How does the Brake Booster Solenoid work?

[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

[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.

[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]

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.

[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

[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.

[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

[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.

[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.

[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.