srq34231

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

danetbra63

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.

Red-Ford

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

srq34231

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

srq34231

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.

Red-Ford

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

Red-Ford

Here is all I have on the ABS:



















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

srq34231

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?