ABS wheel speed

[hex]

Hi guys, i have a GL 450 2007 and I am having issues with the ABS, first issue was i was driving along at about 20-30 MPH little yellow triangle came on and all the brakes applied brought me to a dead stop in the middle of the road!
Get it on the scanner (DCI150) and found the front left wheel speed sensor had an open circuit, so replaced that. take it for a test drive and go above 3 MPH all the brakes apply again, back on the scanner shows the rear right is intermittent so replace that as well.

Clear all the faults, start off on a drive, and get ABS fault and lights on the dash, but the car will now drive and change gears. So back on the scanner and bring up the faults again front left wheel sensor, clear the faults, and go to the "flight recorder "on the DCI scanner load up the 4 wheel speed sensors onto the chart go for a drive and the chart shows that every so often there is a bit of a dip for a split second on the front left .

Again clear the faults and load up the 4-wheel speed sensors onto the chart, notice there is an additional chart for wheel speed left and right so load that up as well , off for a drive ABS warning comes back on, checking the charts there is a dip for a split second on the front left but the wheel speed left starts off ok then flat lines the wheel speed right chart is up and down with the speed.

Is there a separate speed sensor for the wheels on the front?
anybody got a wiring diagram or where can i get one

should i be looking at something else?

 

[FlorinM]

Could it be a problem with the wiring did you do a continuity test?

Or the magnetic part of the wheel bearing could be damaged/dirty/rusty which is preventing the ABS sensor from reading the wheel rotation consistently.

 

[hex]

Could it be a problem with the wiring did you do a continuity test?

Or the magnetic part of the wheel bearing could be damaged/dirty/rusty which is preventing the ABS sensor from reading the wheel rotation consistently.

I am thinking it is the wiring, on the scanner I am getting a reading for the front left ABS sensor and it charts it as expected with the blips as in the pic attached.

the wheel speed chart drops off for the left wheel then flat lines and i think that's whats causing the issue but i don't know if there is another sensor for that , the spedo is working fine as well

 

[FlorinM]

Here is list of all the components of the ABS/ESP system. There are only 4 speed sensors, 1 for each wheel.

I couldn't find a simple electrical diagram for it though.

 

[hex]

must be the esp unit then

 

[FlorinM]

Not sure about declaring the ESP unit as defective.
It's pretty expensive, and it will probably need to be coded to the car using Star Diagnostics.

After all the investigation you have done, maybe you can take to a MB specialist and pay for 1 hour diagnostics, and get their opinion on this issue.

Maybe other forum members, can recommend a place close to your location or other things to check.

 

[hex]

its been suggested that it could be the magnetic ring, can you just replace the ring or is it a full wheel bearing?

 

[alexanderfoti]

Sensors are common on these. I'd be inclined to fit two fronts sensors.

 

[Jobsworth]

must be the esp unit then

Not necessarily. The reluctor ring that the sensor is reading is inside the wheel bearing. The fault could still be there, you haven’t proved it isn’t. I have made up a couple of extension looms, purely for testing, so I can connect the left wiring to the right sensor and the right wiring to the left sensor. I tie them up under the car and test again exactly as you have already. If the fault remains where it was the input channel in the ABS module is faulty (or the wiring). If the fault has swapped sides it’s the wheel bearing. You can test the wiring with resistance meter end to end. ABS module faults aren’t unheard of, but they are not common.