How does the Brake Booster Solenoid work?

[bigjae46]

I made this diagram that hopefully clears up how this system works. The Solenoid just controls if there is airflow to power the venturi/sucking jet pump. Normally the booster is harvesting engine vacuum and storing it by way of the check valve and check valve seal.

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.

[samthejam]

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

[eacmen]

Unsolicited nit: I would swap the direction of the red arrows so that they're consistent with the direction of fluid flow.

not gonna lie. i did waffle back and forth as to how to draw those red arrows.

[heinzboehmer]

I made this diagram that hopefully clears up how this system works. The Solenoid just controls if there is airflow to power the venturi/sucking jet pump. Normally the booster is harvesting engine vacuum and storing it by way of the check valve and check valve seal.

Unsolicited nit: I would swap the direction of the red arrows so that they're consistent with the direction of fluid flow.

[eacmen]

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.

I made this diagram that hopefully clears up how this system works. The Solenoid just controls if there is airflow to power the venturi/sucking jet pump. Normally the booster is harvesting engine vacuum and storing it by way of the check valve and check valve seal.

[sapote]

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..

Now you’re more conscious about releasing gas pedal before braking and this should build up boost vac.

im left foot braking in 2-pedal cars and right foot braking in 3-pedal too

[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

[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.

[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.

[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.

[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.

[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.