The tale of two faults

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.

After we cleared the codes and drove the vehicle for several miles no codes returned, but we did feel the sudden pull/dip sensation to the left while turning a corner. We looked at the repair history of the

vehicle, then interrogated the owner some more. We found that a right front WSS sensor had been replaced. The sensors cost around $250 each, but to replace the front sensors you have to pull the bearing/hub from the knuckle to get at it. The rear sensors are simple, remove a single bolt and pull the sensor out.

The owner had said before that the vehicle acted up more after a rain, and now I am starting to pay attention. They had not noticed anything while driving in the rain, but the next day there was usually a better chance the vehicle would occasionally be harder to stop with pedal vibration. And they thought they heard a vibrating noise sometimes. My thoughts now became, “Was the ABS unit itself having issues?”

We checked all powers and grounds for voltage drops while cycling the ABS pump, but they all read around a few hundredths of a volt drop. Now it was time to sit down, list the symptoms and codes to see if there was a connection. The only connection we came up with is the temperature from driving and the humidity after a rain. Time for another visual inspection on all the WSS connections and

wiring but nothing stood out.

After spinning our wheels for a few days, it rained so we hooked up the scan tool and set it to graph the WSS, then went for a drive. We were about to give up when just as we were turning to go back to the lab it did the jerking and the ABS light popped-on. The graphing showed a sudden 20 mph difference between the front two sensors and the back showed a 23 mph difference between them. Now it is getting interesting.

Knowing the scan tool is slow to display and update it was time to hook up a scope. We have a micro amp probe that reads milliamps. Why not hook it up so we can look at both amps and volts? It did not dawn on me at

the time that this playing just for the fun of it would provide the key to the diagnosis. By the way, when we checked the ABS DTCs we now had all the original codes back plus a C0200 (LF WSS circuit) and C0205 (RF WSS circuit). This was getting more interesting by the minute.After some discussion about why all the codes were appearing, we hooked up the scope using the micro amp clamp and also back probed for voltage on the right front WSS at the Skid Control ECU connector. The other two channels of the scope were back probed for both rear WSS voltages. Now it was time to go for another drive to see what we could find. We had a good pattern for a few miles and then the front WSS began to show an increase in amperage and a decrease in voltage. The right rear WSS’s voltage showed an

increase in voltage over the left rear with a decrease in milliamps. When we slowed and turned a slight left hand corner, the car dipped and jerked to the right slightly.

Now we are thinking the sensors are bad but why would one have an increase in amperage while the rear showed an increase in voltage? Time for some more thinking with the pencil and paper.

Writing it Out
Question, “How do you get amperage up and voltage down?” Answer, “A change in resistance.” So now the question became “Where is the resistance?”

We tore the wiring loom apart from the Skid Control ECU to the right front WSS but found nothing. We decide to raise the car on a lift and spin the wheels to check scope patterns. We noticed the right side WSS amperage and voltage were different. The left front and left rear were the same but the right front showed an increase in milliamp pattern.

The pattern differences were small but it was there. The low was 8.67 milliamps to a high of 16.12. The voltage was lower with the low side being 8.68 volts and the high only hitting 9.67 but the difference in rise was very close. We decided to look closely at the connectors. At first we did not see anything with the connector, wiring or sensor side, but we still had the key on and the scope hooked up. As the student unplugged the sensor the amperage dropped, but not to zero.

I just happened to notice the scope still showed about 4 milliamps. An open connection should have no amperage flow, so my first thought was the amp clamp was not zeroed. I removed the clamp to zero it but it went to zero. When the clamp was reinstalled around the wire the milliamps draw returned. This answers where the extra 4 mA draw came from. Remember less resistance equals more amperage plus this pulls the voltage down because of the extra load on the ECU.

We used a magnifying glass to inspect the connector more closely but almost missed it. I snapped several pictures but never could get the white dust on the white connector to show. Just rubbing the connector with my thumb stopped the amperage draw. The student sprayed the connector and sensor thoroughly with contact cleaner. After plugging the connector back in the patterns matched side to side. What the white dust (corrosion), aided by moisture, looked like was an old fashion carbon track in a distributor cap, but white instead of black. Remember this WSS had been replaced.

With three patterns looking the same we turned our attention to the right rear. Now the patterns on the rear matched. Thinking about it, we used a heat gun on the right rear sensor, which brought the lower voltage and irregular pattern back. This particular style of sensor mounting actually have the top and part of the middle of the sensor exposed so it was easy to heat the entire sensor. Apparently the sensor did not like the higher temps from driving and braking. The milliamp pattern was the same on the low reading but was two milliamps down on the high reading. We inspected the connector hoping to find corrosion as the reason for the lower amperage. Using the scope and turning the wheel showed resistance had to be in the circuit but when you cannot measure any with an ohm meter then you have to use Ohms Law.

Volts down slightly and amps down then resistance has to be somewhere or the output device is

defective, right? In this case that would be the Skid Control ECU. Thinking back about these sensors design, we remembered that the MREs are resistance elements which influence the IC chip. The IC chip is what changes the resistance of the sensor to cause a change in amperage flow through it. This is the reason for the voltage and amperage changes (toggling) of the amount up and down. So we swapped sensors side to side (they would not bolt down) and the bad scope pattern went with the old right side sensor. We installed a new $251 sensor in the right rear, cleared the codes, zeroed the yaw sensor, and went for a ride.

I was thinking about the other codes as we drove around for a half hour turning corners and stopping. The vehicle drove great with no problems or ABS light. We drove the car around for the next few days with no problems or irregular patterns so it went back home. It rained a few days later but we did not receive a call from the owner until about a week later. He was happy to report that the Lexus was driving great.

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