ABS Sensor? 02 Tahoe?

[Matt C]

My ABS keeps kicking in on dry pavement, unprovoked, and is quite scary at times as I can't stop when its kicking in.

My google searches have mainly said cleaning the sensor and or replacing it. Where is this said sensor located at?

Thanks!

[jeffreyd]

you need to figure out which wheel. you can pull the tires off the front and possibly access them from the back side of the hub. they slide into the back of the hub and do gather dirt and metal shavings from the rotors. if you remove the brake pads, the rotor you will see it right there. you could take the tires off, put it on jack stands for safety, take some brake cleaner and spray between the rotor and backing plate. wash all the trash out of it and try and push it in to contact the rotor, it will adjust itself when you drive. if you are not familiar with the system i would take it to someone who can diagnose the issue. the ABS light should not do anything except say that the abs is not working and the computer is picking up a trouble code. It should not hamper braking at all.

[Matt C]

I replaced the front pads myself about 10k ago, so I am pretty ok with pulling the stuff off and nosing around.

Thanks for the fast response! Looks like I have a beer drinkin garage project tomorrow!

If cleaning the sensors off at the wheel's doesn't help, is there a 'master' sensor somewhere to replace?

I should also note, the ABS light has never come on.

[quickfish6]

See if there is an update(flash) for the abs computer. There is a very good chance that there is an update for it. Had a friend that had a problem with that in his chevy and he had the same problem. Got the update for the abs computer and he never had a problem again. You will have to call the dealership to see if they are able to flash your computer for an update.

That is probably the problem . Good luck

[Macgyver55]

There was a service bulletin on these several years ago. The sensor mounts in the hub assembly and rust gets between the sensor and its mounting surface causing the gap to change. But, before doing anything, make sure your tire pressures are correct! Even a low tire can set these off in some instances. You'll need to pull off the brake caliper and rotor to access it, then remove the sensor. Its a good idea to plug the sensor hole with a piece of paper towel to keep the crud from getting into the bearing assembly when you are cleaning. Next, clean the surface down to until you see shiny metal with no rust scale. Before reinstalling the sensor, (dont forget to remove the paper towel) then put a light coating of waterproof grease on the surface to prevent rust from reforming. Reassemble, then do the other side. Unless you have an ABS capable scanner you'll have a tougher time figuring out which side is the problem anyway.

[troutkingmaster]

Quote:

There was a service bulletin on these several years ago. The sensor mounts in the hub assembly and rust gets between the sensor and its mounting surface causing the gap to change.

I would bet $1000 that this is your problem. VERY common issue with many early 2000s GMs. Here is the text from the TSB

"MAKE/MODELS: MODEL/BUILD YEARS:

Chevrolet/Avalanche 1999-2002

Chevrolet/Silverado 1999-2002

Chevrolet/Suburban 1999-2002

Chevrolet/Tahoe 1999-2002

GMC/Sierra 1999-2002

GMC/Yukon 1999-2002

GMC/Yukon XL 1999-2002

MANUFACTURER: General Motors Corp.

NHTSA CAMPAIGN ID NUMBE : 05V379000 MFG'S REPORT DATE: August 29, 2005

COMPONENT: Service Brakes, Hydraulic: Antilock

POTENTIAL NUMBER OF UNITS AFFECTED: 804000

SUMMARY:

Certain pickup trucks and sport utility vehicles may experience unwanted Antilock Brake System (ABS) activation. This condition is more likely to occur in environmentally corrosive areas. This recall will be launched in the "SALT BELT" states of Connecticut, Illinois, Indiana, Massachusetts, Maine, Michigan, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Vermont and West Virginia only.

CONSEQUENCE:

This can cause increased stopping distances during low-speed brake applications, which could result in a crash.

