ABS Brake problem - Pulsating. Electric or Hydraulic?

[94_explorer]

Well, after not driving my truck for a month because of a pulsating pedal, I finally got around to working on it a bit. I replaced the ABS sensor on the differential housing, thinking that could be it. However, that didn't fix the problem.

Symptoms are: When driving at slow speeds, the brake pedal pulsates and I hear and feel a buzzing sound. Truck becomes hard to stop.

Clearly there is a problem with the ABS. I was worried it was something like the Power Booster or Master Cylinder. However, I had a bright idea. I yanked both ABS related relays and low and behold, the brakes work just fine. Sooo, that's great news. I can drive it without ABS if I have to. However, I would like to fix the problem. So, I'm thinking maybe it's the hydraulic pump? But I suppose it could be a front ABS sensor that is confusing the pump? Any ideas? Maybe I can get an error code that would help diagnose this issue.

 

[mrboyle]

From the description you have a sensor that is sending an erratic signal. First thing I would do is lift the front tires off the ground and check the wheel bearings (loose wheel bearings can cause the problem). If the bearings are tight, remove the wheels and calipers and take a look at the tone rings to be sure they aren't rusted badly, chipped, teeth missing, packed with mud, etc... If the rings look good, look at the front sensors and see if one has a large amount of metal shavings built up on the end or if it has any obvious damage. If you can't find anything with a visual inspection it is likely one of the sensors is damaged internaly. If there is no ABS light on it will be impossible to tell which one is bad other than finding someone that has a scope that can graf the square wave form from the sensor (it may be cheaper to shotgun new sensors than pay someone to diagnose each individual sensor).

 

[94_explorer]

Thank you Mr Boyle!! I will definitely pull the front wheels and do an inspection. Maybe I'll also try to pull some ABS codes. I wonder what would happen if I simply disconnected one or the other up front, and then tested driving it. Any idea?

 

[94_explorer]

What is the deal with the ABS Hyrdaulic pump, anyone? They don't seem to be available really. I did find one listed on oreilly, but the price was almost 700 bucks!?!?! Wow. Do these things rarely go bad? Is there a way to test them to see if they are bad? I was messing with the ABS relays and I swear I heard a buzz for just a few seconds coming from the pump.

I haven't had a chance to pull the wheels yet and inspect the front ABS sensors. The interesting thing is that when it pulsates, it doesn't pull right or left. I would think it if was a sensor, it might pull.

There's also the ABS electronic module. Those aren't cheap either. Is there an easy way to test it? Anyone?