How does the Brake Booster Solenoid work?

[George Hill]

What is the failure mode?

Is it hard like no assist all the time or intermittently?

If intermittent what is the pattern?

[eacmen]

More info:

I removed the hose feeding the brake booster that has the sucking jet pump and the one way valve. In an attempt to test each component:

- Cleaning it real good and using my mouth verified that the one way valve seems to be working (though my lungs likely cannot generate the same amount of vacuum as the booster holds). However, if the one way valve was defective I would assume the booster would leak vacuum soon after shutting engine off. Vacuum stayed in the booster for several days.
- I verified the sucking jet pump was working by sending compressed air down the A inlet (diagram above) and holding my thumb on the valve that goes into the booster. I could feel vacuum on my thumb.
- I verified that at least under normal scenario the brake booster solenoid is closed with no voltage applied and open when voltage is applied. Even though I could not test the actuation of the solenoid itself. Maybe there is a tool32 entry I could find to do this?

Really this all just left me with more questions than answers. Was hoping one of these components would be more obviously faulty. Though I am not surprised as this condition only happens with a hot engine deep in a track session while pushing hard. Steady state on th the bench in the garage hard to replicate. Maybe I can put some of these components in the oven and replicate heat soak and test again?

Any ideas are welcome

[eacmen]

I think your booster can is leaking either at the front hose connection or the rear seal.

To do: park it overnight and test the brake with engine off; good sealed can should still have vac and brakes fine but leaking can will cause hard pedal.

Car has been parked for about a week. I just went out and there was still vacuum assist in the booster. So I think the booster is at least holding some vacuum.

However what I did discover is when trying to command the "Sucking jet pump" on and off with the Shwaben scan tool the voltage to the solenoid did not change. It was constant 12v - meaning that solenoid is always open.

Can someone else with the schwaben tool confirm that commanding "Sucking jet pump" on and off in the Active Test menu commands the brake booster solenoid on and off? If so, then either a defect with the ECU or its been coded out with a tune.

[sapote]

I think your booster can is leaking either at the front hose connection or the rear seal.

To do: park it overnight and test the brake with engine off; good sealed can should still have vac and brakes fine but leaking can will cause hard pedal.

[eacmen]

I think sapote is spot on. Sucking jet pump helps generate additional vacuum for the booster and DME closes the inlet (labeled "A" in sapote's diagram), when additional vacuum is not needed. I also don't know what specific conditions drive the DME to close it though...

Have you tried logging the brake pressure sensors on track? I can't really think of any other telemetry that might help you find the problem, but at least that would be a start.

Edit: Not M3 specific, but this is what TIS has to say about the system:



A simple disassembly of the DME FW should tell you what conditions need to be met for the valve to be closed

Brake pressure sensors dont yield any meaningful conclusions as when it happens I seem to make up the difference with additional pressure. So peak pressures don't really change but the brake rate seems to be more gradual build up to the same pressure.

I don't think the DME can know how much vacuum is in the booster so it must simply open or close the solenoid based on other parameters.

As much as I would like to log values this condition can only be replicated on track so opportunity to test it is limited. I might just need to fire the parts cannon and hope for the best!

[heinzboehmer]

I think sapote is spot on. Sucking jet pump helps generate additional vacuum for the booster and DME closes the inlet (labeled "A" in sapote's diagram), when additional vacuum is not needed. I also don't know what specific conditions drive the DME to close it though...

Have you tried logging the brake pressure sensors on track? I can't really think of any other telemetry that might help you find the problem, but at least that would be a start.

Edit: Not M3 specific, but this is what TIS has to say about the system:



A simple disassembly of the DME FW should tell you what conditions need to be met for the valve to be closed

[sapote]

How does the suction jet pump work? Is it powered via venturi?

.

It works using the same principle that fast moving air above the wing lifts the airplane up.
The intake pulls air out at I port of the valve, and the same air mass is coming in at the A port connected to near the air filter. The air channel is narrowed down in the mid section of the valve which is also where the brake booster port B connected to. Air must be moving much faster through the narrow section ( I wish the same happens on the road when FWY closed some lanes but stupid human do the opposite compared to air or liquid and caused traffic jam) , and fast moving air creates lower pressure than at I port, and this lower pressure pull air out of the brake booster can to create vac needed.

Our car has a solenoid valve connected at A to be able to close this port when needed, but I am not sure under what condtion.My guess is that at wide open throttle, most of the air flow thru the opened throttle and not much flow thru this valve and so it's useless and the DME closes the valve to minimize booster leaking from B to A as the delta pressure is high.

[eacmen]

Thanks sapote! How does the suction jet pump work? Is it powered via venturi?

Odd that other E46 M3 owners at the track don't encounter this. Maybe I am the only one left foot braking?

Maybe the booster is actually bad and not holding enough vacuum in the reservoir.

I should probably inspect check valve as well.

[sapote]

It seems to happen deeper in the session when everything is hot and/or when pushing.

I'm not surprised at all.

Racing and pushing throttle all the time will lead to empty booster vacuum, and hard pedal symptom.

wide opened throttle has low vacuum and so not much help from the booster. Must close the throttle sometime to build up brake vac.

The solenoid is for close-off the suction path from the intake under some condition.
You can bypass the valve -- let it open by unplug the connector to it and the suction jet pump is enabled all the time, but still runs out of vac if wide open throttle most the time.

[eacmen]

We have been struggling for a year or so with brake pedal going rock hard while on track. Originally hypothesized that this was ice mode. This past weekend at VIR i’m starting to think it might be brake booster related.

The problem is intermittent. It seems to happen deeper in the session when everything is hot and/or when pushing.

My question for the brain trust is does anyone know the purpose for the solenoid that seems to drive brake booster vacuum? I would think that with an intermittent problem like this its unlikely that the booster itself is failing. It seems to hold vacuum even after the engine is off.

On normal E46 chassis it looks like the brake booster vacuum is driven directly off the intake vacuum. But on our car there seems to be a solenoid between engine vacuum and the booster.

Does anyone know the purpose of this? Under what conditions does it open/close? Is it normally open or normally closed?