Weird Brake Thumping

[rangerlocal]

So I just did a complete brake job and got all the air out of the system. The truck stops fine, but when gradually stopping, like when coming up to a stop sign, right before the truck completely stops, I feel several thumps from the brake pedal under my foot, and it feels as if the pedal sinks slightly and the truck's braking ability diminishes. No ABS light, and it does not do this under extremely light or very heavy braking. Master cylinder is fine, no bubbles in fluid. Calipers put on the right side up. No expanding brake lines. No fluid leaks. Any ideas? The last thing I want to happen is to glide into someone because my brakes let go right before I came to a stop.

Thanks in advance guys!

 

[TheTwanksta]

Did you bed in the brakes when you finished installing? What kind of rotors and pads?

Sent from my computer phone.

 

[rangerlocal]

Pads are semi-metallic centric pads. Rotors were resurfaced. Bearings checked, repacked, and tightened. I rebuilt the front calipers with rebuild kits, and replaced a leaky rear wheel cylinder, so I did both rear cylinders. New shoes and hardware. Rear brakes adjusted.

Edit: I thought you meant "bled", but yes I did bed in the brakes. They seem fine until right before I stop.

 

[rangerlocal]

This right here! http://www.therangerstation.com/forums/showthread.php?t=3989

This guy has my exact issue, and it pretty much confirms my suspicions of what it could be. It felt like the ABS was wanting to engage at low speed. The thing is, mine is only a 2WD and only has rear ABS, not front. I have no ABS light or codes. Maybe I could try cleaning the sensor in the rear, but usually dirty problems are from the rings on the front hubs. What do you guys think?

 

[rangerlocal]

Got it narrowed down. Pulled passenger compartment fuse #14 to disable the ABS, and it brakes fine now. So I guess I need a new sensor on the pumpkin. Good news is I know my ABS still activates! Any idea why this hasn't triggered an ABS light if the sensor is bad?

 

[rangerlocal]

Measured the resistance of the sensor on the pumpkin. Almost exactly 1800 ohms, which is the same values as a new one at the parts store. Putting a magnet close to it momentarily changes the values, so I know it's working. So I guess it's not the sensor, but I pulled both rear wheels and drums today to inspect the new brakes, and they are wearing in evenly and are adjusted correctly. The self adjuster cable is a tad loose, but I think it's just slightly stretched from age.

Still not sure why it wants to engage the rear abs right before stopping. Could it be the ABS control module going bad? I would think it would trip a light, just like the speed sensor would, but who knows. Any insight?

 

[rangerlocal]

I can't be the only one with this problem...

 

[shane96ranger]

I've seen some guys clean the RABS module because it does things like you are talking about. I've never experienced any issues with any of the RABS equipped RBV's I've owned though - so I'm not much help there. Try searching for RABS sensor on here.

EDIT: Here you go.... maybe this will help?

http://therangerstation.com/forums/showthread.php?t=88318&highlight=clean+RABS

 

[rangerlocal]

Hi Shane, thanks for the suggestion!

Yea, I saw a post similar to that that showed the modulator torn apart. Full of nasty stuff. I contemplated doing that, but never did, because I always got good fluid flow to the back when bleeding and never had an ABS light. I think I'm going to try to pull the codes myself with the connector ground trick to see if there are any stored codes since I don't have a light. Will let you guys know what develops; any other suggestions keep them coming! I'm about to just unplug the fuse and pull out the bulb. Not worth messing too much with, but it's nice to have it when hauling stuff.

 

[rangerlocal]

Twas the speed sensor! The resistance of the old was 1720 ohms, the new was just over 1800, so I guess you can't really tell from that. What really gave it away was the fact that 1. The speedo was doing something funny at slow speeds. Barely noticeable since it was only at about 15 MPH, so I almost missed it. And 2. No AC current from the sensor when spinning the rear tires. $12 Dorman part at Advance, ABS is perfect.

Moral of the story is, don't go on what the computer tells you! I had no lights or any stored codes, since the computer basically thought the sensor was working properly and I was just having a lock-up situation. Glad I didn't decide to jump into tearing apart the RABS valve since it would be free...$12 well spent!

Thanks guys!