REMEDY:

Dealers are to remove the wheel speed sensor and thoroughly clean the wheel speed sensor mounting surface on the bearing, apply rust inhibitor to the cleaned surface, grease the mounting surface, reinstall the wheel speed sensor, and check the peak-to-peak output voltage to ensure the wheel speed signal is within specifications. The manufacturer has not yet provided an owner notification schedule. Owners should contact Chevrolet at 1-800-630-2438 or GMC at 1-866-996-9463. "

I believe dealers are required to clean these free of charge but I may be mistaken. In the meantime, I would consider pulling your ABS fuse to ensure that this does not cause an accident.

[BoxMN]

PS - this also occurred on my 98 k1500. It was scary when you got to a stop sign, no doubt! It started off only occassionally, and then got worse. I had them cleaned once by the local small shop, but it didn't work. Then I was gettting tires, and the shop knew all about this, and fixed it for me, I think it was about $100 or so for them to do it, and it has been flawless since. Not sure what they did, maybe the full full cleaning and rust inhibitor thing mentioned about. GM would not do mine for me for free, at the time, as I was not on the list of "affected vehicles"... heh.

[Jeremy airjer W]

If this is happening at slow speeds than the problem you have is referred to as "overactive ABS"

A scan tool is the best way to narrow down the wheel that is causing the problem. I wouldn't have any problem removing the fronts and cleaning as described above. More often than not is one of the fronts causing the problem. be careful when removing them. If the break off in the hub you'll be spending a lot of extra time digging them out if you can even get them out. The sensors are sold by themselves so if one does break, you wont necessarily nave to replace the whole hub/bearing assembly.

[Matt C]

If this is happening at slow speeds than the problem you have is referred to as "overactive ABS"

Thats exactly it! Never goes off at high speed braking. Thankfully!

My truck has had the recall fixed a few years ago.

I will bring it in and have em scan to see what wheel is the issue before I get to active in tearing stuff apart.

Thanks ya'll!

[1968]

A scan tool will not tell you anything if it's a sensor gap issue. It won't set a code. Even monitoring wheel speeds is not very helpful.

When the hub is rotated by hand, the sensor should produce a minimum of .5 volts ac on a multimeter. Around 1v is better.

[Jeremy airjer W]

A scan tool is very help full! The data it shows will help you determine the wheel that is dropping out. I will usually graph all 4 sensors on the scan tool, drive it, and look for the wheel that is dropping out. Then I can go to that wheel first and start investigating.

Takes 5 minutes to hook up a scan tool and figure this out.

[1968]

A scan tool is very help full! The data it shows will help you determine the wheel that is dropping out. I will usually graph all 4 sensors on the scan tool, drive it, and look for the wheel that is dropping out. Then I can go to that wheel first and start investigating.

Takes 5 minutes to hook up a scan tool and figure this out.

Not denying that a scan tool is essential, but a multimeter will be more useful on this one. Maybe with monitoring solenoid activation it could be narrowed down. I have never had wheel speeds indicate anything definitive and no codes either. Voltage measurement is THE diagnostic method on this symptom.

Which front wheel doesn't really matter to me anyway. Unless I see a newer hub assy, both sides get fixed. It may be only one side, but the other will be right behind it. A voltmeter will back that statement up.

[Jeremy airjer W]

Every GM, Nissan, and Ford I diagnose with this problem I have no problem seeing which wheel speed drops out first with a scan tool.

[1968]

The first ones I encountered (all GM) had intermittent "overactivity". Wheel speed data looked completely normal even when they malfunctioned. I have since not gone any farther than pulling codes when this symptom rolls in and grab the meter right away.

A voltmeter,preferably one that can record, is the only way to verify a weak signal and also to verify an effective repair. An oscilloscope works pretty good too.

I was wrong earlier. Mimimum voltage is 350mv ac while rotating the hub by hand.

[Matt C]

Update*

I replaced my front rotors and break pads last night. The inside's of both rotors just trashed and pitted.

I did just replace the pad 20k ago and everything was nice and smooth.

