How does the Brake Booster Solenoid work?

[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]

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]

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]

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]

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]

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]

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]

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]

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.

[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.

[bigjae46]

IIRC...when the car is off, the solenoid is open. Once you turn the car on, the solenoid energizes and closes which isolates the booster from the engine vacuum - the engine pulls air through the intake, solenoid, booster, two way valve, and the air rail behind the throttle body butterflies. When you step on the brake, it opens the hall effect switch, ECU sees the open circuit and degenergizes the solenoid which provides power assist.

Unplug the solenoid, drive the car until the brakes lock on (won't take long) and then with the car running pull the vac hose off the booster, there will be vacuum. Do the same thing with the solenoid plugged in and car running, there will be no vacuum. As soon as you step on the brake, you'll have vacuum on that line.

The system is designed to have power assist if something fails.

There is no sucking jet pump in this...its a form of a check valve. I discovered this when mine fell apart. But I would check it to make sure its not falling apart. Mine came apart at the seam.

If the valve is good, the pedal is hard and the brakes are working - it's a booster, master cylinder, ABS pump, or caliper issue.

[sapote]

Unplug the solenoid, drive the car until the brakes lock on

brakes locked up even not touching the pedal?
I don’t think this can happen unless the rear seal is leaking in the booster can and allows air to come in and pushing on the diaphragm forward.

[maupineda]

IIRC...when the car is off, the solenoid is open. Once you turn the car on, the solenoid energizes and closes which isolates the booster from the engine vacuum - the engine pulls air through the intake, solenoid, booster, two way valve, and the air rail behind the throttle body butterflies. When you step on the brake, it opens the hall effect switch, ECU sees the open circuit and degenergizes the solenoid which provides power assist.

Unplug the solenoid, drive the car until the brakes lock on (won't take long) and then with the car running pull the vac hose off the booster, there will be vacuum. Do the same thing with the solenoid plugged in and car running, there will be no vacuum. As soon as you step on the brake, you'll have vacuum on that line.

The system is designed to have power assist if something fails.

There is no sucking jet pump in this...its a form of a check valve. I discovered this when mine fell apart. But I would check it to make sure its not falling apart. Mine came apart at the seam.

If the valve is good, the pedal is hard and the brakes are working - it's a booster, master cylinder, ABS pump, or caliper issue.

This does not makes sense, most of the times when stepping on the brakes you release the throttle, this negates any vacuum in the intake manifold. There needs to be an active air path for vacuum to build up. Under load very likely the ICV is closed, that means no sucking in the vacuum rail so the vacuum path is through the intake plenum. Make sense the solenoid opens under load.

[eacmen]

Went for a drive with the solenoid disconnected (meaning OPEN) to test the sucking jet pump solenoid.

If you maintain partial throttle with the engine under load you get maybe 3 boosted brakes in a row before you lose most of the assist.

Same scenario coasting off throttle you never lose the assist.

I will admit that it may be my braking technique causing these issues. I do left foot brake. So its possible there may be some crossover between fully off throttle and brake application.

One of the things about driving on track is that you want that pedal to be a consistent force. You don't want to be relearning each corner how much brake force is required for the level of boost you currently have.

Right now I'm seriously considering deleting the booster and switching to one of Lee's dual MC balance bar setups. Cost wise its not that much more expensive than buying the booster, check valve and solenoid.

[bigjae46]

This does not makes sense, most of the times when stepping on the brakes you release the throttle, this negates any vacuum in the intake manifold. There needs to be an active air path for vacuum to build up. Under load very likely the ICV is closed, that means no sucking in the vacuum rail so the vacuum path is through the intake plenum. Make sense the solenoid opens under load.

It’s possible I have it backwards. I dealt with this 4 years ago when I deleted the LCM