anti lock break system (ABS)

oceanfrontview · October 10, 2004

Hello:

I'm new here first post.

I have 2000 Silverado with 50K miles. About two week I noticed that at low speeds when I was slowing down to stop my ABS was kicking in. I was on dry pavement and barely had my foot on the break. Since think I have almost crashed into two building because of the ABS kicking in when it should not have.

I order a service manual for the truck (yes the $135 one). I found a lot of info. in it but little that can be done with out a scan tool.

Let me describe my findings.

1. ABS light is off, the ABS computer is not detecting any faults.

2. ABS seems to function normally at higher speeds.

3. According to the service manual ABS events can only accrue at speeds equal too or greater then 8mph.

4. All of my ABS event problems accrue at speeds less than 8mph.

5. Both front speed sensors measure 1000 ohms & 1100 ohms. Manual states this range need to be from 700 to 2000 so seems to be good.

6. Both front speed sensors product AC voltages while spinning the wheel at voltage levels up to 200mV AC. According to the manual this voltage needs to be above 100mv AC. Seems to be good.

7. If I unplug either front speed sensor and start the truck the ABS light turns on.

8. Speedometer seem to function normally which mean the speed sensor on the drive shaft is functioning.

9. I think there is an error in the manual which talks about how to test the speed sensor on the drive shaft. Manual states to "unplug" sensor cable and measure sensor wire to ground. Igniting turned on. Value should be 5-7 volts DC. If the sensor is "unplugged" the is no way for it to have any DC voltage there for you should get a value of 0 which I do. I did an AC measurement across the two sensor wires and did measure about 2VAC while the drive shaft was rotated.

I think the real kicker here is number 3 & 4 from above. ABS should not turn on at these low speeds. It acts like the ABS computer thinks the truck is going much faster than it really is. At low speeds less than 8mph I believe the front wheel speed sensor become unreliable because of the type of sensor used "Variable Reluctance Sensor" so if the ABS computer thinks it going faster than what the truck really is and the computer is getting invalid readings from the front wheel sensors because the speed is too low for the Variable Reluctance Sensor to operate. I believe the ABS would do like it is and kick in when it should not.

That said What could the problem be? I know there is some sort of wheel size calibration number stored in the ABS computer, could this number have been messed up some how and now causing the computer to miscalculate the speed?

If so wouldn't it also cause the speedometer to show a false reading as well? Maybe or maybe not, depends on how the software works. Its my understanding all truck speed comes from the ABS computer through a serial data link.

Any thoughts?

I think I'm going to try to get my hands on a scan tool to verify the speed the ABS computer thinks the truck is going.

I have also removed the ABS fuse until I can find a solution.

There may be errors in the above statements because I’m not a mechanic. As stated above some of the statements are “I believe” or “Its my understanding”.

Thanks

David

budhayes3 · October 11, 2004

This seems to be a pretty common problem for the NBS trucks...do a search in this forum and I'm sure you'll find alot of info on it. Seems that cleaning the junk off the sensors seems to help, and I've also heard of some replacing wheel bearings w/ the reluctor ring. I'd try cleaning the sensors first though.

357mag · October 11, 2004

Had a similar problem with my 94, although my speedo would drop to zero at about 15 mph road speed. Then the !@#$%!! ABS would pump the brakes whilst I continued to move forward thru the stop sign. What a PITA. I too had voltage from the speed sensor on the trans, however, the ohm reading was zero with the sensor out of the truck. Chilton said 900- 2200 ohms or so should be a good sensor, so I bought a new one. Lo and behold, problem solved. That and a new vss buffer (DRAC) module to adjust the speedo for larger diameter tires. This may or may not help; ghood luck.

Tim sends

oceanfrontview · October 11, 2004

Had a similar problem with my 94, although my speedo would drop to zero at about 15 mph road speed. Then the !@#$%!! ABS would pump the brakes whilst I continued to move forward thru the stop sign. What a PITA. I too had voltage from the speed sensor on the trans, however, the ohm reading was zero with the sensor out of the truck. Chilton said 900- 2200 ohms or so should be a good sensor, so I bought a new one. Lo and behold, problem solved. That and a new vss buffer (DRAC) module to adjust the speedo for larger diameter tires. This may or may not help; ghood luck.
Tim sends

<{POST_SNAPBACK}>

I have found out there is a service bulletin for this problem. Turns out you need at least 350mV AC from the front wheel speed sensors. The problems are usually caused by a rust build up on the (I will call it) gear teeth which is where the sensor get the speed from.

I'm going to try to clean mine.

David

mhraracing · October 12, 2004

This is the EXACT same thing that is happening to my buddies 01 Dually Dmax/Ali. We trailered our Two place car trailer to the dragstrip this weekend and the stupid abs would come on when I just about came to a stop. I ended up pulling the fuse so the truck would stop correctly.

We are going to replace/inspect the front brakes this weekend - and will try cleaning the "gear teeth" to see if that fixes the prob.

Let me know if it works for you !!

Thanks

camaronut · October 12, 2004

Had the same problem on my 01 Sierra 4x4. The front speed sensors needed cleaning. You have to remove the brake calipers, then the rotors to get to them. There is a allen headed screw holdimg them in. Remove the screw, then CAREFULLY remove the sensor. It can be a real b*tch getting them out due to the wire that goes through the dust shield. Just be real careful because once you break them.......that's it...you're done....time to visit the dealer for parts. Believe me..I know..... been there done that.

And to make matters worse, the sensors are on national back-order which means at least a month before you get them, if you're lucky.

Once you get one out, clean the end of the sensor and if you really want to do it right, clean the "tone ring" inside the hub with a small screwdriver by removing the grease in between the "teeth".

Hope this helps.....

mhraracing · October 12, 2004

Thanks camaronut - I just talked to my buddy about this post and he told me he couldnt wait - had the day off today and did all the brakes - front and rear today. The pedal is back - not as mushy - but the ABS still comes on. He is going to try the cleaning method this weekend - or if he is lazy - may jsut bring it to the dealer.

macifre · October 12, 2004

I had the same exact problem, cleaned the ABS sensors and it did not work, replaced them and it worked to some extent, it did it in rare ocations. Anyway, I could not leave it at that because you cannot trust faulty brakes, and believe me... it is pretty annoying trying to halt your truck from 3mph to 0 and not being able to. What I finished doing is taking the whole thing appart, calipper, disc, hub... and when the hub was out I noticed that it was starting to go bad, so I bought a new one (wich comes with a brand new sensor anyway, about $170) and installed it. I have driven for about 800 miles since and had no problem at all. Brake confidence almost fully restored.....

Hope this helps...

What I realized is that the sensor by itself cost me $95, hub+sensor $170... next time I rather replace the whole hub+sensor than having an almost new ABS sensor in my garage....