ABS Light ON - Left Rear Wheel Speed Sensor "Open"

[lasturbo]

I finally checked out why my ABS light is on (from past threads, I thought it was the dreaded computer failure and delayed checking it). It has been on since I purchased her in July 2008 and the fault is Left Rear Wheel Speed Sensor "Open." Anyone have any issues or feedback on probs. with speed sensors?

[johnny010]

just replace it?

[mdwyer]

the fault is Left Rear Wheel Speed Sensor "Open."

I hope I'm not stating the obvious... forgive me if I am.

Speed sensors transmit speed info to the computer by opening and closing a switch as the wheel goes around. The computer counts how many times the switch opens and closes within a time period, and calculates a speed from that.

The computer is reporting that the switch appears to be stuck open, and isn't closing anymore.

So, here's what I'd look for:

• Broken or disconnected cable - check both ends of the cable, between the wheels and the headunit.
• Bad sensor - You might be able to hook up a multimeter to the sensor and spin the wheel and see if the continuity is changing. It depends on the type of switch, though.
• Bad encoder - something passes in front of the speed sensor to cause the switch to change. It might be a magnet, but it is probably something that looks like a gear.

The tech manual on Tone's page will probably help. I can't get to it right now, or I would give you more specifics on the ABS system. Testing for a bad sensor is probably tricky. You'd need to have the whole VX up on jack stands to safely spin the wheels, and even then if the sensors are hall effect sensors, they're probably more complex than the simple switches I described... and so they might need more than just a multimeter.

Bottom line, check the cables, then read up in the tech manual.

[lasturbo]

Good info. and thanks for the time to put it together. I could not find in the vehicross shop manual how to diagnose it other than how to replace it. It could very well be a bad sensor but wanted to get some general feedback to see if the speed sensors was one of the problem areas of the VX which it sounds like it is not. I do have access to a "rack" to test the car so it just a matter of getting the time to do it. I have not checked out Tone's tech manual yet and will soon. Tone, Lisa(VX Crazy) and I have been working with each other on transplanting her rear diff. out of her RIP VX into mine as well. So I need to add that to my "VX to do" list. Again, I appreciate it.

[VehiGAZ]

mdwyer's analysis sounds right. I would definitely try to replace the LR speed sensor. They are not necessarily prone to failure, but there certainly have been some. If the ToD or ABS can't tell what the wheels are doing, they cannot function correctly. Good luck!

[mdwyer]

Well I found a few minutes to go through the manual -- this is the one called IV00_01.pdf. The sensor is indeed a hall effect sensor. The good news is that the tech manual includes tests that can be done with a multimeter.

From page 5A-12:
It says, keep the car turned off and unplug the the cable that goes into the ABS control under the hood.

Set meter to 2k ohm, pins (22, 23) it should be 1.3k to 1.9k Insulation Resistance
Set meter to 200k ohm, pins (22, 15) more than 100k Internal Resistance
Set meter to 200mV AC, pins (22, 23). You should get 200mV while you spin the wheel at 1 RPS.

Page 5A-15 has pictures of the plugs. The one under the hood is a C-4 connector. The one at the wheel is an F-3. You can probably do the 1st and 3rd tests right at the wheel to rule out the cable or the sensor.

Page 5B-4 shows how to replace a wheel sensor.
Page 5A-19 says to look for iron filing stuck to the sensor probe.
Page 4C-3 shows a picture of the sensor's rotor.

Finally, a quick note: The vehicle has at least six speed sensors. I think a lot of people confuse the TOD sensors (two on the drive shafts near the transmission) with the ABS sensors (one on each wheel). Be careful when you're searching that you're looking at info for the correct sensor.

[Tone]

The VX has 3 speed sensors total on the tranny/TOD, 4 ABS sensors and a deceleration sensor on the ABS controller. The two rear ABS (L&R) sensors are only sold as a $400 set as they combine to a single connector.

Solid lit ABS light means there is a problem (short, open, etc) but the ABS system is still working. FLASHING ABS light indicates a serious problem with the system.

ABS does NOT engage until the vehicle travels above 6 MPH and automatically disengages and resets at 3 MPH.