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2006 chevy trailblazer_lt
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Discussion Starter · #1 · (Edited)
Took my 06 LT in for service yesterday to our mechanic for what I figured to be a brake issue.

- At very low speeds usually nearly stopped the brakes will pulse and make noise. Much like ABS is being engaged. It's not always at the same speed but usually this is when it happens.
- No ABS light shows up on the dash during this.
- Peddle becomes slightly mushy during this.

Mechanic says the front rotors are warped, and they are replacing under warranty because I had them changed there not long ago. They call me say it's all set to go. My wife gets into the car and at the very first stop light it does it again and does it all the way home. I jump in and take it around the block and get it to do it a couple times, but then it goes away for a while. Snow is on the ground and at high speeds if I slam on the brakes I can feel the ABS engage but again there is no light on the dash saying so.

Any thoughts on this? ABS light comes on when the ignition is turned on and vehicle goes through it's checks but it doesn't seem to actually be working when it is being engaged during operation.

Said mechanic also replaced pads/rotors on the rear, replaced a windshield washer pump and did an oil change during this service. Pretty upset that when I got the truck back I noticed on the paperwork there was 5.5 quarts of oil used for the change (I know the truck requires 6.9) also the oil change light was not reset. :hissy: This automatically tells me the mechanic was in a hurry and did not pay much attention to his work. Which is also probably why they went straight to warped rotors for the brake issue.
 

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2005 chevy trailblazer_lt_ext
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I can only comment that the ABS light only comes on at start up and if there is an issue with the abs. Not when it engages.
 

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2004 gmc envoy_slt
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The problem is with the front wheel speed sensors. Either one is defective, but more likely there is a build up of rust or corrosion under the mounting plate.The rust raises the sensor just enough to affect the sensor's voltage output. Have your mechanic inspect the area under the mounting plate. If he is a good mechanic, he may also have a way to measure the sensor's voltage output. That will tell him where the problem is.
 

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2006 chevy trailblazer_lt
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Discussion Starter · #4 ·
The problem is with the front wheel speed sensors. Either one is defective, but more likely there is a build up of rust or corrosion under the mounting plate.The rust raises the sensor just enough to affect the sensor's voltage output. Have your mechanic inspect the area under the mounting plate. If he is a good mechanic, he may also have a way to measure the sensor's voltage output. That will tell him where the problem is.
A friend of mine just text me and said the same exact thing. He said likely they need to be cleaned. I'm actually going to have him look at it in his dad's shop instead of taking it back to where I had it. It's a hassle for us to get it back to this particular place, so I'm not going to deal with it.

They also mentioned that the tech notes show 7 quarts of oil were used for the change, and that the lady up front had a typo. Oil level is fine too. So just a mistake on the paperwork for that.
 

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2005 trailblazer_lt
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I've encountered this problem on both my Trailblazers; It's a real uncomfortable feeling when the brakes 'let go' as your coming to a complete stop !

The 2004 I owned had minor pedal 'mush' when coming to a complete stop and I was able to correct the problem by removing the ABS sensor from each hub and cleaning it and by blowing compressed air into the hub. In the case of that truck there was some grease on the tooth ring inside one of the hubs. Put it all back together and no more brake fade when stopping at red lights !

I bought a 2005 to replace the 04 after the 04 was wrecked last spring. It had the same brake fade problem at low speed - but MUCH more significant. ( almost felt like I was going to crash into the car in front of me when stopping at a light. ) I removed the sensors and used compressed air again, but after reinstalling everything the problem persisted. I took both sides apart again and replaced both ABS harnesses with used parts and this time made sure to clean\scrape all the rust corrosion from the HUB around the ABS sensor hole. Don't know if the rust corrosion around the hole or the original ABS sensors were bad, but the brakes don't fade - even a tiny bit - anymore.

