I have read all of the posts I can find on ABS lights, but found nothing like this so hopefully this is a fresh topic. I have a 2003 F150 4WD with four wheel disks. I ran a scan and got no codes. Here is what I know.


When I turn on the key, the ABS light is illuminated as it should be.
When I start the truck, the light goes out.
I can back out of my driveway and the light stays off.
I can drive slowly around the neighborhood, use my brakes, light stays off.
I can drive faster down city streets and the light stays off.....until I touch the brakes for the first time. Then it lights up and stays on until I turn off the truck.
As best I can tell, the speed that results in the light turning on (when I touch the brakes) is somewhere around 30 mph.
If is absolutely predictable, is not weather related, and has nothing to do with rotors or fluid because I changed the rotors and flushed the system.....problem didn't change.


Anyone have any ideas or suggestions? Thanks

 

More likely a bad sensor from what I was told. You may be able to take it out of the caliper and clean it but I'm not 100% sure.

 

You're sure that you have enough fluid? I'd point to an ABS sensor though.

 

I have an owners manual and have checked codes. Neither helped, which is why I posted the question here. I got information from another source that it may be the ABS relay, which could explain why it resets in a predictable way. The part was only $15, so worth a shot. The problem is, where does it go. I have checked the manual and it is not shown in either of the fuse/relay boxes. Seems a common question on the forums, but the answers are usually, "look in your manual" or "take it to Ford". Hopefully someone can aim me. Thanks

 

I rechecked fluid and it is fine.
I replaced the rear sensor. No change in the behavior of the light.
Tested resistance in both front sensors. Each is right on about 450 ohm. Each bounces around if I spin the wheel.

Lots of checks, but no closer to understanding.

 

Did you check the speed sensor on the trans? I'll check my books to see where the relay goes.

 

Here is all I have on the ABS:



















Ignore the circled ones, I got this book out of the junk yard lol.

 

Thanks for the input. All sensors checked out. In the end, I ran to the junkyard and got a module for $40. Relatively quick and easy swap, and light seems to be fixed.


Now to a new post to figure out why my horn won't go off.....contact in airbag or clockspring?