
2) The stroke of the piston is 10mm.
3) The strength of the spring determines the level of vacuum up to what is practically achievable.

An absolute vacuum (hypothetical) would exert a forward pull of about 90lbs on the piston rod. This determines the max. strength for the spring, 90lbs to compress 10mm(length of stroke), and achieve equilibrium (i.e. park it) at a hypothetical 29.92”hg. A stronger spring would achieve nothing, just overload the mechanism and bearings and cause the pump to stroke vainly and continuously. It would be idiocy to design a pump to run continuously that is only required to work for around 5% of the time the engine is running.

A pump can be set to park at any chosen level of vacuum by adjustment of the spring strength. For example - a spring set to 45lbs to compress 10mm would park at about 15”hg. The pump starts pumping at full stroke and as the vacuum builds, the stroke progressively shortens until the vacuum pull from the piston equals 45lbs (+/-15’’hg) and equilibrium prevents the spring from pulling the piston back. There it will park and be held fully forward until vacuum falls below 15"hg allowing the spring to lower the rocker gently into the cam again. To park at 25" the spring strength is calculated to balance the vacuum "pull" at that level. It’s not rocket science!

If you drain the vacuum system, start the engine and listen with a stethoscope to the pump you should clearly hear the thump- thump of the pump on full stroke change to a tick-tick over 30 sec or so as the rocker lifts out of the cam and the roller just nicks the cam crests.

My own car still topped out at 25” recently and I am at over 2000ft here. I had changed the plate valves and bearings twice since I bought the car.

This is not correct and a guess I made off the top of my head just to illustrate a point. 35kg would have been closer to the mark – Apologies.

Well that’s it folks – I’m outa here. Once again - Adios Amigos!

I am modeling this, and will test it on an engine shortly.

On a new or low mile vacuum pump attached to a good sealed vacuum system, this sounds correct.

Sadly many systems leak, and the vacuum pump is tired.

.

funola · #13 06-24-2012, 09:35 PM

Quote:

Originally Posted by whunter

I am modeling this, and will test it on an engine shortly.

On a new or low mile vacuum pump attached to a good sealed vacuum system, this sounds correct.

Sadly many systems leak, and the vacuum pump is tired.

.

May I suggest:

1. Start with a brand new vac pump, ideally a piston type.

2. Drill a hole on the casing such that a borescope can be inserted, focusing on the cam and cam follower and shoot a video of the test.

Seeing is believing!