How does the Brake Booster Solenoid work?

[sapote]

The solenoid valve is between the sucking jet pump inlet (A) and the airbox. Even when the solenoid valve is closed the booster should still be getting vacuum from the air rail - right?

If valve is closed, booster = intake vac, which is not enough to brake, and less vac as throttle inc opening.

[eacmen]

It doesn't make sense if the valve is closed most of the time but WOT, as there is no booster with the closed valve during street driving.

The solenoid valve is between the sucking jet pump inlet (A) and the airbox. Even when the solenoid valve is closed the booster should still be getting vacuum from the air rail - right?

[sapote]

Well i haven't been able to setup logging of the voltage while driving and the scan tools I have aren't able to log the status of this solenoid. But so far I cannot ascertain the conditions required to open this solenoid. It might only be during WOT.

It doesn't make sense if the valve is closed most of the time but WOT, as there is no booster with the closed valve during street driving.

You measured between the connector 2 pins or one pin and chassis gnd?

[eacmen]

You never see the voltage changed to zero volt under different condition?

Well i haven't been able to setup logging of the voltage while driving and the scan tools I have aren't able to log the status of this solenoid. But so far I cannot ascertain the conditions required to open this solenoid. It might only be during WOT.

[sapote]

The solenoid is held closed at idle. Blipping the throttle, and pressing brake pedal had no effect on the solenoid voltage..

You never see the voltage changed to zero volt under different condition?

[eacmen]

1. I don't see the reason for open or close the booster solenoid based on the brake pedal actives or not active. The booster should always be pumped unless there is not enough vac in the intake such as with WOT. Why would the DME disables the pump during braking, or during not braking (how would it get vac boosted if pump not enabled during not braking)?
2. Solenoid is opened when no power applied to it. TIS says so. I don't think the state of the brake booster has any effect on why your brake was locking up.

3. It's not just a 2 way valve, but it amplifies the vac of the intake to create higher vac for the brake booster. The intake vac level is not strong enough for the booster to work.

1. Agree. I started the car with a multimeter on the solenoid connector. The solenoid is held closed at idle. Blipping the throttle, and pressing brake pedal had no effect on the solenoid voltage. I did not try WOT since I was in a garage and valued my hearing. The voltage drops near 0 right after the engine is started, I suspect that is due to the starter pulling all the voltage.

2. My experiment verifies this. With no voltage the solenoid is OPEN. 12v will CLOSE the solenoid.

3. The valve you're referring to is the sucking jet pump or the one way check valve? If this is the case i'm not sure why the solenoid valve is closed by default.

[sapote]

1. When the hall effect switch is de-energized (you step on the pedal) it opens the circuit to the ECU. Forget what it is, its a blue wire with red dots, might be a bus loop, IIRC it feeds into the ABS, ECU, and LCM. It is a blur but the ECU is reading the hall effect switch position and then the ECU opens and closes the solenoid based on the hall effect switch.

2. Awhile back, I was trying to delete the LCM so I replaced the hall effect switch with a momentary push button to activate the brake lights. After driving about 100 feet the brakes would start to stick on and lock. If I shut the car off and restarted, the car would go another 100 feet and then the brakes locked. I believe the solenoid is closed when it is deenergized - that where I fixed the issue and stopped investigating.

3. The T is a 2 way valve, I looked inside of it. It is not pumping anything.

1. I don't see the reason for open or close the booster solenoid based on the brake pedal actives or not active. The booster should always be pumped unless there is not enough vac in the intake such as with WOT. Why would the DME disables the pump during braking, or during not braking (how would it get vac boosted if pump not enabled during not braking)?
2. Solenoid is opened when no power applied to it. TIS says so. I don't think the state of the brake booster has any effect on why your brake was locking up.

3. It's not just a 2 way valve, but it amplifies the vac of the intake to create higher vac for the brake booster. The intake vac level is not strong enough for the booster to work.

[heinzboehmer]

It's not an active pump, but rather works on the venturi principle, just like the stock suction jet pump in the gas tank.

See the "Air Ejector" section here: https://en.wikipedia.org/wiki/Vacuum_ejector

[bigjae46]

So far in this thread...it describes how the solenoid valve works incorrectly. The solenoid is open or closed so it controls flow from the air rail to the booster back to the intake manifold.

The solenoid is connected to the ECU. Then it controlled by the hall effect switch on the brake pedal, LCU, and ABS unit. When the hall effect switch is de-energized (you step on the pedal) it opens the circuit to the ECU. Forget what it is, its a blue wire with red dots, might be a bus loop, IIRC it feeds into the ABS, ECU, and LCM. It is a blur but the ECU is reading the hall effect switch position and then the ECU opens and closes the solenoid based on the hall effect switch.

Awhile back, I was trying to delete the LCM so I replaced the hall effect switch with a momentary push button to activate the brake lights. After driving about 100 feet the brakes would start to stick on and lock. If I shut the car off and restarted, the car would go another 100 feet and then the brakes locked. I believe the solenoid is closed when it is deenergized - that where I fixed the issue and stopped investigating.

The T is a 2 way valve, I looked inside of it. It is not pumping anything.

[George Hill]

Does it happen after a long WOT scenario?

Does it happen after low throttle scenarios?

My car did this intermittently, the booster would bleed down when WOT for extended period of times and lose assist. Once I got into situations where the engine was in a state building vacuum the assist would come back. The booster would hold vacuum with the car off as well. It also made a little bit of a swoosh sound intermittently.

[eacmen]

What is the failure mode?

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

If intermittent what is the pattern?

Rock hard pedal, pressing harder does yield more braking force.

Happens intermittently.

Typically starts 10+ minutes into a session when things are nice and hot. Sometimes the following braking zone it gets better, sometimes it takes 3 braking zones.

[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.