I'm slowing working through a few issues with my 2011 MCS. I had a rear wheel vibration which I figured out was badly scalloped tires, a telltale sign of worn rear dampers. I replaced the dampers on all four corners with Koni yellows, figuring the fronts were probably pretty worn as well. While I was doing the dampers, I thought I might as well replace the wheel bearings too, since the car has about 90k on the odometer. Fast forward to now and the wheel bearings are all replaced.

The issue I'm having now is the right front wheel speed sensor is registering as returning no value according to my scanner. I got a replacement wheel speed sensor from Pelican (Febi brand) and installed it over the weekend. I cleared the fault code, and took the car out for a quick test drive, only to be greeted by the ABS fault light about 25 feet down the street.

I pulled the car back into the garage and decided to do a little recon on the sensors. I pulled the connection (blue plug) for the new sensor (passenger side), attached a multi-meter to the two leeds, set the multi-meter to AC volts and rotated the wheel to see what the output was: nothing. I did the same thing to the driver's side and got the same thing: nothing.

Am I completely off base trying to view the AC voltage output from the sensor as the wheel rotates? I read through an article on pelican parts: http://www.pelicanparts.com/techarti...or_Testing.htm where they were diagnosing the same thing, but they were back probing with the electrical connector from the sensor still connected and reading DC volts.

I'm at a loss for how to actually test the wheel speed sensor itself, since reading AC volts wasn't working last night. My other courses of action are to swap the driver and passenger side sensors and see if I can get the error to reproduce on the driver's side, but that entails 30 minutes of work for 15 seconds of actual testing. Any other ideas?

 

Try rotating the wheel slowly while monitoring the sensor for DC volts. Some DVOM's load the circuit, while a scope won't, that's one reason they suggest a scope.

 

I am still confused why the sensor output would be DC volts and not AC. I cannot imagine the sensor has a power supply that would output a DC signal voltage. Other wheel speed sensors I've seen are a passive magnetic sensor which outputs an AC voltage based on the rotation of a magnetic or toothed ring on the hub.

 

You want to read AC volts, you gotta turn the wheel really fast and read a very low voltage --- I'm guessing 1 - 2VAC, maybe less. The Pelican article specified DC and a scope, and one of their pics showed a waveform. Can't read the scope scale setting, but max DCV is probably less than 5. Chances are, the ECU reads the frequency, not the actual voltage, to determine speed.

Sensor power should come from the ECU (one lead) --- similar to the MAP sensors. Sensor output will be the other lead. Both connections must be made for the system to function. This means to monitor the system, you gotta splice or somehow tap into both wires while it's ready to run. Sounds like you expect the sensor to provide it's own power --- not likely. Probably needs to be mounted to a good solid electrical ground too.

In the computer world, AC is not as easy to work with as DC. Try following the pelican article but substitute a good DVM for the scope, looking for voltage swings of zero to 5DCV max. Obviously, the ignition needs to be "ON". Might be other "interlocks" too --- like tranny in gear, or something.

Finally, a disclaimer --- all this comes from lotsa years of electronics test / troubleshooting experience. I've never messed with these speed sensors.