Brake Master Cylinder Pressure Sensors

A quick background: I am a Software Engineer / Process Manager for a company that makes a *lot* of the electronics for Toyota.
I have no particular knowledge of the exact brake system / ABS system used in the Prius, but I do have extensive knowledge on automotive sensors, electronics and software.

My guess (but again... I do NOT know this particular system) is that the sensors send their signals digitally over a high-speed CANbus and thus do not give out any voltage signal you can measure.
Both systems are possible though, I'm just basing my guess on the general high-technology electronics in the Prius.

You could *try* to get a hold on the wiring diagrams from the company providing the sensor. You'd probably have to look at the sensor itself to see the manufacturer.
Be aware though that manufacturers usually don't quite toss their datasheets out there to random people, so it might be rather hard to get them.

Hope I was of some help

Edit: Since you asked if there was a way to get the data you need, assuming my guess is correct... You'd need a tool that can read out the CANbus, then figure out which can messages are the ones being sent out by the sensors you're looking for then and then find out which part of the CAN-message contains the data you need.
This information will also be on the datasheet of the sensor, which again, might not be provided easily.

 

You may want to review your assumptions about how the ABS and stability control systems work. It is enough to have an accurate rotation sensor from each wheel. By knowing the exact wheel rotation and angular acceleration rate, the control computers can "bang the stops" to modulate individual wheel speed.

Good Prius friend Hobbit tapped all four signals to make a relative tire pressure detection system. Based upon monitoring the relative wheel rotation rates, Hobbit has successfully detected asymmetrical tire rotation.

Bob Wilson

 

are the four pressures TIRE pressures?

 

Would a scan tool or specialized app like devtoaster allow reading this information off the CAN bus?

 

The sensors are there for monitoring purposes. Making sure the pressure are between the min. and max. values for that system. Their signals are processed by a brake system controller that then sends low, or OK data via the can bus to the cars main computer. They work the idiot light for the brakes mostly which isn't so stupid after all. This stuff is made by contract by Bosch, Denso ect.
Robert Bosch GmbH - Antilock Braking System ABS

Robert Bosch GmbH - Brochures and media

Robert Bosch GmbH - AE / Automotive Electronics

Can bus pdfs I think.
Robert Bosch GmbH - Automotive Semiconductors and Sensors

 

As established by others in this topic, apparently these sensors are not CAN-connected. So in this case we cannot read out these sensors from the CANbus.

Just to hook onto what someone else said about CANcommunications only going on an ECU-to-ECU level... This could be true for the Prius, but be aware that there are several types of sensors that nowadays have their own CAN-controller and thus get hooked up to the CANbus directly. SmartNOX sensors for example.

Ok back to the question... I do not know devtoaster, but any tool that allows you to hook up to a CANbus and read out CANframes will allow you to trace these messages indeed.
At my job we usually use professional tools (CANalyzer, CANoe, ETAS systems, etc), but there are many semi-professional tools that will do the same.
The problem usually is that without specs and datasheets it kind of a PITA to find out which message (from the hundreds you will see) is actually the sensor you are looking for

Sorry to go offtopic by the way

 

The Prius is a dual-circuit braking system. It's basically 'brake-by-wire' except that hydraulic fluid and pressure sensors are used to provide the input, rather than electrical position sensors as in the 'drive-by-wire' accelerator.



In normal use, the hydraulic lines from the master brake cylinder go only to a 'stroke simulator' - a piston that permits the brake pedal to move and provides braking feel. The master cylinder pressure sensors, PMC1 and PMC2, measure the driver's braking pressure. SMC1 and SMC2 are closed, so for the moment ignore the lines leading from there to the wheel cylinders.

The Skid Control ECU takes those sensor values and computes how much braking force to apply to each wheel. It asks the HV ECU to provide as much of the braking force to the front wheels as possible through regeneration, and the HV ECU reports back how much it achieved (this is done via CAN). The Skid Control ECU then distributes the remaining braking force required across the friction brakes, turning on the SLAxx solenoid valve for each wheel to Apply the brake further (increase pressure) and the SLRxx valve to Release the brake (decrease pressure). For example, SLAFL and SLRFL control the Front Left wheel and SLARR and SLRRR control the Rear Right. It monitors the pressure achieved with each wheel's Pxx sensor (PFL, PFR, PRL, PRR).

Pressure changes are achieved by having a chamber (accumulator) that stores fluid at high pressure. When an Apply valve is opened, the high pressure fluid flows to the lower-pressure wheel cylinder, applying the brake. The Accumulator Pressure Sensor (PACC) measures the pressure in the cylinder so the ECU can decide when to run the pump motor, to keep the accumulator pressurised.

The reason that hydraulics are used in the sensing side is so that in fail-safe conditions, the master cylinder is connected to some wheel cylinders, operating them directly from pedal force. This is where the Skid Control ECU cannot operate the brakes for some reason. Two valves that normally stay closed, SMC1 and SMC2, open, and the stroke simulator is closed off (valve SCSS). Instead of moving a piston in the stroke simulator, pedal input now directly controls the front wheel cylinders.

There is also an electrical brake pedal stroke sensor (a potentiometer), which is used to determine how fast the pedal is being applied. If the ECU detects a fast application it does Brake Assist, where it actually applies more force than the driver's pedal pressure indicates. It's been found that in emergencies, drivers tend to brake sharply but not actually hard enough.

The fact that the wheel cylinders are completely cut-off from the brake pedal, in normal use, means that the normal air bleeding procedure of opening a bleeder valve and pumping the brakes does not work. Instead you have to use the diagnostic computer to ask the ECU to bleed the brakes.

Back to the original question, the 'brake pressure indicator' that Bob talks about can be found at http://www.techno-fandom.org/~hobbit/cars/bp/ (which is where I stole the top diagram from). PMC1 is found on pin 30 of Skid Control ECU connector block S7, and PMC2 on pin 27 of block S10. Or, alternatively, PMC1 is pin 38 and PMC2 is pin 43 of the brake actuator. According to Hobbit's description, the sensors are powered by the ECU (VCM1 and VCM2, respectively) and grounded by chassis ground - GND1 and GND2, pins 31 and 32 of the actuator, which are common ground points EF and EE with the 6 ground points of the Skid Control ECU. Common chassis grounding is the norm in vehicle electrics.

 

Mike, that was a great writeup! (And a great link.) Thanks!

 

Just as a comment about posting etiquette, the OP stated that this question was in regard to the 2008 Prius, but posted this on a Gen III thread. All Gen III Prius are 2010.

Tom