ABS engaging at low speeds Technical Investigation

[mngopher22]

Hello all. I am posting to give back to the community because I have learned so much from here. I have been battling an ABS issue for the last six months. The ABS would kick in at low speeds and braking. Recently it got really bad wehre it would engage at higher speeds when braking etc. Also i started to notice the death howl of a wheel bearing going out. I thought that the abs was going because the wheel hub was going bad. I diagnosed that the front left wheel hub was the bad one by administering the turning test. I changed out the front driver sides wheel hub and the howl was gone but the ABS was still acting up. So i decided to dig into it more and determine what the cause of the ABS issue was. I used an oscilloscope on the new front wheel hub. I disconnected the abs wire, measured the output of the senor while rotating the tire. The output of the sensor is shown in frontdriver.png. The sensor output looks good. I then went and measured the output of the sensor on the front passenger side while rotating the tire. The output was zero and is shown in frontpassenger.png. The output of the front passenger sensor proves that the front passenger sensor is bad. Luckily i still had the bad bearing that i replaced on the front drivers side. I measured that sensor and it was still good. I removed that sensor from the bad bearing and put it in front passenger side wheel hub. WALLAAAAAA I don't have anymore ABS issues anymore. If you have access to a oscilloscope it can make the world of difference when diagnosing various issues with your vehicle. You could also do this test with a multimeter but a oscilloscope gives much more info.

Hopefully this helps future GMTNationers in diagnosing ABS issues. Cheers!

 

[MDBT]

Thanks for sharing your testing, I didn't realize the sensors could be swapped. I had this problem with my GMT330 but it eventually went away after I had to swap the front bearings out twice (bad ball joints tearing them up).

 

[mngopher22]

Thanks for sharing your testing, I didn't realize the sensors could be swapped. I had this problem with my GMT330 but it eventually went away after I had to swap the front bearings out twice (bad ball joints tearing them up).

Yeah they can on the trailblazer, its one hex screw, so you will need a hex bit and I am forgetting the size off the top of my head. Once the screw is out, it takes quite a bit a pressure to remove it from its fitting. It has a seal and that lip kinda makes it hard to remove and put back in but rest assured you can remove and replace just the sensor.

 

[The_Roadie]

Ahhhh, thanks! So the ABS controller sees a flatlined signal, and concludes that the wheel isn't spinning, so it fires the ABS on the other wheels. Hmmmmm, we're lucky they didn't design it to fire at higher MPH.