ABS speed sensors.

I have a 199 GMC Sierra 4 Wheel Drive. When I am on the brake and go below 5 miles per hour. The pedal vibrates, like when the ABS kicks in. I have checked it on 3 Computers already. While driving and sitting. Nothing shows
up, All sensors are in sync. I pulled the fuse for now. I know you can clean the sensors. I only have front ones. What and how do they clean the sensors?
I was told to get the whole ABS unit under the chassis from a junk yard and replace it?
The only thing I did was change all my brake lines to ones that are rust resistant. A month or so after this . The ABS acted up.
I do not think it could be from not bleeding fully? The pedal is hard not soft at all. I bled it using the computer. I did it at least 10 times. I am going to bleed them manually just to try it. I am at a lost for solutions.
I appreciate any suggestions.
Thank You Doug
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Doug wrote:

What does the computer show when the unit is acting up? The common item with the front sensors is that they get rust under them which raises them just enough that at slow speeds (2-3 mph) they have a bad pulse output train which the ABS thinks means the tire is locking. To clean them isn't hard. Pull the wheel off so you can see them easily. Then use a wire brush all around them and clear out any crud. Then remove the retaining bolt and pull the sensor out. Plug the hole and clean off any rust/dirt from around the hole. Also clean the sensor as well. Don't use a lot of force and try to stay away from the tip. When you install it use a small dab of silicone around it to help slow the rust. I also like to use a small brush and paint sealer around the sensor to keep water away and slow/stop the rust.
You have three sensors FYI, that system uses the speed sensor in the trans for the rear signal, unless it is one of the later ones that has an actual sensor on the rear axle housing.
If this doesn't help then it id likely that some dirt or rust got into one of the valves in the unit and is causing it to leak internally. That happens sometimes when you replace lines and stir up the crud.
--
Steve W.

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Thanks Steve for the reply. I ordered new sensors, got a good price. Both fronts for $39.99 AC Delcos. I figured for that price I would change them. I think I will first try to clean the old ones. Just to see if that is the problem. I have the other sensor in the tranny. Should I clean that one too?? If the unit is leaking, would I see it?? If it was leaking Brake Fluid I know I would. Are you talking about the pump, vacuum leaking? I have a unit off another Pickup, same model. It is in working condition. It would be easier if the sensors do fix my problem to change the whole unit? Instead of the module and or the pump, motor,etc?
The computer finds nothing wrong all readings are good. I have a unit I checked with, Chevy Dealer AND A MECHANIC ALL PUT IT ON THEIR COMPUTERS, ALL THE SAME RESULTS, NOTHING FOUND THAT WOULD CAUSE THE PROBLEM. @ OUT 0F 3 SUGGESTED TO GET THE abs UNIT FROM A JUNKYARD AND REPLACE..(Ooops hit cap key)!!
I appreciate your knowledge Steve and for the help. If you can add more to my reply to you. I would really appreciate it. Thanks Doug

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Doug wrote:

Not if your speedo works OK. They don't usually get rusty.

No the leakage is internal. You won't see it but it can cause problems.

I would clean the sensors and mounting points first. It doesn't really take much rust to cause the problem. The sensor tip only clears the reluctor wheel in the hub by a couple thousandths and the signal drops real fast as the distance increases. Add 2-3 thousandths for the rust and the signal drops so far that at low speeds the signal drops out and triggers the ABS.

I wouldn't swap out the ABS unit till I eliminated the sensors first. Rust under them is a VERY common issue.

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That was my plan to clean the sensors and mounting point. If that is the problem, Great. If not I was going to change whole unit. If it is just rust. I have the new sensors, I will probably change them if cleaning works.
I hope it is sensor, but after changing all brake lines is when it happened, so I think maybe as you said. Some dirt or rust may of gotten in one of the valves. Then it would be leaking correct?? Is it leaking fluid in the unit?
Thanks Again Steve
Doug

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Doug wrote:

There are a few small passages and valves inside the controller that can cause problems. Then the valve doesn't seat properly and you have problems. All of this happens inside the unit.
I have had them act up and flushed them using a scan tool to lock open the valves and power bleeding the system with the fluid flow reversed.
--
Steve W.

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Steve
I have one more question if you do not mind. I bleed my brakes after changing lines with scanner. I did not notice you can do a reverse Bleed.
Do these systems have to be power Bleed or can you manually bleed them? (curious)
Thanks Doug

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Doug wrote:

I meant that you connect up a power bleeder and push the fluid through in the opposite direction.

Either way will work IF you pay attention. Power bleeding is just a LOT faster and easier for one person. Plus it usually pushes out more crud due to the pressure of the fluid. The additional bonus with a power bleeder is the ability to push the fluid through backwards if you need to.

--
Steve W.

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Thanks for all the help Steve!!!!!!
Doug

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