How does the Brake Booster Solenoid work?

[sapote]

Again, unplug the solenoid and the brakes will start to progressively self-apply as the engine runs. .

For a normal working brake system this should never can happen because the pressure on both sides of the boost diaphragm are the same, giving ZERO force on the diaphragm and the master cylinder push-rod. The only way this could happen is there is a small air leak on the valve located on the driver side of the booster, leading to this side has high pressure and the front side of the diaphragm has high vac, leading to the diaphragm being pushed forward on the master.

[sapote]

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.

Interesting, but the hose connecting the solenoid valve to the air box doesn't subject to high vac pressure -- it's almost same as atmospheric and so I wonder why it has any effect to the brake boosting.

[eacmen]

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.

Ok I did an experiment to verify.

There is no obstruction in the Y-pipe/venturi/sucking-jet-pump whether the solenoid is open or closed. So there is an unobstructed path between the air rail and booster check valve at all times.

What I did was pulled the check valve from the booster. With engine at idle you can feel vacuum whether that solenoid is open or closed.

[bigjae46]

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

A professional shade tree repair...lol

And you're missing some nuts...hahaha

[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.