Anti-Lock Brake Question

[TooManyTrucks]

On my 2000 ranger when you come to a stop...just before you stop all of the way the anti lock releases and you get the feeling that your brakes just gave out...just for a brief second and if you push harder it continues to grab...is it a bad sensor? I have no idea...

4x4, 4 wheel anti lock, 4.0 liter

 

[XLT]

Hey i am in the same boat as. Mine only does it once in a great while. When i asked for help on it, i was told to try cleaning the sensor. Cant tell you if it worked or not becuase i never got around to it. Good luck.

 

[Massman]

Hey i am in the same boat as. Mine only does it once in a great while. When i asked for help on it, i was told to try cleaning the sensor. Cant tell you if it worked or not becuase i never got around to it. Good luck.

 

[Massman]

I'm having the same issue with my 2004 Sport Trac. . Mechanic told me to wait for a warning light to come so he can diagnose the problem. After 3 months no warning light but problem persists.

 

[J_C]

^ You may never get a warning light, because it's losing the signal from one of the speed sensors only intermittently so it thinks a wheel is locked up. If only happening at low speed, it won't set the light because it doesn't know the difference.

It could be the hub going out (increasing distance to the ring the sensor uses), or wiring, or connector to the speed sensor, or the sensor itself. With 2WD vehicles that use an external sensor ring on the CV shaft, the ring could have excessive corrosion, but with 4WD it is inside the hub. Since it only happens at low speed it is probably not the wiring or connector, nor the sensor bad but could still be contaminated.

If you live in an area with winter salted roads, rust may result in breaking the sensor trying to remove it, but cleaning it is definitely something to try, yet if it is fouled from metal particles in the grease covering it or the sensor ring (when ring is internal on 4WD or AWD vehicles), this probably means your hub bearings are worn out, and it may get contaminated again.

You can figure out which sensor circuit to focus on, using a scan tool capable of live data including ABS. If you expressed to the mechanic that this is easily reproducible, like near every time you stop and s/he didn't know to do this, I'd find a different mechanic. The scan tool will show (which) wheel RPM signal dropping to zero during the ABS activation event, though if it pulls to one side with your hand off the steering wheel that is a good clue that it's a front sensor, on the side opposite the direction it pulls.

If you have (roughly) 100K+ mi on the hub, it's probably due for replacement. New hubs usually come with new sensors. Rear speed sensor is on the differential on a 2nd gen, not sure about an '04 Sport Trac.

 

[Pete Deering]

^ You may never get a warning light, because it's losing the signal from one of the speed sensors only intermittently so it thinks a wheel is locked up. If only happening at low speed, it won't set the light because it doesn't know the difference.

It could be the hub going out (increasing distance to the ring the sensor uses), or wiring, or connector to the speed sensor, or the sensor itself. With 2WD vehicles that use an external sensor ring on the CV shaft, the ring could have excessive corrosion, but with 4WD it is inside the hub. Since it only happens at low speed it is probably not the wiring or connector, nor the sensor bad but could still be contaminated.

If you live in an area with winter salted roads, rust may result in breaking the sensor trying to remove it, but cleaning it is definitely something to try, yet if it is fouled from metal particles in the grease covering it or the sensor ring (when ring is internal on 4WD or AWD vehicles), this probably means your hub bearings are worn out, and it may get contaminated again.

You can figure out which sensor circuit to focus on, using a scan tool capable of live data including ABS. If you expressed to the mechanic that this is easily reproducible, like near every time you stop and s/he didn't know to do this, I'd find a different mechanic. The scan tool will show (which) wheel RPM signal dropping to zero during the ABS activation event, though if it pulls to one side with your hand off the steering wheel that is a good clue that it's a front sensor, on the side opposite the direction it pulls.

If you have (roughly) 100K+ mi on the hub, it's probably due for replacement. New hubs usually come with new sensors. Rear speed sensor is on the differential on a 2nd gen, not sure about an '04 Sport Trac.

I have the problem on my 95. I can't read the ABC system. I replace the front hub and sensor, service the brake and calipers and replaced the rear sensor. No luck. So I purchase a ABS module. Need to install. You can use a volt meter to measure the resistance of the sensor as you spin the wheel. If you are OBD 2, you can a high end scanner to review the sensors as you are moving.

 

[J_C]

^ If it is only happening when you slow almost to a stop, it is not likely to be the ABS module itself. I suppose it's possible but would be the less common problem.

The resistance of the sensors can be measured at any time, no need to spin the wheel. I measured my factory sensor and an aftermarket front, both were around 420ohms. The rear I've never measured but vaguely recall it should be a higher resistance, maybe closer to 1K.

Spinning the wheel, you measure for low voltage AC, it will create a pulse from each ring tooth that passes the sensor, but it will be so momentary a pulse that with only a basic multimeter, you're just looking for a changing value, not necessarily what that specific value is, then not knowing for certain if that value is high enough to be accepted by the ABS module, so a scan tool capable of it or an oscilloscope, or a multimeter that can graph or log is better than one that can't, so you can see this very momentary signal strength... not sure how that would show up on a meter with RMS, maybe that would help if you compare to the other front wheel.

I used to tell people that they can use an ELM327 dongle with Forscan app to see the speed sensor values and you can, but it is not always straightforward. When my front wheel sensor stopped signaling on my '98 w/ODB2, Forscan stopped reporting values (exited out of the test loop) because it is not written correctly to handle this no-signal fault, so you may not be able to measure and watch all 3 sensors simultaneously, instead watching one at a time to see which causes the error state.

 

[Massman]

Thanks for the replies and info. I will definitely use the info in hopes of correcting this issue and will share with my mechanic. He is under the impression that the ABS warning light should come on. Hopefully this will change his mind.