RX300 Anti-Lock Brake Issue
05-24-06, 12:58 PM
#1
Driver School Candidate
Thread Starter
RX300 Anti-Lock Brake Issue
Wanted to start a new thread regarding this issue. Lexus 1999 RX300 AWD. As mentioned previously, the anti-lock brakes are actuating sporadically when braking. It’s not consistent when it happens, mostly under slow speed gentle braking. I am hoping to generate some discussion on this issue. (Tammy thanks for your previous comments, I will have that on my list)
The Anti-lock brake actuator is controlled by an ABS ECU. The inputs to the ECU are the brake pedal switch and speed sensors. The outputs are signals which control the ABS actuator (solenoids and pump).
Once you hit the brake pedal, the brake switch (in addition to completing the circuit for the brake lights), provides a 12 volt signal to the ABS ECU. This tells the ECU that braking is occurring. The ECU then monitors the speed sensors and the variation between them. If there is enough variation, then the ABS ECU tells the ABS actuator to apply and release the hydraulic pressure individually to the front wheels and combined to the rear wheels. This prevents wheel lock-up.
In my case, the brake pedal switch is telling the ECU that braking is occurring, and that’s good. The fault appears to be the speed sensors input telling the ECU that a large enough variation is occurring such that the ABS actuator operates when not needed, not so good.
So my first task is to see if I can pull the business end of the speed sensors out of the wheel hubs and inspect for cleanliness. Perhaps one is dirty or damaged. I hope I do not damage one; a replacement is over $200.
If the speed sensors look okay, my next step would be to disconnect the brake pedal switch input the ABS ECU. Drive the vehicle and prove that the ABS actuation goes away.
Other things I could do are to check the resistance of the speed sensors to ensure they meet specs. It could also be a faulty ABS ECU. Has anyone heard of a Technical Service Bulletin or other information from Lexus on this issue? Any comments welcomed.
The Anti-lock brake actuator is controlled by an ABS ECU. The inputs to the ECU are the brake pedal switch and speed sensors. The outputs are signals which control the ABS actuator (solenoids and pump).
Once you hit the brake pedal, the brake switch (in addition to completing the circuit for the brake lights), provides a 12 volt signal to the ABS ECU. This tells the ECU that braking is occurring. The ECU then monitors the speed sensors and the variation between them. If there is enough variation, then the ABS ECU tells the ABS actuator to apply and release the hydraulic pressure individually to the front wheels and combined to the rear wheels. This prevents wheel lock-up.
In my case, the brake pedal switch is telling the ECU that braking is occurring, and that’s good. The fault appears to be the speed sensors input telling the ECU that a large enough variation is occurring such that the ABS actuator operates when not needed, not so good.
So my first task is to see if I can pull the business end of the speed sensors out of the wheel hubs and inspect for cleanliness. Perhaps one is dirty or damaged. I hope I do not damage one; a replacement is over $200.
If the speed sensors look okay, my next step would be to disconnect the brake pedal switch input the ABS ECU. Drive the vehicle and prove that the ABS actuation goes away.
Other things I could do are to check the resistance of the speed sensors to ensure they meet specs. It could also be a faulty ABS ECU. Has anyone heard of a Technical Service Bulletin or other information from Lexus on this issue? Any comments welcomed.
05-24-06, 01:41 PM
#2
Lexus Champion
Just one more thought ...
It might be worth the investment to take your RX to a Toyota dealer (or Lexus) and have the ABS brake system analyzed by their diagnostic computer. As it talks to all the components of the system, you not disturbing any part that is not failing. Then you know if this is an electronic or hydraulic failure. It could very well be both.
As this is an electronic system; and you cannot see it work as if it were mechanical. And the parts are too expensive to replace till it's fixed.
Good luck with fix !
It might be worth the investment to take your RX to a Toyota dealer (or Lexus) and have the ABS brake system analyzed by their diagnostic computer. As it talks to all the components of the system, you not disturbing any part that is not failing. Then you know if this is an electronic or hydraulic failure. It could very well be both.
As this is an electronic system; and you cannot see it work as if it were mechanical. And the parts are too expensive to replace till it's fixed.
Good luck with fix !
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