Ford Fiesta ABS fault diagnosis to exclude two cases

Published on by Ford

Cases of a failure phenomenon:

A Ford Fiesta Fatal crash in motion, so that the right front side of the vehicle ABS pressure regulator assembly and piping severe damage. After replacing the pressure regulator and hose assembly, the car repair. Test and found that some of the wheels have locking phenomena, and some wheels without braking.


According to the above phenomenon, suspected to be a problem with the ABS system, first check the car dashboard settings ABS malfunction indicator. Showed normal (ie launch vehicles off after about 2s). Diagnostics using WDS. Found no fault code appears, commissioning and during braking. There elevation of the brake pedal feeling of pulsation described ABS work. The ECU will pull out the plug to test the conventional manner, the results show four wheels were normal (ie drag and India). According to the above inspection, the final ABS system determines the fault remains, the self-diagnostic system ECU but it can not detect a fault. Therefore still on the ABS system for analysis.

From the principle of the brake system architecture carnival look, the car with ABS electronically controlled independently controlled by the electronic control system, hydraulic system and conventional brake components. ABS electro-hydraulic control unit (HECU) electrical control part (ECU) to get the car's speed signal from the arranged inside each wheel by ABS wheel speed sensor harness to determine the operating condition of the wheel. When braking, the brake pedal is applied to a sufficiently large force when the wheel tends to locking, ECU 即向 hydraulic control unit (HECU) instruction issued by a hydraulic adjustment, HECU adjustment instructions wheel braking the brake fluid pressure by a solenoid valve dynamic pressure regulator (ie, the pressure built, maintained, blood pressure). According to the principle structure of the above analysis, the car is a four-wheel independent control. ECU receives a wheel in a wheel speed signal to determine how the pressure regulator, i.e., a wheel speed signal and the wheel pressure regulating correspondence. If a received signal is a wheel speed, the other wheel away control of the pressure regulator, i.e., when a locking tendency of the wheels has issued the wheel speed signal received by the control ECU, slow locking away Another hydraulic wheel, without being reduced so that the locking; At the same time, should the locking wheel speed control signal has been away ECU receives a wheel locking tendency, so that the pressure, hold dead. As a result, some completely locking the wheels, and some wheels are no brakes.


From the above analysis, but also on each pipeline eleven checks. In this process, found that although there are differences pipe joints, but due to lack of attention and repair process to pick the wrong pipe, which appeared above the fault. Adjust the pipeline, troubleshooting.

Example Two Symptom:

A Ford Fiesta sedan. Traveling over 70,000 kilometers. The car ABS malfunction lamp is lit, the owner of the service station to repair my vehicle.


First with WDS read fault codes, display 00295, G45 is the right rear wheel speed sensor fault. This failure is generally caused by the following conditions:

● When the speed exceeds 10km / h, there is no speed signal is transmitted to the ABS control unit

● When the vehicle speed is greater than 40km / h. Speed ​​signal exceeds the standard value

● recognizable presence sensor circuit or positive, Ground wire short circuit fault

So should focus on the following tests:

● Line connection with the ABS wheel speed sensor control unit

● The wheel speed sensor is mounted and a ring gap, the installation location, and dust contamination or impurities

● fault sensor itself

In the car troubleshooting process, the first did not rush to check the speed of data. Engine idling, choose to read the data. After rising vehicle lift, and then observe the display data.

Wheel when stationary, the display area are displayed 0km / h, right-hand rear wheel is rotated, the second display area 8km / h. And the other wheel is rotated, a display area corresponding to observe that the basic line. Down vehicles, with WDS clear fault codes, ABS light is off, road test everything is normal.

WDS data reading, the second display area display group into the right rear wheel speed, both in the acceleration, deceleration, braking, the data is consistent with the other three wheels. ABS lamp is not lit, it can feel ABS braking system actuation, failure did not occur. As we prepared to return to the factory, the fault light illuminates again, out of the right rear wheel fault code occasional failures. According to the diagnostic status of the car, the only one possible, and that is to produce instantaneous short circuit or open circuit between the right rear wheel speed sensor and ABS control unit. According to the circuit diagram examination revealed, ABS control unit in the 24-pin connector pin 8 has a slight corrosion. Cleaner Repair, clear fault codes, test, everything is normal ford vcm ii.

Fault summary:

Asked the owner that, after washing the vehicle with high-pressure water cleaning the engine compartment, the reason can be imagined, the water splashed into the ABS control unit, resulting in the instant case of disconnection has occurred. It reminds us once again, in the diagnosis of such a failure, first consult the owner recent car situation, so detours.

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