I searched through and didn't find this specific problem addressed.

Help.......anyone seen this problem?????

My 2002 F150 4X4 w/ 4.6 V8 125,000 miles will have the antilock activate even if the brake pedal isn't touched.

All the sensors are fine (scanner shows no problems), the control module is fine (replaced it no change), replaced the rear sensor just to check and it didn't change anything. No error codes and No ABS light gets lit. Disconnect the abs motor wire to disable the ABS motor and the ABS light gets lit so that part of the system is working. Brakes working fine other than when the antilock activates. It activates under very light breaking as well but not all the time. No loose connections.

Went from 255x70x16's to 265x75x16's in Oct of last year with no problems till now so I don't think it is the tire size doing it. Tire shop said they have many other customers that have done the same tire size change w/F150's and no problems. I drive 3k a month so after 9k miles I haven't had a problem with the tire size making any difference, doubt that it is it.

Happens in reverse as well as forward. Generally does it at low speeds but it might be doing it at highway speed and I might not be hearing the antilock hydraulic motor going due to noise in the cab.

No TSB's or Recalls address this.

Love the challenge but everybody is stumped

 

A good diagnostic scanner should be able to monitor the signals from the VSS on the diff and the wheel speed sensors on the front hubs. Odds are you'll see one of the front ones dropping out, triggering the ABS.

Somewhere in the system, when monitoring it, you should be able to determine what's causing it. I've got my money on an ABS wire between the sensor and the connector that got damaged or broken.

-Joe

 

Thanks Joe.
We have had a scanner on the pickup and even when the antilock is malfunctioning it shows the wheel speeds to be fine. They are not dropping out when the antilock kicks in.

I think I'll have the chip reprogramed to the exact tire size and then see what happens. The speedo isn't off enough to bug me but as long as I'm this far into the problem .......

Your point about a damaged wire is well taken. This truck sees it's share of rough terrian and off road travel. I'll get in and trace all the wires to look for physical damage.

Thanks,
Lane

 

The scanner monitoring the WSSs already checked all the wires. If the controller is activating ABS when all the wheel speeds are normal, the controller must be bad.

Scroll about 3/4 down this page & read under "Unwarranted ABS Activity":

It's for an older style of 4WABS, but that particular section applies to you.

 

Steve,
Thank you. That is what we came up with but after replacing the controller yesterday the problem is the same. So I guess it is possible that we got a defective replacement controller (what are the odds of that happening?) but I wouldn't think so.

I can't get in to reprogram the new tire size until Monday so I think over the weekend I'll try dropping the air pressure down to lower the 'apparent' tire height and then at low speeds see if it is still doing it. My mechanic says the ABS brain doesn't care about the tire size and he thinks it won't matter. Generally he has been right but I need to eliminate the variables.

I might be imagining things but the more I drive around town slow (under 30) the more often it seems to do it. Spend time at 55 and then drop the speed and it seems to take a bit longer before it starts unwarranted ABS activity. That usually happens between 15 and 20 mph w/o touching the brakes.

These are all great trouble shooting thoughts so keep them coming I really appreciate the help.

Lane

 

He's right. Wheel SPEED has no influence on ABS activity (other than to disable it at low speed); it's all about the CHANGE in speed, i.e., acceleration. Something is causing a fluctuation in a wheel's speed signal that's apparently too fast for your scanner to read.

 

I had an ABS issue once - pulled my hair out , couldn't find the source of it for a couple months . Well , I changed the brake fluid and the problem went away completely??? Only thing I can figure is that I must have had air in the valve somehow - everything else checked out..

 

Well the re-flash didn't fix the problem. Not that I had high hopes it would but what the heck.

Even though the front wheel speed sensors are not showing any problems that is the only remaining item to replace. Well that and what I call the hydraulic dump valve/pump. Replaced the controller, replaced the rear wheel sensor(had one from a sister truck), reflashed the brain, all that is left is the front wheel sensors. At $115 each I think I'll prowl the junk yards instead of going brand new.

Still no error codes and it is consistent in doing it at speeds under 30mph. Mechanic says it won't do it over 20 mph nor under 12mph. But I've had it do it under 12 mph and just over 20. Normally on start up the ABS system operates once at 12 mph per the manual just to circuit check the system.

If anyone has a solution I'm all ears.......still not fixed.

Lane

 

I have a similar problem with my 2001 f150 4x4.
The ABS starts pulsing at 5 MPH when slowing down ALL the time. It sometimes pulses without pressing the brake pedal when going slow. Replaced the rear diff speed sensor, both front sensors. My mechanic replaced the master cylinder. I replaced all four rotors in the last year with power slots & new pads. All the brake hardware is loose and the brakes work perfect except for the ABS under 5 mph.

I'm going to send the ABS module to modulemaster.com next and see what they come up with.

can anybody add anything?????

This is starting to get to me!!!! ( I could of used a few different words XXXX !)

Thanks,

Jerry

 

If you are having a ABS Problem at have replaced everything, check your front wheel bearing, this caused my ABS problems is my 97 f150, The problem really showed up with the ABS when you where turning slowly into something like a parking spot, or a driveway.Also check at advancedautoparts.com , as they are the only place I have every seen you can buy abs censors that are reasonable..

 

We checked the front hubs, they're tight.
The 2WD sensors are a lot less than the 4WD front ABS sensors. I got them from motorcraftdirect.com for $107 each, they're usually $125 each.

Anybody else have this problem???

Jerry

 

I have been having the same problem with my 01 Screw XLT 4x4. Just before the truck comes to a stop, the last 10feet or so the ABS activates. I don't have the ABS light on. After reading this thread it would seem to be a sensor? Where is the sensor located in the rear end? That sounds like the cheaper sensor to start with.

 

Rear differential sensor is on top of the rear end punkin.
one bolt holds it in, and unplug the cable which has the usual locks on the sides. It's $15 or $20. It has a reputation of failing and is a good place to start.

I'm starting to think my front hubs/bearings are wore and its causing the front ABS sensors to walk away from the tone ring just enough to false trigger. they're about $180 each.

Jerry

 

Thanks for the information on the rear sensor, I will start there and with any luck it will be a fix!

I was told by a friend that a bad hub bearing could be the cause also, but it sounds like yours are tight and you replaced all the sensors. Not sure whats left? When you get there let me know the fix, my 01 could have the same.

Good Luck!

 

Had the speed sensor go on my mom's 2000 dodge grand caravan and it would ABS and only go like 20MPH, replaced the speed sensor and bingo, all fixed. $15 at dealer.