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  #46  
Old 11-22-2015, 07:24 AM
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MTU, I hope you read my balanced posts.

Does anyone have a diagram and description on how the SBC system works?

From what I've seen so far, the SBC pump actuates the brakes when you push the pedal and not the master cylinder. Apparently there is a master cylinder connected to the pedal that applies the brakes if the SBC system fails. Someone posted DOT requirements for pedal pressure / decel rate in the event of power assist failure and I'd expect a "failed" SBC to meet that.

My question is: If the SBC system fails, is the car any more difficult to stop than a traditional vacuum booster system.

Also, in the late 80's, GM has a psudo non active SBC system where a electric pump moving brake fluid provided power assist through what looks like a hydraboost system. Other than accumulators failing, there was never a big panic over this system that I'm aware of.

( Hydraboost is a power steering pump driven brake power assist that is very common on 1 ton and up trucks and somewhat common on some select cars. Hydraboost is great, once you take up a bit of pedal slack, the pedal more or less stops moving. It responds to pedal _pressure_ not motion. )
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  #47  
Old 11-22-2015, 09:08 AM
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Here's how it's SUPPOSED to work.

https://www.youtube.com/watch?v=RtWU17NLgfA
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  #48  
Old 11-22-2015, 09:58 AM
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Quote:
Originally Posted by 97 SL320 View Post
MTU, I hope you read my balanced posts.

Does anyone have a diagram and description on how the SBC system works?

From what I've seen so far, the SBC pump actuates the brakes when you push the pedal and not the master cylinder. Apparently there is a master cylinder connected to the pedal that applies the brakes if the SBC system fails. Someone posted DOT requirements for pedal pressure / decel rate in the event of power assist failure and I'd expect a "failed" SBC to meet that.

My question is: If the SBC system fails, is the car any more difficult to stop than a traditional vacuum booster system.

Also, in the late 80's, GM has a psudo non active SBC system where a electric pump moving brake fluid provided power assist through what looks like a hydraboost system. Other than accumulators failing, there was never a big panic over this system that I'm aware of.

( Hydraboost is a power steering pump driven brake power assist that is very common on 1 ton and up trucks and somewhat common on some select cars. Hydraboost is great, once you take up a bit of pedal slack, the pedal more or less stops moving. It responds to pedal _pressure_ not motion. )
Yes it is more difficult than a vacuum booster system.

The emergency backup master cylinder is small and doesn't give much pressure multiplication like a full size master cylinder does. It doesn't give as much braking force as a standard system so it takes DRASTICALLY longer to stop the car.
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  #49  
Old 11-22-2015, 01:16 PM
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Quote:
Originally Posted by mannys9130 View Post
The emergency backup master cylinder is small and doesn't give much pressure multiplication like a full size master cylinder does. It doesn't give as much braking force as a standard system so it takes DRASTICALLY longer to stop the car.

Actually, a small bore master cylinder will generate Higher hydraulic pressure than a large bore MC given the same force on the piston.

Any specs on MC diameter? Pedal ratio would also come into play so there might not be a 1 to 1 SBC / Vac boost correlation.

The u tube vid won't work for me because I'm on a slow connection.
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  #50  
Old 11-22-2015, 02:49 PM
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Quote:
Originally Posted by mannys9130 View Post
Yes it is more difficult than a vacuum booster system.

The emergency backup master cylinder is small and doesn't give much pressure multiplication like a full size master cylinder does. It doesn't give as much braking force as a standard system so it takes DRASTICALLY longer to stop the car.
So I can understand exactly what you mean, when you say "more difficult than a vacuum booster" you DO mean a FAILED vacuum booster?
Just want to get an idea exactly what to expect if I ever experience this situation. Which would be unlikely, since I don't plan to acquire a used Mercedes new enough to have SBC.

Happy Motoring, Mark
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  #51  
Old 11-22-2015, 04:06 PM
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For all practical purposes it is like having no brakes. Certainly there will be a little effect but usually not enough to save the situation.

Ninety percent of our general driving requires at least fifty percent of the braking force remaining. Seldom do I feel that is the real case in many cars when the primary system has difficulties.

The other item is usually you get a little warning something is not right. This has saved me a few times over the years.

I suspect this particular Mercedes system gives no worthwhile warning. All of a sudden it just is not there. You may still have the contents of the master cylinder but unlike some situations it is again of little use. Even if a dash light comes on it takes time to process the information.