IMO it's a good idea to get/have replacement ABS harnesses available and ready to install before taking the wheels off and cleaning everything up real good. They are cheap enough and while I'm sure that in most cases a good cleaning of the original harnesses will resolve the problem,.... you might just save more time and $$ ( if your not doing the work yourself ) to simply replace the ABS harnesses when you first get into this repair. Otherwise - you might be doing this job twice !

:cool:
 

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I think EnvoyBoy2004 has got it right. Had the same problem and cleaned the mounting surfaces and put the sensors back. Of course I broke one so I got another from Rock auto. The pulsing at low speed problem went away but then my ABS light came on and stayed on. Also the Airbag light also came on and stayed on. And the Ebrake light and seatbelt light never came on. Finally got it all worked out and I think what fixed it was this...
Measured the ohms on the wheel sensors to see if they were any good. Should read about 1K ohms. Both sensors appeared good.
Then hooked up a volt meter to the same place where I measured the resistance and spun the rotor. Set the meter on AC volts.This is a bit of a pain because you have to take off the brake caliper and the bracket that holds it. Mount the rotor backwards with a couple of lug nuts and you can spin it sorta easily with a big screwdriver stuck in the rotor webbing. Spinning about 2 revs per second should give you about 3 millivolts.
The trick is to have each side give the same voltage. So how do you do this you might ask. By adjusting the distance between the sensor and the reluctor ring inside the hub. To do this you add or remove shims from the sensor. I had to get some shims from the local pick your own part junkyard. Fun day. Shims seem to range from 10 mils to 40 mils. Also just putting a sensor in using the shim that was there may or may not work. The voltages need to be about the same on both sides. And of course make sure the connection is good and snug when you plug the wheel sensor harness back into the wiring harness.
 

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2004 Trailblazer LS 4.20 L
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cant you make the distance between the rotors and sensors equal for both sides and forget all the backwards rotor spinning screwdriver bit? pretty sure that's how they are assembled. equal distance equal speed should equal equal sensor output. keep it simple. same parts, same torque, both sides would probably eliminate all the guessing. i like the information but seems a bit more effort than necessary.
 

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cant you make the distance between the rotors and sensors equal for both sides and forget all the backwards rotor spinning screwdriver bit? pretty sure that's how they are assembled. equal distance equal speed should equal equal sensor output. keep it simple. same parts, same torque, both sides would probably eliminate all the guessing. i like the information but seems a bit more effort than necessary.
Mr. Sparky...What you say should be true and makes sense. The problem may be that wheel sensors, hubs, etc come from different manufacturers and the tolerances are all different. I did try keeping the distance equal between the sensor and hub ring and that didn't work. Adjusted the distance to get the same voltage on each side and that worked. I saw to do this on another post so apparently other folks dealt with the same issue.
 

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2004 Trailblazer LS 4.20 L
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good reason to change both sides at the same time i guess. like we should do. but hey, everybody gets jammed up or rushed once in awhile. ever heard of the one guy that doesn't ever have the same problem everyone else has because he's the problem? i think we've all been "that" guy. nice you found a foolproof way to fix that one. someone has to be the one to get it right.
 

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Not on Trail Blazers --- but I had an ABS problem on a Cadillac some years ago where the wires from the pickups were twisted in different ratios and that generated an imbalance too.

The twists are to keep stray RF from impacting the system. Without twists - strange things can happen.
I know my add-on LEDs really mess with my radio and that's because they throw out a lot of RF "junk" and it's plays badly with my car.​

I always look to see if the twisted wires look about the same - and if they are equal or very near to it, there is a different impedance which I believe is causative enough to create havoc with the AABS computer.

It's not just "resistance" but "impedance" that makes a big difference, although if the wiring or sensors are different manufacturers - they could be a problem in and of themselves.

BTW --- I have never heard of shimming them to get even voltage outputs. Interesting!

But --- if everything's clean and the air gaps are equal and the twists are pretty much identical - then I think it should all work as designed.
:\
I'm still interested in this shimming technique.
Old dogs can learn new things.​
 
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