ABS is coming on? - Brake pedal judder?

When you pull the fuse, you disable the ABS system and make it standard brakes, that is why the light is on and they will lock up. Has the light ever come on when it is malfunctioning? Does it do this over bumps or train tracks? A scan of the ABS system may be in order to see if the computer saw a failure and stored a code

It sounds like the ABS is actuating. First, check the wiring to the sensors. If they are good, make sure the sensor and startor is clean. If they are caked with dirt, it could cause the ABS to actuate. Also, if you can, have the computer scanned to see why the ABS light is on. It should point you in the right direction.

Let me know what you find.

Joe

Hello,

In the diagrams down below I have included the factory diagnostic troubleshooting guide for your vehicle for Direct Trouble Code (DTC) C0265. Please go through this guide and get back to us with what you are able find out for your vehicle's problem.

Thanks,
Alex
2CarPros

The wheel speed sensors on GM trucks generate rather wimpy signals to start with, and anything that reduces that even more will make it look to the computer like that wheel stopped turning. In response the ABS kicks in to try to get that wheel turning again. On most truck brands the ABS stops activating below 3 - 7 mph, but even when they still do, most don't reduce stopping distances like on GM vehicles. The purpose of anti-lock brakes is actually only to maintain steering control, not shorten stopping distances, but most vehicles DO stop quicker when the ABS kicks in.

I assume you have four-wheel anti-lock brakes, not rear-wheel, and it's a 1500. GM has had a common problem where rust builds up between the hub and wheel speed sensor. That pushes the sensor away from the tone ring which weakens the already weak signal. Many people think they solve the problem by replacing the speed sensor, which will work, but the better fix is to remove the two front sensors and clean the rust off the mounting surfaces on the hubs, and to clean off any rust or metal filing on the end of the sensor.

If there was an electrical problem with a sensor, typically a broken wire, that would be detected by the computer which would set a diagnostic fault code, turn the system off, and turn the yellow warning light on to tell you. That hasn't happened so we can assume electrically the sensors are okay. They just aren't generating a solid signal. Anything that generates a voltage generates a smaller voltage as the speed decreases. That's why you get the false activation only at lower speeds. As the problem gets worse, the false activation will occur at higher and higher speeds.

If this only happens at low speed stops, (like in a parking lot) it is probably corrosion buildup under the front speed sensors. If it is just at random, it could be the module going out. Yes, they can have intermittent failures. You should monitor the front wheel speed data on a scan tool when the fault occurs. Look for the signal to drop out on one of the sensors.

Subject:
Antilock Brake (ABS) Activation At Low Speeds (Clean Wheel Speed Sensor Mounting Surface)

Models:
2002-2006 Cadillac Escalade, Escalade EXT
2003-2006 Cadillac Escalade ESV
1999-2006 Chevrolet Silverado
2001-2006 Chevrolet Suburban, Tahoe
2002-2006 Chevrolet Avalanche
2003-2006 Chevrolet Express
1999-2006 GMC Sierra
2001-2006 GMC Yukon, Yukon Denali, Yukon XL, Yukon Denali XL
2003-2006 GMC Savana
2003-2006 HUMMER H2

Supercede:

This bulletin is being revised to provide additional warranty information. Please discard Corporate Bulletin Number 03-05-25-007B (Section 05 - Brakes).

Condition

Some customers may comment on ABS activation at low speeds, usually below 8 km/h (5 mph). Upon investigation, the technician will find no DTCs set.

Cause

The cause of this condition may be an increased air gap between the wheel speed sensor and the hub reluctor ring due to rust and debris built up on the sensor mounting surface.

Correction

Measure AC voltage and clean the wheel speed sensor mounting surfaces.

1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in the General Information sub-section of the Service Manual.

2. Disconnect both the front wheel speed sensor connectors at the frame and harness.

3. Place a Digital Volt Meter (DVM) across the terminals of each wheel speed sensor connector.

4. Rotate the wheel clockwise approximately one revolution per second. The minimum reading should be at least 350 ACmV's. If the reading is less than 350 ACmV's, remove the wheel speed sensor. Refer to the applicable Wheel Speed Sensor Replacement procedure in the ABS sub-section of the Service Manual.

5. Plug the wheel speed sensor bore in order to prevent debris from falling into the hub during service.

6. Clean the wheel speed sensor mounting surface using a wire brush, sand paper, emery cloth, scotch brite, or other suitable material. Be sure to thoroughly clean the wheel speed sensor surface. There should be no rust or corrosion.

7. Check the sensor head to determine if it has been warped/distorted due to the corrosion build up or other causes. Check the mounting surface on the sensor head for flatness by placing it on the edge of a metal machinists scale or other suitable straight edge to measure the flatness. Check the sensor for flatness in multiple (minimum 3) positions/directions. If the sensor head is distorted, replace the sensor.

8. Apply (spray) two thin coats of the specified rust penetrating lubricant (corrosion inhibitor) to the complete sensor mounting surface on the bearing hub. Allow to dry for 3-5 minutes between coats. Use ONLY Rust Penetrating Lubricant, P/N 89022217 (Canadian P/N 89022218).

9. When the corrosion inhibitor is dry to the touch (about 10 minutes), apply a thin layer of bearing grease to the hub surface and sensor 0-ring prior to sensor installation. Use ONLY Wheel Bearing Lubricant, P/N 01051344 (Canadian P/N 993037).

10. Install either the original sensor or a new one in the hub. Ensure that the sensor is seated flush against the hub. Refer to the applicable Wheel Speed Sensor Replacement procedure in the ABS sub-section of the Service Manual.

11. Place the DVM across the sensor terminals and recheck the voltage while rotating the wheel. The voltage should now read at least 350 ACmV's.
This was a common problem on these vehicles.

If the ABS light is off, are you turning the wheel as this happens? If yes, check hub/wheel bearings for play.

I had the same issue with my 2000 3/4 ton. The problem was that the ABS sensor, which is located at each front wheel bearing assembly (only one side caused the problem) was going "Out of range" due to corrosion. It was a simple fix (ie no money). What happened/happens is that the ABS sensor (which is a magnet in a plastic housing), the item is bolted to the wheel bearing housing, and if you live in an area that uses salt on the roads will cause corrosion. Rust will begin to form on the top of the wheel bearing (between the housing and the sensor). That little bit of rust will push the sensor up and the magnet will not read the toothed ring gear inside the wheel bearing housing. There is one bolt (8mm I think) that retains the sensor, take it off and carefully twist and pull the sensor straight up out of the housing. When it is out, put a cotton ball in the hole and use a knife or other instrument to scrape the rust off the housing (the cotton ball is to keep the rust from falling in the open hole of the wheel bearing). Once you get the big chunks of rust off, use a wire brush to clean the area real well. Put a film of grease on the outside of the sensor (that goes into the hole), take the cotton ball out and re-insert the sensor back into the hole. Affix the retaing bolt and (after you put the wheel back on) take it for a test ride. You will say."Holy monkey-butt, that was easy and cheap". It worked for me.

Let us know how you made out with the repair.