So hoping this was the issue, it's not. I now have (started yesterday before i changed it out) a grinding noise coming from the front end, like breaks being stuck...and the ABS is still kicking in at slow speeds.

Could it be a caliper issue?

[Matt C]

Here are a few pictures.

[Macgyver55]

Did you ever clean the sensors?

[Matt C]

I did while in there last night. Looks well used for a truck with 170k on it, but no grease on it.

After 3 more ours of google searches today, I am pretty narrowed down to a faulty sensor, so the cleaning wouldn't do anything, or the calipers

I am however on my way to the auto store to just buy a new speed sensor/abs sensor. $77 bucks, 5 minute install. If that doesn't work, I'll replace the calipers.

[Jeremy airjer W]

You might want to check the wheel bearings in the front for play. With the tires of the ground push pull at the 12 and 6 o'clock positions on the tire. If you feel movement it is likely the wheel bearing that has some play.

[Jeremy airjer W]

I wioll aslo add that it is not uncommon for the front rotors to look like they do in your photo's. Make sure the caliper bracket that the pads rest on are nice and clean. You can also apply a little high temp anti seize to this area to help keep the pads moving freely.

The other thing is don't be afraid to get on the brakes hard every once and a while. This will help boil off any moisture on the rotors and help with the rusting as well.

[Macgyver55]

I did while in there last night. Looks well used for a truck with 170k on it, but no grease on it. After 3 more ours of google searches today, I am pretty narrowed down to a faulty sensor, so the cleaning wouldn't do anything, or the calipers

I am however on my way to the auto store to just buy a new speed sensor/abs sensor. $77 bucks, 5 minute install. If that doesn't work, I'll replace the calipers.

Jer is correct in that a loose/worn wheel bearing could give a similar symptom, but are you saying you removed the sensor, cleaned it and it still does it, or it "looked" ok so you didnt remove it? I wish I could tell by looking..... but if you do end up putting in a new sensor, make sure to clean it well before installing the new one. 5-Minute install? The mounting bolt sure, but not the other stuff you need to remove to get to it. I just wouldn't put one on until I knew that was the problem.

Calipers will not cure the ABS problem.

[jeffreyd]

no one asked if you pushed the sensors back in all the way before installing the new rotors? as you install the new parts, it will push the sensor out and when you drive it the air gap will be set to the new rotor. As far as a multimeter to check the sensor that is fine if you dont have a scan tool however the system is on a .5volt system so if you have 1 volt that is too high.

[Macgyver55]

This vehicle uses a sensor mounted in the hub and bearing assembly. There is no involvement with the rotor.

Here is the exact part, I just finished putting one in.

[1968]

You need a multimeter! ABS reluctor/sensor's are nothing more than an AC generator. I'm not happy till they produce around 1.0 volts ac when rotated by hand. The brains of the system only counts the voltage pulses. If they aren't loud enough, the brain doesn't hear them. Weak enough signal=no wheel speed. The sensor gap needs to be measured electrically.

An actually faulty sensor will always set a code and turn the light on.

If this is all too frustrating, just throw some hubs on it instead of calipers.

If you decide to fix it, don't forget to stuff some paper towel in the sensor hole to keep debris out of the hub while you are cleaning up the sensor mounting surface.

[mechanico]

McGyver55 and 1968 are right. Any how, with that kind of mileage, I wouldn't hesitate to replace all that stuff; rotors, hubs, pads and sensors. (rubber break lines too) If that doesn't do it, you will have to check all the electrical connections on the system that you should have checked to start with. You shouldn't have to put anything like oil or grease on the sensors. They are meant to take the abuse, but knocking the loose rust out can't hurt. If you must, use lithium arisol grease. It will dry on the surface and won't collect as much junk as other greases. And it repells water very well. I use it on lotsa stuff to keep water out...even door locks. When I'm testing a brake job I take it out to the mall during off hours and slam it around to give it a good thorough test. Just watch out for the mall cops.