ABS pump activates when coming to a full stop

Albert Owner/Technician Pennsylvania Posted   Latest  
Resolved
Braking
2006 Chevrolet Trailblazer 4.2L (S LL8) 4-spd
Abs Pump Activates When Coming To A Full Stop

Hi Guys and Gals,

Here are the details: the complaint is that the brakes make a thumping sound when I put my foot on the brake pedal.

On the test drive, I noticed that the ABS pump was being activated on the test drive when coming to a full stop on the test drive

I've test drove on and in all road conditions and still the same results. 

Back in the shop, take all the wheels off and find all of the pads and calipers are in good condition.

I bled the ABS system of air because the pedal did seem soft, and the problem persists.

NOTE (Just under a year ago, I performed a complete flush of the brake system, and about six months to a year before that, I put new brake pads and rotors on the vehicle Also replaced shocks and strut assemblies two days ago)

There are no codes related to the ABS system.

My question is where or what should I be looking at to get to the solution, or do I need to bleed the ABS system again?

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Andrew Technician
Colorado
Andrew
 

Check the wheel speed sensors and wheel bearings first. And the tone rings.

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Roger Mechanic
United Kingdom
Roger
 

I’ve had something similar a few times , check the tone rings for cracks , if they are damaged they can give a false ( too low ) speed output to the abs system. Can you check the live data for individual wheel speeds when this occurs??

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Jackie Technician
Massachusetts
Jackie
   

Front wheel bearings(speed sensor) common on these check live data should see one dropping out

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Robby Technician
Alabama
Robby
 

Hi Albert, I doubt it's anything related to air in the system. I suspect this is false ABS activation due to a dropping out wheel speed sensor. That usually won't set a DTC and can be a little tricky to pinpoint depending on what scanner you're using and how many PIDs are selected. I would use the scanner to monitor each wheel speed sensor. However, only select and display the speed sensors in…

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Jeffrey Mobile Technician
Illinois
Jeffrey
 

Definitely what this guy says!!

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Tian Technician
New York
Tian
 

Hi there It sounds like there is sensor wheel have a broke tooth where the ABS sensor is reading off, I will check all sensor wheel near the hub and see if there is tooth broke off.

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Benjamin Diagnostician
Connecticut
Benjamin
 

If you can view live data, you might be able to see one or more of the speed sensors drop to zero before the others. Of course, the ABS interprets this as wheel lockup, not a malfunction (no DTC), so it releases the brake to “free up” the wheel. Once you determine which wheel seems to be “locking up” before the others, you can investigate that one. The usual culprits are cracked tone rings, rust…

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Glenn Owner/Technician
Texas
Glenn
 

Hello Albert, GM has multiple TSB's on this issue. The most common issue is a problem known as “rust jacking”. Rust/ corrosion develops under the wheel speed sensor mount areas and begins to lift the speed sensor. This distorts the signal to the ABS controller and the controller interprets that the wheel is skidding, so it reacts. The other problem, that others have mentioned is the tone ring…

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Bob Technician
Massachusetts
Bob
 

The issue is likely caused but an accumulation of rust underneath the front sensors. Remove the rotors and then pull the sensors out. Put a piece of rag or something in the hole to keep debris out. Then, take a chisel and small hammer and “gently” knock all the rust off the sensor mounting pad. Clean the surface up nice and smooth. Use a flat file if needed. Apply a dab of grease around the…

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Aldi Owner/Technician
Albania
Aldi
 

Hi I have the same problem with my 2004 opel meriva 1.6 petrol. I have checked with live data and the front right abs sensor doesn't read the speed under 4 km/h. I think this is the reason in my situation but honestly hadn't time till know to work with it. Maybe next week I find the time to replace abs sensor.

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Jared Diagnostician
Maryland
Jared
 

I wouldn't watch wss pids first for this one. I would watch which wheels' solenoid is being driven during your false abs activation, then spend your time on that corner looking for airgap or worn bearing.

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Benjamin Diagnostician
Connecticut
Benjamin
 

Hmm, yeah, I should work that into my diagnostic routine.

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Stuart Mobile Technician
Illinois
Stuart
 

This is very common with that vintage GM trucks and Suvs​.​Like the others mentioned of rust build up under the speed sensors sending false info to the system because of creating a larger air gap​.​Since the tone ring is incorporated in the hub if it has one, it just might be a magnetic ring, unless there is some play in the front wheels i wouldn't think that would be the…

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Ronald Technical Support Specialist
British Columbia
Ronald
 

When you connect with scanner to read abs data the abs system is disabled and you will not see any application of solenoids.

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Ed Owner/Technician
Pennsylvania
Ed
 

I don‘t recall this happening with my Modis? I know I’ve been able to see the solenoids activating.

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Jared Diagnostician
Maryland
Jared
 

This has not been my experience on GM products with a Tech2

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Paul Mechanic
Illinois
Paul
 

That's the warning on the tech 2, but never found it to be true. Just did an 03 envoy xl and rf sensor was last to respond and first to drop, abs was active during.

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Dwight Owner/Technician
California
Dwight
 

Nice tip. I would never have thought to check the ABS system solenoid for activation. Better than grafting the WSS. I'm going to try it out myself to see if works. Thanks!

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Albert Owner/Technician
Pennsylvania
Albert Update
 

Thank you all. I've looked at live data did not see anything dropping out but I was not looking at it in the graph mode. Again Thank you all for your advice.

