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The tale of two faults

Yes boss, there really was more than one bad part.
Monday, October 1, 2012 - 07:59
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We came across a 2006 Lexus IS 350 with 56,345 miles on it that had a complaint of poor handling and unpredictable braking habits along with the occasional Antilock Braking System (ABS) light. This Lexus was optioned with Vehicle Dynamics Integrated Management (VDIM), meaning it has more than just ABS.


 

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Toyota’s VDIM is a system that monitors and combines vehicle motion control which integrates brake, steering, and drive force controls. This vehicle would unpredictably jerk to the right while going around a corner or operate the ABS for no reason while braking. The climate conditions were 

a big influence on how and when the system reacted, something that went right by us to start with. I sure wish I had paid more attention!

The system had four Diagnostic Trouble Codes (DTCs) logged when we tested the car, so we thought this should be straight-forward even though other techs had already had a stab at it. I had an independent study student who wanted to research this type of electronic stability control system, specifically researching this type of wheel speed sensors so I decided why not. The student dug right in and pulled four ABS DTC codes. He sounded disappointed when he said, “This should be easy.” Then he added, “I wondered why the other techs had such a problem diagnosing it.” Boy were we both going to get an education!

Diving In
First, let me explain that this vehicle has Active Wheel Speed Sensors meaning they have voltage and amperage applied to them so they are very sensitive and can even detect forward or rearward wheel speed down to wheel position at zero RPM. The rearward wheel movement is for the “Hill Assist” feature incorporated into this system. When you pull your foot off the brake, the system will not let the vehicle move by holding the brakes on until the gas pedal is pushed.

These sensors return a square wave digital signal toggling between about 6.8 to 14.4 mA with voltage toggling between 9 to 10 volts. They also have a different pattern for reverse, sort of a half wave with ears. The sensors react with magnetic tone rings integrated into the inner bearing race.

These sensors have built-in electronics to enhance both input and output signals. The 48 North-South or South-North poles, depending on wheel rotational direction, cause the two Magnetic Resistant Elements (MREs) in the sensors to toggle the line voltage and amperage by changing resistance between power and ground. Amperage will prove to be a key to diagnosing these issues along with an understanding of Ohm’s Law.

The interesting thing is the DTCs we found. After the student pulled the four codes, I began to think this would be a unique diagnosis. The Toyota Techstream Lite scan tool displayed a C0210 for a right rear Wheel Speed Sensor (WSS) circuit and a C0215 for the left rear WSS circuit. He also found a C1210 code meaning the Yaw sensor was not calibrated, but the interesting one was the C1223 code. This means the ABS unit itself has a malfunction requiring replacement of the Skid Control ECU (ABS controller). I definitely did not want to go there first.

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