The brake pedal may even feel normal and this requires more time again before any action is taken. It seldom feels normal in typical systems when they have a real problem.
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  #52  
Old 11-22-2015, 04:18 PM
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Quote:
Originally Posted by 97 SL320 View Post
Actually, a small bore master cylinder will generate Higher hydraulic pressure than a large bore MC given the same force on the piston.

Any specs on MC diameter? Pedal ratio would also come into play so there might not be a 1 to 1 SBC / Vac boost correlation.

The u tube vid won't work for me because I'm on a slow connection.
While the multiplication is still there, the small fluid volume doesn't push the caliper piston far or hard enough before reaching maximum travel. It is a very undersized MC. Really, it's meant to be used only once. It's there to stop the car at the moment the SBC fails, and MB assumes at that point you'd call a tow truck and have the SBC repaired. It's like a handbrake.
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  #53  
Old 11-22-2015, 04:20 PM
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Quote:
Originally Posted by Mark DiSilvestro View Post
So I can understand exactly what you mean, when you say "more difficult than a vacuum booster" you DO mean a FAILED vacuum booster?
Just want to get an idea exactly what to expect if I ever experience this situation. Which would be unlikely, since I don't plan to acquire a used Mercedes new enough to have SBC.

Happy Motoring, Mark
Yes, a failed vacuum booster with no assist. Even with no assist, a standard vac boosted MC still has the ability to apply the same braking power if the human behind it can push hard enough. Even without pushing as hard, it will still stop the car in a reasonable distance if you stand on the pedal.
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  #54  
Old 11-22-2015, 04:24 PM
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There is a bit of information and posts of SBC failures on BW in the w211 forum. Barry12345 has it correct- it's like having no brakes at all; the car stop only slightly better than coasting and the only "warning" you get is at the time of failure is the red light on the cluster. That's the limited agreement of w211 owners over there.
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  #55  
Old 11-22-2015, 06:42 PM
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My complaint with the SBC system is that this is not a FAILURE of a part, it is a shutdown of the system when the "service threshold" is reached. MB should issue a recall, interrogate the computer for number of activations and tell the owner how close to the threshold the counter is. I am perfectly willing to pay to have it replaced.

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  #56  
Old 11-22-2015, 07:52 PM
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Originally Posted by engatwork View Post
You have to mash on the brake pedal substantially harder. Disconnect SBC module if you want to give it a try.
This.......
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  #57  
Old 11-22-2015, 08:00 PM
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I haven't read through this yet, will when I get time.

SBC product introduction
Welcome to the W211 E- Class!

Hydraulic diagram
Welcome to the W211 E- Class!
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  #58  
Old 11-22-2015, 08:35 PM
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Awhile back, I decided to not own any vehicles with ABS. While ABS often malfunctions by simply shutting down, leaving the brakes to function normally without ABS, sometimes it can malfunction by reducing braking power if the computer 'thinks' it's needed, even when there's no slipping or skidding condition present. A friend has a '95 Camry with ABS that activates every time the car slows to about 3-4 mph. It feels like it's suddenly sliding on gravel. Very unnerving!
Other friends have occasionally had the ABS on their vehicles activate needlessly at much higher speeds, without warning. Then the system resumes functioning normally when the vehicle gets to the repair-shop.
As I buy used vehicles long past warranty, I have no desire to gamble on aged or neglected electronic safety technology. I'm adding SBC to my 'to avoid' list.

Happy Motoring, Mark
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  #59  
Old 11-22-2015, 08:47 PM
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It is not like a traditional booster failure. When a booster fails its harder to push the pedal but all motion is turned into braking force. In SBC the backup master cyl isn't engaged until approx. the bottom 1/3 of pedal travel. What you normally feel as pedal resistance in an SBC car is only a dummy resistance generated by the SBC to simulate pedal feel. When an SBC fails fight the urge to pump the brakes, it won't do anything. You have to slam the pedal through the floor as hard as you can.
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  #60  
Old 11-22-2015, 09:55 PM
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Quote:
My complaint with the SBC system is that this is not a FAILURE of a part, it is a shutdown of the system when the "service threshold" is reached.
Seriously? So instead of just a service soon warning it shuts itself off?!?! if this is true I would replace it as a maintenance item upon buying a car with SBC and just chalk it up to a cost of ownership\necessary safety.
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