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Ed Owner/Technician
Pennsylvania
Ed
 

Definitely graph them. I usually test drive very slowly… less than 10mph And let it coast to a stop looking for spikes or one sensor to drop quicker than the others. If you can’t spot the drop out then trying graphing the output of the abs module looking for which valve is dumping (opening). This will tell you which wheel it thinks is locking up. I’ve used this method on a couple I couldn’t…

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Bob Technician
Massachusetts
Bob
 

Scanner resolution is not the greatest for this. If you hooked a scope to all the sensors I'm sure it would be obvious which one has the issue. If you don't see any glitching of the signal on a scanner then I wouldn't bother scoping them, I would simply clean the rust out from underneath the front sensors.

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Paul Mechanic
Illinois
Paul
 

Depending on your choice of scanner, the update or refresh rate may not be fast enough- don't watch while you drive, snapshot it and playback. Easiest is to watch which wheel responds last when you start moving. I believe they don't read below 4-5kph. Also, one last thing, while it is most likely a wheel bearing reluctor air gap to sensor issue, confirm all tires are same size and air pressure…

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Alan Owner/Technician
Mississippi
Alan
 

Had forgot about the tire size being a possible suspect I have seen it cause premature activation on a few Ford products in the past. Very good suggestion !

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Stuart Mobile Technician
Illinois
Stuart
 

That would only affect it if they were mismatched like mixing 225s with 215s

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Glenn Owner/Technician
Texas
Glenn
 

Hi Albert, The problem with graphing on a scanner is what is known as latency. In reality the scanner has to go through processers in the unit itself. This is why a scope is the best way to test wheel speed sensors. A scanner is good for reference, a scope measures in real time. I have been down the road of chasing scanner graph data many times. When it was actually tested with a scope, the…

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Rick Technical Support Specialist
Missouri
Rick
 

Please graph those ABS WSS.

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Mark Owner/Technician
Pennsylvania
Mark
 

What we have seen, especially on the full size trucks is, as some have said, one of the signals is dropping out at slow speed. Usually we are able to remove the effected speed sensor(be careful, they break easily). Some are riveted and cannot be unbolted. If it has the bolt in it, remove the bolt and gently remove the sensor. You will more than likely find a rust build up on the hub/wheel…

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Bob Technician
Massachusetts
Bob
 

Agreed, the rust pushes the sensor away from the tone ring. Removing the rust is all that's needed to solve the problem.

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Mark Engineer
Colorado
Mark
 

Albert Great advice so far. I agree with check wheel bearings first. Then get something (scanner/scope) to graph WSS and after you know which sensor has a speed loss then concentrate there. Could be broken wires inside insulation (although that would probably set a code). Looking forward to seeing your RESOLUTION post!

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Jesse Owner/Technician
Pennsylvania
Jesse
 

I agree with what you guys are all saying about the rust jacking and possibility of a bad wheel bearing I once checked distance from the tip of the sensor to the tone ring and the Gap was nearly an eighth of an inch with that little special washer under it I clean the surface and eliminated the washer and got it down to a ⅓2 of a gap my favorite method of testing the sensors is to set my digital…

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Stuart Mobile Technician
Illinois
Stuart
 

The only ones that I know of that have the washer under the sensor are Ford trucks​.​GMs don't have washers that i've known.

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Alan Owner/Technician
Mississippi
Alan
 

I have to agree with the other technician on the wheel speed sensor even though it is registering does not mean is working properly for causing frequency glitches. Unplug all of the wheel speed sensor and take it for a ride see what happens. That will eliminate half of the systems. Better yet just. Unplug front two first because generally that's where the problem lies because of weight and…

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Phillip Educator
Ontario
Phillip
 

What you are experiencing is likely an unwarranted ABS actuation. Likely one of the front sensor signals is weak and drops out before the other side. Monitor wheel speeds as you are slowing and note if one drops out before the other side. If you're in the Salt belt then rust is likely pushing the sensor out of the hub casting. If you can remove the sensor and clean the rust to restore the air…

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Timothy Owner/Technician
Wisconsin
Timothy
 

To assure yourself that the whole issue is related to the ABS, remove the fuses for the ABS system and repeat the test drive. If the issue is totally gone, you can be assured. You'll not have codes, as the ABS module ‘believes’ that one of the wheels is locking up and is trying to release that wheel. You can also just release the steering wheel as you come to a stop and see which way the wheel…

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Curtis Mechanic
Florida
Curtis
 

Hey Albert, connec ur scanner and record wheel speed sensors at the end of your stops too c which relucter ring is cracked, split, or just corroded

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Louie Technician
New York
Louie
 

This exact same thing happened to me when I replaced a left front wheel sensor on a jetta . It wasn’t seated properly because of rust buildup removed it again cleaned the hole with a wire brush reinstalled sensor with antiseize worked perfectly

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Bruce Owner/Technician
Massachusetts
Bruce
   

It's likely a wheel speed sensor dropping out causing false activation. A couple of ways to attack this. Find an empty parking lot, use a scan tool to monitor wheel speeds PIDS, and see if you catch the one dropping out. Alternatively, take your hands off the steering wheel, come to a slow stop, and observe which way the vehicle pulls when the false activation occurs. It will pull away from a…

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Albert Owner/Technician
Pennsylvania
Albert Resolution
 

Thank you all for the advice. I put the scanner in graph mode and checked the sensors, but no luck; however, I did find that the front right isolation valve was picking up the problem. So I checked the right front sensor, and the only thing I found was grease build up on the sensor, so I cleaned off the grease, put it back together, went on another test drive, and the ABS pump did not…

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