SOLVED! Brake pedal ABS-like response below 10 mph w/o ABS light on

SpitShine_PL

Original poster
Member
May 26, 2023
29
Poznan, Poland
Well, a little mystery here, pestering me on and off since the Burb (2002 K1500) got a new set of Powerstop pads and rotors with new rear caliper pistons.

1. The brake pedal can do the ABS-like jitter (pulsing) when the road speed is below 10 mph, especially with the front wheel turning left or right.
2. The ABS light does not come on whet this happens. It is operational, it comes on with the dash light test (ignition on and engine start).
3. The live PID readings from the ABS speed sensors on the wheels during driving, and when the brake pedal does the ABS-like jitter, is somewhat weird. The wheel speed values from the sensors while driving straight ahead may vary between each other (and the speedo reading) by as much as 6 mph.
4. When this mysterious ABS-like brake pedal reaction happens and I continue to brake to park, the ABS speed sensors on the front left and rear right wheels keep reading more than 0 mph with the car stationary (the reading can be 1 to 4 mph, this varies); as if they output a false speed signal.
5. The brake system was bled after the brake job.
6. This thing happens erratically - some times 2 times in a row, sometimes every 6th brake operation to less than 10 mph.
7. No ABS trouble codes present.

Is it possible that at least 2 wheel speed sensors are shot? What about the ABS tone rings?
 

Sparky

Member
Dec 4, 2011
12,927
Yes entirely possible. So different platform but on my old Trailblazer, I was having phantom low-speed ABS triggering when almost at a stop, similar to how you're describing. A little while later, my driver side bearing started making noise so I replaced it. The phantom ABS triggering stopped after the hub replacement. I'm not the only one who has run into this. Something going on in the hub before it started making noise started messing with the ABS.

Now again I know, it is a different platform, but the system is still similar. I would be suspecting tone rings gunked up somehow, and/or sensor issues. Maybe you've got a bearing giving a little bit of play just enough to distort the wheel speed sensor signal.
 

mrrsm

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Supporting Donor
Member
Oct 22, 2015
7,349
Tampa Bay Area
The VOP (Video Original Poster) demonstrates a practical way to Test the ABS at the Wheel Hub for the very same symptoms you describe using a DMM set to Ohms to Test the Hub Side Harness for Resistance (which should be about 1,000 Ohms) ...and then set to A/C Voltage and taking the readings while Hand-Spinning the Tire at Low RPM from the ABS Wheel Hub Harness (which should read around 450 Millivolts). Then he shows what additional R&R actions to take for an effective repair... AND of course, he performs a Re-Test confirming "The Fix" has worked:

 
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TollKeeper

Supporting Donor
Member
Dec 3, 2011
7,949
Brighton, CO
Had the same issue on my Envoy when I bought it from the dealer..

Dealer just replaced the Hubs with sensors and wires. Problem fixed.
 
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SpitShine_PL

Original poster
Member
May 26, 2023
29
Poznan, Poland
Thanks, guys!
I researched this while waiting for someone to chime in here and reached similar conclusions.
The GMTs are not super popular where I live, but owners in Poland also suggested hub/sensor replacement.
To be 200 percent sure, I'm gonna remove the ABS module fuse and replicate the conditions which trigger the issue; I am fairly confident there's no air in the brake system and that no brake hose has failed, but you never know, right?
I'm also leaning towards hub play... I can hear the wheel noise change when the front wheels turn to left or right; the change is not very pronounced, but it's definitely there. Had something like this happen to a few of my previous cars (worn hub bearing play/noise pitch change consistent with cornering).
 
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TJBaker57

Member
Aug 16, 2015
2,846
Colorado
Unfortunately, I only got Torque Pro hooked up via V-Link Bluetooth to OBDII interface.

This is exactly why I asked. While I cannot say with absolute certainty for your specific truck platform I can say for TrailBlazers and my 2005 Yukon th Torque Pro setup for wheel speeds is completely BOGUS.

It will "appear" to give wheel speeds but that is not what is really happening.

It is a bit complicated but what Torque Pro is showing in my experience is the Vehicle speed, not the individual wheel speeds.

The issue is that the EBCMs do not support the reading of PIDs in the manner that Torque Pro and other such apps use.
 
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SpitShine_PL

Original poster
Member
May 26, 2023
29
Poznan, Poland
I suspected as much. Torque Pro has four separate PIDs for "ABS wheel speed sensor speed", and they are tagged as 'GM-specific', but I couldn't tell how reliable the readings were, but they were there. Other readings from the ECU checked out fine, though I noticed it took about 2 minutes for the app to display some of the readings; cannot say why.
I tested the same V-Link BT to OBD adapter with Torque Pro on my '16 Challenger SXT, noticed the same pattern - some PIDs took more time to be read than the rest; it might be because of the way the vehicle modules process and output the data, or it might be the app itself. I understand the app does not provide the best performance or diagnostic capabilities; that's why I used Forscan (the full version) when I owned a '02 Grand Marquis.
The BT to OBD adapter I got (available as V-Link or V-Gate, this varies) is a great product, anyway - had zero issues with it using Forscan on the MGM, both in data reading mode and transmitting commands to vehicle modules.
 

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