Electric Car Power Brakes

by Ben N on May 19, 2014

DSC_1569I was recently asked how brakes work on my electric Geo Metro. I also noticed that every once in a while lately, my vacuum pump wasn’t powering up, so I thought it was about time to take apart the electric power brakes system and examine it. While I did, I took a few photos to show you exactly how the system works.

The system only has a few components – a vacuum reservoir, a vacuum switch, a vacuum pump, and a vacuum gauge. Hey, that’s a lot of things with the word “vacuum” in them, what’s up with that?

Power brakes on modern cars are really VACUUM-assisted brakes. Vacuum is just the LACK of air. It really does suck! In a regular gasoline car, the action of the engine creates a suction, or vacuum, which then PULLS on the back side of the brake diaphragm, giving you assist, or “power” brakes. (If you want to learn more, you can always check out How Stuff Works.)(Technically vacuums can’t pull. They just get air out of the way so that air pressure can push.)

Of course, one of the first things I did on my electric car conversion was to REMOVE the engine, and thus the source of power for “power” brakes.

So, what was needed in its place is a small electric vacuum pump and a few other accessories to go with it.

I have one wire in the car that is switched with the key. So, any time the car is on, there’s 12V+ to that wire. That wire goes to a vacuum switch, which will pass through that 12V (or not) depending on the state of vacuum. If there is NOT enough vacuum, the switch passes power through to the vacuum pump, which runs, pulling out air.

Both the pump and switch are connected to a vacuum reservoir, which is nothing more than a sealed aluminum bottle, about a liter in volume. As the pump runs, it pulls air out of the bottle until the vacuum switch reaches its set-point, and then turns the pump off. (There is a one-way valve between the pump and bottle, so that air can’t leak back out when the pump stops.)

DSC_1577On the top of the vacuum bottle is a tee so that the switch, pump, vacuum gauge, and connection to the brakes all can connect at the same time. When the brakes are pressed, a small amount of vacuum goes to the brakes, assisting in pushing the brakes. The vacuum gauge inside the car shows the driver how much vacuum there is. Even in case of vacuum pump (or switch) failure, the brakes can be pressed about TEN TIMES before the vacuum in the reservoir runs out. A typical gas car is designed to press the brakes three times without vacuum (such as would happen if, say, you ran out of gas.)

Let’s take a closer look at my setup. The vacuum bottle is held onto the fire-wall using “plumber’s tape” – metal strapping with holes evenly spaced in it for nuts and bolts. Mounted to the top of the bottle is a brass tee. Coming off of that is a short pipe nipple that the hose to the brakes goes on. Also, the thin clear or white plastic tube runs through the firewall and to the dashboard for the vacuum gauge. The vacuum switch is mounted directly to the tee on top of the bottle. The last port runs to the vacuum pump.

The switch has three electrical connections. NC, NO, and common. Common is where the wire off to the pump goes. NC and NO stand for Normally Closed and Normally Open. This is fancy electrical speak for “unless something else happens, the circuit is complete and turns something on (NC)”, and “most of the time, the circuit is NOT complete and DOESN’T turn something on (NO). On a vacuum switch hooked to a pump, it will turn the pump on or off ABOVE or BELOW the set-point, depending on which connection is used. In this case, I used the Normally Closed connection, so the pump fires up as soon as the car turns on. Once the set-point is reached, the switch turns the pump off. The set point can be adjusted with a regular screw-driver.

DSC_1576I found that there was some corrosion on the switch terminals while I was inspecting it, so I cleaned off the terminals. I also cut off the old female spade connectors on both wires and replaced them with new insulated connectors that should hopefully help keep the weather out. I also put a dab of electrical grease on before re-installing the electric connections.

The vacuum pump is a 12V DC unit that has two little cylinders in it, just like a tiny engine. An electric motor spins, making the two cylinders pump air. There’s actually FOUR air connections on this unit, one each on both air cylinders for air in and air out. I ran tubing to a tee to connect both of the air IN connections to the vacuum bottle through a one-way air valve. The pump is fairly low power, so it runs directly off the switch. If a more powerful pump was used, the switch could control power to the pump through a relay.

The pump, vacuum bottle, and brass connections were all salvaged materials, scrounged from my friend’s “save these for building robots” parts pile. The air hose from the vacuum bottle to the brakes is just a few feet of old air compressor hose. The ¬†gauge is a vac/boost guage ¬†purchased new from the local auto parts store. The vacuum switch was purchased by mail order from an industrial supplier. I used an Airtrol F-4200-X30, although there are plenty of other switches that would work just as well.

So, power brakes on a DIY electric car aren’t that complicated. You just need a way to create a vacuum with a pump, and turn that pump on and off. That’s about it. One of my favorite things about electric cars is that they AREN’T rocket science. You just need a few simple parts and the ingenuity to put them together.

Stay charged up! And remember if you can go, it’s good to be able to stop as well!


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