How does the Brake Booster Solenoid work?

[sapote]

rocked the check valve slightly and there was apparent hissing from the booster, so at least one issue appears to be the gasket. BMW is proud of that sucking jet assembly... which part number for an 01? part number 11617836745 or 11617831663 ? FCP lists both as fitting...

While engine running or off? The hissing means there’s vacuum in the booster can as expected. If the booster doesn’t provide power assist then the seal/valve where the pedal push rod enters is bad, blocking air to enter into the rear chamber.

leaking air when you rocking the hose doesn’t mean the seal is bad as it’s distorted by the force.

[fattycharged]

It was working and suddenly went bad, or after you did something?

I have not driven the vehicle in a very very long time, just moved in and out of the garage over the past 18mo.
Only thing recently done is installing SS V1's>Sec3, new motor/trans mounts, spare diff and ds, so I shouldn't have messed with anything on that side.


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[sapote]

Can you answer post 46?

[fattycharged]

I think there is no vac delivered by the intake to the booster. Connection to the jet pump is bad?

"Engine off, I can vacate the booster and pedal goes stiff with hydraulics."
This means the booster can is fine.

rocked the check valve slightly and there was apparent hissing from the booster, so at least one issue appears to be the gasket. BMW is proud of that sucking jet assembly... which part number for an 01? part number 11617836745 or 11617831663 ? FCP lists both as fitting...

[sapote]

So I've got a an issue related to this right now...NO brakes at all when engine is running. Engine off, I can vacate the booster and pedal goes stiff with hydraulics. Is it actually possible to have the booster fail and create a no brakes scenario? per ChatGPT it is, but that is a wild design if so...

I think there is no vac delivered by the intake to the booster. Connection to the jet pump is bad?

"Engine off, I can vacate the booster and pedal goes stiff with hydraulics."
This means the booster can is fine.

[sapote]

So I've got a an issue related to this right now...NO brakes at all when engine is running. Engine off, I can vacate the booster and pedal goes stiff with hydraulics. Is it actually possible to have the booster fail and create a no brakes scenario? per ChatGPT it is, but that is a wild design if so...

It was working and suddenly went bad, or after you did something?

[sapote]

Again, unplug the solenoid and the brakes will start to progressively self-apply as the engine runs. When you shut the engine off the brakes release.

Unplug the solenoid with engine running creates higher vac inside the booster as compared to plugged in. But the higher vac applied to both sides of the diaphragm and so zero net forward force on the master rod. Your booster has a small leak at the pedal push rod/booster seal to cause the self lock-up at higher vac.

[eacmen]

I do not, I have a CSL box

Remove the vaccum hose going into the brake booster and cap it off and see if it changes. I could see if the check valve failed and you ended up with positive pressure in the booster it might cause the symptom you describe.

[fattycharged]

Maybe the check valve went bad? Do you have a turbo or supercharger?

I do not, I have a CSL box

[eacmen]

So I've got a an issue related to this right now...NO brakes at all when engine is running. Engine off, I can vacate the booster and pedal goes stiff with hydraulics. Is it actually possible to have the booster fail and create a no brakes scenario? per ChatGPT it is, but that is a wild design if so...

Maybe the check valve went bad? Do you have a turbo or supercharger?

[fattycharged]

So I've got a an issue related to this right now...NO brakes at all when engine is running. Engine off, I can vacate the booster and pedal goes stiff with hydraulics. Is it actually possible to have the booster fail and create a no brakes scenario? per ChatGPT it is, but that is a wild design if so...

[sapote]

Again, unplug the solenoid and the brakes will start to progressively self-apply as the engine runs. .

For a normal working brake system this should never can happen because the pressure on both sides of the boost diaphragm are the same, giving ZERO force on the diaphragm and the master cylinder push-rod. The only way this could happen is there is a small air leak on the valve located on the driver side of the booster, leading to this side has high pressure and the front side of the diaphragm has high vac, leading to the diaphragm being pushed forward on the master.

[sapote]

Sort've related. Whenever I got my car it had this homemade contraption for the hose from the manifold to the valve mentioned in this thread. The car had an overly soft and inconsistent brake pedal, always felt like I could never bleed it properly. However with car off, pedal was rock hard and firm.

Luckily I was able to find one of the correct plastic lines used as the price new is eye watering. I mainly swapped it out for visual reasons but was very pleased to be greeted with a firmer and consistent brake pedal. After reviewing this thread, still confused how it all works but just thought I'd throw this into the pot.

Interesting, but the hose connecting the solenoid valve to the air box doesn't subject to high vac pressure -- it's almost same as atmospheric and so I wonder why it has any effect to the brake boosting.

[eacmen]

I think the booster is NOT harvesting engine vacuum unless the hall effect switch is opened. Again, unplug the solenoid and the brakes will start to progressively self-apply as the engine runs. When you shut the engine off the brakes release. Unless the booster adds boost when not under vacuum.

Ok I did an experiment to verify.

There is no obstruction in the Y-pipe/venturi/sucking-jet-pump whether the solenoid is open or closed. So there is an unobstructed path between the air rail and booster check valve at all times.

What I did was pulled the check valve from the booster. With engine at idle you can feel vacuum whether that solenoid is open or closed.

[bigjae46]

Sort've related. Whenever I got my car it had this homemade contraption for the hose from the manifold to the valve mentioned in this thread. The car had an overly soft and inconsistent brake pedal, always felt like I could never bleed it properly. However with car off, pedal was rock hard and firm.

Luckily I was able to find one of the correct plastic lines used as the price new is eye watering. I mainly swapped it out for visual reasons but was very pleased to be greeted with a firmer and consistent brake pedal. After reviewing this thread, still confused how it all works but just thought I'd throw this into the pot.

Vanos and wheels by Kurvenkamph Motorsport, on Flickr

A professional shade tree repair...lol

And you're missing some nuts...hahaha