On the newer trucks the ABS will read 3mph when the truck is stopped. All 4 should say 3mph then going up from there. I am wondering if the LF has an interminant open in the wiring between the sensor and the computer. Have you ohm checked the wires and looked to see if the terminals are tight? If you have swapped out sensors then I would look closely at the wiring.

I have seen a truck that exhibited the same symptoms with new sensors installed and the fix was a new ABS module. Looks like that's where you need to go.:(

I also saw this problem after a brake job that I did not do they hung the brake calper by the abs wiring and had a wire haning by a thread it would drop the abs siginal at slow speeds check the wiring from the sensor plug to the module with a meter and the wheel bearing must be tight or it can do it also. Dont ask me why it turns the abs on I think it only does this under 10 mph or so call me if you want and I will walk you through it Dennis 573-808-0598

Heres your fix, guaranteed!
 

Mr Gadgets...
This is my 1st post, and I hope the attached images work, as they are worth 1000 words... I am an advanced automotive electrical tech, and travel all over the country to help dealerships and independant shops solve complex computer engine and body control problems. This on is on the money, and usually I can get it done without travel about 98.9% of the time... Sure wish they were all like this...lol Now, on to your problem, and how you will solve it!

A VR (Variable Reluctance) ABS sensor uses a rotating toothed ring to change the magnetic flux within the sensor. The sensor can generate an AC signal to be utilized by the EBCM as an indication of wheel speed. The EBCM looks at the frequency of the sensors signal and can determine how fast the wheel is rotating. If the wheel isn't rotating, or isn't rotating at the same speed relative to the other wheels and the brake switch is on indicating deceleration, the EBCM will go to work to try and release the wheel it feels is approaching lockup. What you need to understand is, frequency is not the only factor the EBCM looks at when deciding to engage the ABS. The EBCM also takes into consideration the amplitude of the signal generated by the VR sensor during its wheel speed calculations, and it also needs to see a minimum amplitude before it considers the signal valid. Oddly enough, if you look at your scan tools wheel speed PIDs when the truck is at rest, many will read 3 mph! There are a few different things that might affect a VR sensors ability to produce a valid signal. The number of wire turns in the sensor, the strength of the magnet, the relative speed of the toothed ring, and the sensors distance from the toothed ring to name a few. Now, the 2 things you said that are important to me are, 1... you do not
have an ABS problem during high speed braking, and 2... the problem only appears at speeds lower than 10 mph. What happens is with the increased air gap, the stability or strength of a VR signal is lower at slow rotational wheel speeds not meeting the EBCMs minimum threshold, and the result is the 3 mph difference you said was seen between sensors right before stopping. Other causes of low signal amplitude are, a bad sensor, different tire sizes from side to side, a difference in tire pressures from side to side, or a cracked tone ring inside the brg and hub assy, but these are more extreme cases and what I am about to tell you fixes 98.9% of then...THE CAUSE ?????... Apparently, corrosion under the front wheel speed sensors which upon a normal diagnostic inspection would seem irrelevant. It seems that on these vehicles, rust builds up between the sensor and the hub where the wheel sensor mounts, and the rust actually
lifts the sensor away from the toothed ring as it grows (remember... rust never sleeps).

Click here to see a the problem

Click here to see a cleaned useable hub

Click here to see a cleaned useable sensor

With increased air gap comes decreased signal amplitude. The EBCM can't see the signal even though the wheel is not approaching lockup. The EBCM will engage the ABS to try and unlock a wheel that really isn't locked. The signal is still present, but it's below the EBCMs minimum amplitude threshold.

Click here to see aweak amplitude waveform

Click here to see a good amplitude waveform

All along The EBCM doesn't need to see just a change in frequency from side to side, it will also cycle the ABS if it can't see the signal in terms of amplitude, and the computer sees wheel lockup which is not actually there.

Click here to see 1 sensor @ 3 mph stopped waveform
Notice that 1 is 5 mph and the other 3 mph? This is an actual snapshot of the problem occurring, confirming the problem.

Click here to see both sensors showing good 0 mph waveform
I would also recommend this be done during any normal brake rotor removal because of the labor involved - you (or someone) are already there and you'll be eliminating the same problem at a later date in 5 or 6 months because of the environment the sensors live in. If you only cleaned one side and then looked at the lab scope, the amplitude of the other sensor will actually increase, meaning another problem is soon to follow. One BIGGE I forgot to mention is, if there is ANY play in the front wheel bearings, looseness or side to side play or wobble, it will change the distance from the sensor, and create the same problem. I have seen several that had to have the brg and hub assy replaced before the problem was solved, but most of them that were worn somehow, or had a cracked ring (don't bother looking), were on the right side, and the ones with no visible damage, looseness, or traceable problems are always on the left side, and you can take that to the bank even though I can't explain why. Below are what cleaned sensors and hubs should look like. DO NOT forget to plug the hole the sensor goes in when cleaning, or the rusty crud will fall into your wheel brg grease... and then we know what happens... Good luck my friend, and if you need any more info shoot me a message, if you can "FindMe"...

FindMe,

It all falls into place after your explanation!! The pics are great.. We will tear back into it and check it out.. I am sure that will be the cause. Sensitive little bugger that needs such a close tolerance.. but I can see the resultant low amplitute waveform is proof of the problem..

Sounds like a real neat job.. any openings?? I love to solve problems like that. We have just been strapped for time, and my buddy lives an hour away, other wise I would have hooked up a scope and looked at it.

Thanks again for the help. I will let you all know what we come up with!!

Dick

Good post findme, welcome to oso :)

I'm glad to see this :D .I have a 98 Tahoe with intermittent ABS problems myself:(