Most ABS systems have a standard layout of: ABS ECU; Hydraulic control module; and Wheel speed sensors(all wheels have WSS)
Because ABS is ECU controlled, it is wise to understand the circuitry of operation of the ABS.
In a wiring diagram practice, the point is to have the ability to identify which wire is which: by colors and pin numbers. It is important to identify the control wire from the ECU, or the power side from the ABS relays, or which solenoid is which pin number etc...
In the ABS wheel sensor, the reason for the braided wire is to prevent interference from external radio signal; or inductance from other close high amperage wires.
For each individual wheel, the ABS system pretty much just actuates the Fluid Inlet and Outlet Valve, to control the brake pressure and free wheel under its own programmed style.
Under normal braking: The Inlet valve opens also the Outlet valve opens, this doesn't diminish the brake pressure because the pump is running as well, so brake pressure increases as much as enough for the wheels to lock.
When ABS system kicks in is when the brake system is about to lock the wheels, as the driver depresses the brake up to a point where ABS kicks in. Then, these 3 periods of ABS operation take place as 1 cycle of ABS braking, given that the driver still keeps his foot on full brake during the whole brake length:
1. Reduction: Inlet valve closes, Outlet valve opens in order to reduce brake pressure.
2. Hold: While break pressure is decreasing, it is then necessary to hold that low pressure by keeping both valves closed, so wheels can turn and this provides steerability.
3.Increase: Inlet valve opens, outlet valve closes, this in prior to the pump will accumulate the fluid pressure, causing the wheel to be locked up again, and the whole cycle repeats until the car safely stops.
A misconception that the circuitry part of the ABS is quite troublesome, wrong, the ECU is robust and almost never give any problem, it is the hydraulic, mechanical side that sometimes get clogged up, stuck Opened or Closed that fail ABS. Especially, the wheel speed sensor is the most frequent cause of ABS failure, simply because the if the mechanism is damaged, then the signal is unclear to the ECU.
WSS: same concept into the crank or cam sensor: they can be reluctor, hall-effect, Photo-sensitive etc….
Let’s look at the reluctor WSS:
This consists mainly of a tooth wheel that rotates with the wheel’s speed and a magnetic pickup. Whenever a tooth moves closer and closest and starts moving away, it induces a sine wave signal through the pickup, and that goes to ECU as the ABS’s input.
In order to perform this check, first get a car on a hoist, remove the wheels, bring the car up, then the sensor mechanism can be worked on easily. Then use an oscilloscope to examine the signal that theoretically it should give:
This is a very important step in diagnostic, because the signal waves pretty much tells everything about how good the sensor is working. A visual check is also important, it helps spot out the obvious faults and can be prevented from future failure. When a signal is not as good as it should be, then one of the problem could be the gap between the tooth (at closest position) and the pickup magnet. Using a metal feeler tongue would damage the sensor, therefore a brass tongue should be used. The part that gets damage is the braided cover that prevents interference from other radio signals, also removing noises, this protects the clarity for the signal.
Unfortunately, we were not able to get the Mazda Familia ABS actuator screen on scan tool, but we got the diagnosis screen:
There are 5 faults, mostly dedicated to the 4 WSS. Because this is FWD, and we hoisted the car up to run the front wheels, therefore the rear wheels were not spinning, only if they are on the ground. The ECU normally gets all 4 wheels spinning @ same speed for the test but now it thinks that there is a fault that the 2 rear wheels didn’t transfer any signal.
No comments:
Post a Comment