How does the Brake Booster Solenoid work?

[eacmen]

So I found a document for the MSS54 that states the following:

The Suction Jet Pump is regulated by the ECM to provide sufficient vacuum for the brake booster under all operating conditions. The ECM controls the Suction Jet Pump Solenoid to allow vacuum flow through.

The additional vacuum compensation is activated by the ECM when the idle air actuator is regulated for:

• A/C compressor “on”
• Vehicle in gear and the clutch is released (driving under load)
• Engine in warm-up phase <70º C

Additional vacuum compensation is applied to the brake booster when the circuit is “deac-tivated” (Solenoid sprung open). Vacuum enhancement is limited to the brake booster when the control circuit is “activated” (Solenoid powered closed).

So if I am reading this correctly that solenoid should be open when the A/C is on, when you're in gear and driving, or when the engine is cold. So not sure why my during my test the solenoid was kept closed during cold idle.

[eacmen]

Let's do the math:

29" hg = 14.2 psi
at 20mm hg, boost, there is 29" - 20mm = 28.2" Hg assist, which creates 28.2 * 14.2 psi / 29 = 13.6 psi on the booster diaphragm.
Assuming the booster diaphragm is 7" dia, then the booster force on the diaphragm = (3.14) * 3.5^2 * 13.6 = 523 lbs. Wow this is more than enough (it's like a 500 pound fat person stands on the brake pedal of non-booster car).

I didn't know the M3 air rail has such low vac.

So I think most of the time the solenoid is closed bc the booster has plenty of vac from the air rail, and no need to pump more vac with solenoid valve opened. Only at wider open throttle that DME opens the valve to increase boost.

You can try this: add a toggle switch in series to one of the solenoid wire, and during WOT racing turn the switch to off and this ensures to open the solenoid valve for boosted braking.

Thanks for doing that math!

Its a dedicated track car so my current thinking is to just disconnect the solenoid connector when I get to the next track event which would leave it wide open and see if the problem persists.

[sapote]

When i've measured vacuum on the air rail in the past at idle its been around 19-20mmHg which seems to be in range of the booster vacuum. Aftermarket systems seem to be in the 16-22 mmHg range. Granted as throttle opens the vacuum will drop.
.

Let's do the math:

29" hg = 14.2 psi
at 20mm hg, boost, there is 29" - 20mm = 28.2" Hg assist, which creates 28.2 * 14.2 psi / 29 = 13.6 psi on the booster diaphragm.
Assuming the booster diaphragm is 7" dia, then the booster force on the diaphragm = (3.14) * 3.5^2 * 13.6 = 523 lbs. Wow this is more than enough (it's like a 500 pound fat person stands on the brake pedal of non-booster car).

I didn't know the M3 air rail has such low vac.

So I think most of the time the solenoid is closed bc the booster has plenty of vac from the air rail, and no need to pump more vac with solenoid valve opened. Only at wider open throttle that DME opens the valve to increase boost.

You can try this: add a toggle switch in series to one of the solenoid wire, and during WOT racing turn the switch to off and this ensures to open the solenoid valve for boosted braking.

[eacmen]

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

If i'm understanding correctly the one way check valve on the booster should allow the vacuum to accumulate in the booster.

When i've measured vacuum on the air rail in the past at idle its been around 19-20mmHg which seems to be in range of the booster vacuum. Aftermarket systems seem to be in the 16-22 mmHg range. Granted as throttle opens the vacuum will drop.

What vacuum level is enough to brake?

I might try some careful driving with the booster disconnected and the air rail capped off to see if the pedal feels familiar.

[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.