Toyota Prius ABS C1251 and C1256

Toyota Prius ABS C1251 and C1256

Relearning and matching of Toyota Prius ABS C1251 and C1256 units is necessary. This service can also be availed in case the traction control light has lit up. We also recommend a brake oil flush with this service so that the ABS unit can give a long life. Toyota Prius, Aqua and newer radar variants are available with us. Our techs perfectly match the ABS units with other on board electronics.

C1251 – Malfunction in hydraulic brake booster pump motor
C1256 – Malfunction of accumulator low pressure

The Toyota Prius ABS C1251 light and the Traction light can turn on due to any problem with braking system. Since most of the Hybrid vehicles use regenerative braking to store electricity, any problem in ABS can cause a decline in the efficiency of vehicle.

Your Prius’s ABS system uses a system of sensors to determine wheel speed when braking. The antilock brake system runs through the ABS module. This module physically controls the anti-lock brake system If it becomes damaged, and can no longer control all four brakes, the Anti-lock Braking System (ABS) light will activate. Typically, corrosion is the most common reason why the module fails.

Toyota Prius ABS Warning Do Not Repair

If you hear any hissing noise then be careful with your cars brakes. This usually represents a failing unit with a leaking accumulator. Although, it depends a lot on the DTC,s stored in the car, we recommend a professional scan tool. Improper fitting of transaxle assembly can cause failure to this system as well. This service involves replacement of unit but is referred to as ABS repair service.

If there is some increase to the individual rate of any of the wheels when accelerating, this wheel receives breaking pressure to reduce the difference. During this time, the electronic control unit changes the torque. The Anti-lock Braking System indicator light does not flash before, during and after the above action, i.e. control unit does not perceive the situation as an error, but as the normal operation of ABS brakes.

Toyota Prius - Braking System Repair Issues

The brakes are control by the Brake Control Module (BCM), rather than the BCM regulating the pressure produced by the driver, as is the case with most braking systems. The purpose for this is so that the Hybrid control module and the BCM can work together. Simply put, when the brake pedal pressed, the Hybrid Control Module and the BCM assess and negotiate information about the driver’s intentions in order to satisfy the driver’s expectations. The end result is a mix of regenerative braking and hydraulic system application.

Hydraulic Control Unit

The system appears to be standard, however there is no hydraulic connection between the master cylinder and the brake callipers in normal operation. Instead, a hydraulic pump generates the pressure need to activate the brakes. The pressure at the master cylinder and the brake callipers is monitor via sensors. The brake hydraulic module has several sensors. Fault codes will be generate if there is an abnormality between these sensors.

How it works

The master cylinder isolation valves  closed when the system is turn on, the stroke sensor solenoid valve is open, and the rear brake linear valves V8 and V10 are close (as can be seen in the picture).

When the driver applies the brakes, the BCM gets information regarding speed and location from the master cylinder stroke sensor, as well as pressure sensors 1 and 2 on the master cylinder. The stroke simulator piston travels under pressure to provide the driver with the same feel as a traditional system while also allowing the master cylinder to stroke. The BCM evaluates this information and determines a braking technique.

There may be no hydraulic pressure supplied to the callipers while braking at speeds above 10 km/h, and all braking is done by regeneration (the drag of the big motor/generator in the gearbox as it charges the battery). The regenerative and hydraulic brake applications will share the increase braking demand. The Controller Area Network used to communicate between the Hybrid ECU and the BCM (CAN).

Controlling the linear valves allows hydraulic brakes to be applied. Pressure-apply valves V3, V4, V5, and V6 are pressure-apply valves, whereas pressure-release valves V7, V8, V9, and V10 are pressure-release valves. The linear valves are control by a Pulse Width Modulated Current (PWM), which allows for extremely accurate pressure control in the braking callipers. The apply valves are open, allowing pressure to develop in the callipers. The pressure vented to the master cylinder when the release valve is open. The calliper pressure and wheel speed are both monitor and compare to data in the BCM. This enables the braking effect to be tailored to the driver’s needs.

Prius ABS

Fault Codes

Brake pad replacement on the 2004-09 Prius does not require any special tools, however when the brake pedal is pressed for the first time to bring the brake pads into touch with the brake disc, error codes ‘C134x Hydraulic System xxx Malfunction’ may be produced. This is due to the fact that the BCM does not foresee a large pressure spike in the calliper. The codes can be removed manually at the EOBD socket or with an appropriate scan tool.

The hydraulics on the brake system may be bled without any special equipment, although it’s preferable to use the scan-tool for this. The hydraulic module must be initialised whenever it is removed or changed; this procedure allows the system to determine the linear valves’ properties.

Brake Bleeding System

The hydraulic module’s valves revert to their default position when the ignition is switch off for more than two minutes. The master cylinder cutoff solenoids have now been remove, allowing brake fluid to flow to the front brakes. The two ABS relays are remove to prevent the system from becoming activate. To bleed the front brakes, first pump the master cylinder to build up pressure in the system, then release the bleed nipple to let the fluid out. The pipes leading to and from the master cylinder and the stroke simulator are bled properly when the front brakes are bled this way.

After replacing the two relays and turning on the ignition, the rear brakes may be bled. G-Scan will come in handy here. Choose ‘bleed rear left’ or ‘bleed rear right’ from the menu and follow the prompts. Alternatively, the rear callipers can be bled using accumulator pressure, although this requires extreme caution and will result in fault codes that must be clear.

Brake pad replacement is a straightforward procedure. When the relays are remove, a clean path from the rear callipers to the reservoir and from the front callipers to the master cylinder is formed, as shown in the above diagram. During piston retraction, Toyota does not advocate clamping the brake hose and venting the calliper at the bleed nipple.

Note: the following is not a complete set of instructions on brake bleeding, it’s to demonstrate what is happening in the hydraulic system during the process. Remember you should always follow vehicle manufacturer’s instructions.

Clearing the DTC (Diagnostic Trouble Code)

The ABS/VSC errors cannot be removed by disconnecting the negative battery cable. A scan tool can be use to clear codes. ABS codes can be be obtain as blink codes and clear using an OBD2 connection. Between pins 4 and 13 on the 16-pin OBD2 socket, a Special Service Tool, or SST (a fancy term for a bridging wire), is insert. The ABS/VSC light will flash when the ignition is turn on. All ABS/VSC codes will be clear if the brake pedal is apply eight times in five seconds, then the SST is release.

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