Brake actuator pump... brake accumulator pump...?

The assembly has one pump, one electric motor that drives the pump, and one accumulator that stores pressurized fluid. Those three items are mounted on the valve body housing. The accumulator is replaceable without removing the complete assembly. It merely threads into the a recess in the valve body.

 

one pump

 

The names are continual stumbling blocks for everybody, for a handful of reasons:

• The words ac·cum·u·la·tor and ac·tu·a·tor are confusingly similar, and both are parts (distinct ones!) of the brake system
• Different Prius generations combine them differently into the actual parts you can buy. The details are a mess but there's a four-year-old post where I broke them all down from Gen 1 through Gen 4. (Take those four-year-old prices with a grain of salt; for example, the Gen 3 actuator is now about $700, and folks who complain about that clearly weren't paying attention four years ago when it was $2500.) In a Gen 2 like yours, there is just one assembly with both the ac·cum·u·la·tor and ac·tu·a·tor, so people in the Gen 2 forums are especially prone to mixing them up and it's hardly worth being a stickler about. In the other generations it is more important for deciding which thing you will buy.
• Even in Toyota's own documents and parts catalogs, the assemblies will sometimes be called by the names of the other components they include. Hence in Gen 3 you might see references to "Skid ECU", "Master Cylinder", "Booster", and "Actuator", and those are all the same Gen 3 part. "Booster Pump", though, is a different Gen 3 part, and the same one as "Accumulator". Again, there's the four-year-old post to keep things straight.
• "Booster" and "Booster Pump" are two more extremely confusable terms. In Gens 1, 3, and 4, there is a "booster", which is part of the "master cylinder", and a "booster pump", which is what you hear pumping up the accumulator. Gen 2 is the odd one, as it has no booster (but it still has an accumulator, and a pump).
• Gen 4, for the first time, has more than one pump.

So as you can see, it's a nomenclature mess, people misname parts all the time, and about the best you can hope to do is try to know all the parts well enough yourself that you know which ones other people are talking about even when they don't.

Strictly speaking, the pump is always the "accumulator pump" and not "actuator pump". But in Gen 2 it comes, along with the accumulator, as part of the one assembly that is usually called the "actuator", so there you are.

The fact that you can replace the Gen 2 accumulator (just the actual pressure vessel itself) by finding one of compatible size and threads and screwing it in, is an aftermarket discovery; it's not separately available from Toyota.

 

I find that understanding what the various parts DO helps me to keep them straight in my head.
On a typical non-hybrid car, the brake pedal connects to a booster, which uses some form of power from the engine to amplify the force of your leg muscles. That force is applied to the master cylinder, which sends pressurized fluid (up to 1500psi) through lines and hoses to in turn apply the brakes at each wheel. These brakes convert the mechanical energy of the car's momentum into heat energy through friction. Most cars have some form of ABS system in between the wheel brakes and the master cylinder in order to modulate operation if wheel skid is detected (but most of the time the ABS just sits there because it's not needed). Your foot is "directly" connected to the friction brakes at the wheels.

Now on a hybrid car they don't want to waste the mechanical energy of motion by turning it into heat. Instead they want to capture as much as possible as electricity and store it in the HV battery (aka regenerative braking) then reuse it to propel the car. But regen has capacity limits, can only work on the front drive wheels, and can't operate in an ABS mode. So they still need the hydraulic friction brakes.

When you press on the pedal of a Prius, there is a master cylinder, but that is connected to a device that mimics the feel of "normal" brakes. Sensors measure how far and hard you push to a computer that figures out what your intentions (gentle decel vs a panic stop). That ecu decides how much regen it can do vs friction braking (which is controlled by another ecu). Your foot normally has no "direct" connection to the brakes at the wheels at all. Since your leg and the booster aren't creating the fluid pressure directly, it needs something else.

The Gen2 has a brake hydraulic actuator assembly. It has an electric pump to pressurize brake fluid to around 2000psi, an accumulator which is a special tank to store that high pressure fluid. It also has valves to let the ecu apply some of that fluid to each of the friction brakes, and sensors to monitor pressure in different parts of the system. If the ABS detects wheel lock-up (skid), it will apply and release the friction brakes directly (no regen at all).

The pump in the actuator pressuries fluid (which is stored in the accumulator) to apply the friction brakes at the wheels- all under computer control.

Posted via the PriusChat mobile app.

 

That's a pretty good description for Gen 2, so it fits here in the Gen 2 forum, though it elides some of the differences between Gen 2 and other gens. In particular, every generation other than 2 does have a booster between your foot and the master cylinder, though it's a hydraulic one instead of the familiar giant vacuum diaphragm in other cars.

And Gen 2 is the only generation that combined the pump and accumulator into the same assembly as the actuator. (So when somebody talks about "the pump in the actuator", it's usually safe to assume that's a person who is most familiar with Gen 2, and just needs to get out more.)

... that is, until you get to Gen 4, which still has the usual accumulator pump, but also has two additional pumps and those two really are in the actuator.

 

I only have a Gen2 so that's what I have been learning on. If I could find a (dead-ish) Gen3 for under $1k I might pick it up to play with. Reading about stuff online ain't the same as putting your hands on it and DOING things.

Posted via the PriusChat mobile app.