Help me diagnose ABS problem!

My 2000 Yukon XL, is experiencing some ABS issues, and I can't find any discussion of similar problems, so I thought I'd ask you guys.

I am not getting any ABS warning lights, but when I am stopping, just before I come to a complete stop, on dry pavement, the ABS vibration kicks in. Just for the last 2-3 feet or so. Also, occasionally it will feel like something is preventing me from pushing the pedal all the way down. More force will get the truck to stop, but it feels like the power assist is going away.

If I pull the ABS fuse, the dash lights light up, but the brakes work perfectly. (just no ABS.) I thought it might be a sensor going bad, but from what I can find, that should throw a warning light.

Any ideas?

It could be a failing wheel speed sensor that the ABS unit is not recognizing. Going on a test drive with live data streaming might help you find the bad sensor(s).

I'll try that. I was thinking that if it wasn't showing an indicator light, the scanner wouldn't tell me anything.

Does your "ABS vibration" come with "ABS noise", ie do you hear the pump running?

If a WSS signal was going away, that would definitely not cause the system to begin cycling. In fact, the failsafes in the software would recognize the WSS signal going away and it would: 1. light the yellow ((ABS)) light and 2. inhibit the system from activating.

Also, there is a speed threshold below which the system will not cycle. That speed is typically around 3 mph.

If you feel something that feels like "ABS vibration" at such a low speed (as in the last few feet of a stop), I'm going to suggest that you've got a friction interface issue, ie something going on where the pad meets the rotor. Perhaps the pads are worn down to the underlayer (the part of the friction that is bonded to the shoe plate), which is not as noisy as steel-on-steel but is still much rougher than normal. It is also possible that you've got a wheel bearing going away and it is only noticeable under the conditions you've described.

Does your truck have a vacuum booster or a hydroboost?

There is a TSB for premature ABS activation from rust buildup at the WSS mounting point. the fix was to sand down the pad and replace the sensor. BTDT.

Brian

Ranger50 wrote: There is a TSB for premature ABS activation from rust buildup at the WSS mounting point. the fix was to sand down the pad and replace the sensor. BTDT. Brian

i'm not sure if the last 2-3 feet of a stop (copied from OP) is considered "premature".

You are driving the same basic truck as me. The ABS is hateful and over intrusive on them, but you likely have a faulty wheel speed sensor, or a cracked tone ring, or an almost broken wire going to one of the wheels. The previous posts about scanning is good advice, but you've probably introduced some extra codes by unplugging the fuse. Might be able to pick up variable speeds or pending codes on the scanner.

Silver bullet time: Look for rust on the tone rings, or squeezing the sensor, and tug on the wires going to the front wheels where the wire would normally flex when turning the wheels.

The only thing scarier than driving on snow with an ABS equipped vehicle is driving one that works poorly. "Off" would be a massive improvement. Fix it before it kills you.

In reply to AngryCorvair:

Yes I hear the pump running. Or at east it sounds like what ABS brakes sound like when the system activates. I thought that I should have a yellow light come on if the sensor was going. I'll lift the truck and see if anything feels like it is dragging.

I am embarrassed to say that I have no idea if the truck is vacuum or hydroboost! I am pretty sure that it is vacuum, but I haven't paid attention. I try to ignore this vehicle as much as possible! It is my winter beater / tow vehicle.

all of the conditions Streetwiseguy mentioned would set a fault code, disable the system, and illuminate the yellow ((ABS)) lamp.

rereading Ranger50's post, when he said to "sand down the pad" i'm pretty sure he's referring to the flat surface on the knuckle where the sensor goes through. there might be some value to this, although i've not heard of it before. if it's documented via TSB there's a reason for it.

does it happen at the end of every stop, or could you narrow it down to only when you go over an expansion joint or hit a bump or only when the truck is empty or any other condition? also, please try to notice the speed at which it happens. that'll help with diagnosis.

i've been in the ABS engineering world since 1993. i'm not just making E36 M3 up.

Copied TSB text:

Antilock Brake (ABS) Activation At Low Speeds (Clean Wheel Speed Sensor Mounting Surface) #03-05-25-007 - (09/29/2003) Antilock Brake (ABS) Activation At Low Speeds (Clean Wheel Speed Sensor Mounting Surface) 2002-2004 Cadillac Escalade, Escalade EXT

2003-2004 Cadillac Escalade ESV

1999-2004 Chevrolet Silverado

2001-2004 Chevrolet Suburban, Tahoe

2002-2004 Chevrolet Avalanche

2003-2004 Chevrolet Express

1999-2004 GMC Sierra

2001-2004 GMC Yukon, Yukon XL

2003-2004 GMC Savana

2003-2004 HUMMER H2

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.

Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in the General Information sub-section of the Service Manual (SI Document ID #899664). Disconnect both the front wheel speed sensor (WSS) connectors at the frame and harness. Place a Digital Volt Meter (DVM) across the terminals of each WSS connector. 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. Plug the WSS bore in order to prevent debris from falling into the hub during service. Clean the WSS mounting surface on the hub to remove any rust or corrosion using a wire brush, or equivalent.

Important Make sure that the sensor sits flat on the hub. Check the sensor flange against a straight edge to ensure flatness. If the sensor flange is distorted, replace the sensor.

Apply a thin layer of bearing grease to the hub surface and sensor O-ring prior to sensor installation. Use ONLY Wheel Bearing Lubricant, P/N 01051344. 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. 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.

Now the TSB says 01-04 Yukon XL's, but if the hub has been replaced with current parts under warranty....... Or you are misquoting your year...Or you have a end of model year vehicle, which has newer parts on it after the old parts dried up.

Brian