99 F150 ABS activates when shouldn't
 

Hi guys,


I am experiencing a situation about 50% of the time when driving and apply the brakes to slow down and stop. When I’ve slowed down to about 5 mph, the ABS will activate until I stop completely. No slick road surface or sewer covers. No turning. Just driving straight on dry pavement. Any ideas what is going on and how I need to approach repairing this annoying situation? Thanks for any thoughts.


Lou

Is the light comming on or can you feel it kick in.

I can feel it kick in under foot. Have not seen the light on.

Sounds like you may have one bad wheel speed sensor and/or wiring. Does it have 4 wheel ABS?

Sent from Nexus 4. IB AutoGroup

I believe it's rear wheel ABS only but I need to call the dealer Monday to check for sure. I have rear drum and front disc. What is involved in replacing one of these for drum and for disc?

It would be unusual for a 99 to be RABS. Most likely, it's 4WABS. Look at your front spindles to see if there's a wiring harness going to each of them.

Most likely, you have a dirty/corroded sensor, a cracked tone ring, a damaged wire harness or a worn front wheel bearing that's causing an erratic signal from one of the sensors.

Is there any easy way to isolate which wheel corner has the issue?

You will need an assistant to do one of the tasks.

Select a safe area such as a gravel-covered drive or lot.
Driving at about 10 mph, apply the brakes sharply so as to cause either one of the front or both of the rears to lock up. Repeat several times observing to see if one front wheel (or both rear wheels) locks consistently.
The front wheel that locks indicates a sensor problem on that side or, if both rear wheels lock, the problem will be with the rear diff sensor.

All you have to do is look. I saw the wiring for the front wheel sensors and, I'm assuming, the rear wheel sensor also. Is it located at/in the differential? I saw a connector at the top of the differential housing. is that the sensor location or is it somewhere else on/in the rear end?

Yup, that's it.

If the rear end sensor requires replacement, does it require tearing the rear end open to do it? Also, are there any other parts of the ABS system located at either of the rear wheels or is it all at the differential housing?

The 99 only the single sensor on top and the ring gear inside for the rear wheels. Check the sensor first since getting to the ring gear is messy and smelly.

Is there a way to check the sensors using a DMM set to ohms? Saw a video on Youtube showing unplugging the sensor from the harness, attaching a DMM, spinning the wheel and checking for resistance and resistance changes (it was on a 2002 and he got 535 ohms on the good replacement and 1 on the old bad one he changed out). His ABS light had been on, though, and mine hasn't. Don't know if that means a different issue in my case.

Set the multimeter to VAC and look for several volts when the wheel spins. The resistance check is done when the wheel is stationary and will tell whether or not he sensor is ope (usual) or shorted (unlikely).

Set to volts AC not DC?