help! 98 4runner ABS engaging everytime i brake

[duckrunner541]

I just got a great deal on 98 SR5. it has 200k miles on it and needs some cosmetic tlc but is mechanically solid... or so i thought..

i drove from eugene to portland to get it, had no issues at all on the test drive. drove almost all the way home with no issues and then noticed everytime i came to a near stop (under 10 mph) i could feel the ABS activating. it is happening everytime i come to a stop now. any suggestions? should i try cleaning the sensors? how do i do that? previous owner also said rear wheel bearings were just replaced, could that have anything to do with it? any help would be much appreciated.

 

[pedalquickly]

I would suspect the rear sensors for sure particularly since the rear bearings were recently done. I have an 01 and the sensors are held in with one bolt. Just remove the bolt and pull the sensor out and check the wiring also.

 

[BlackCat914]

yep, check the sensors. Sometimes rust builds up on the mating surface and raises the sensor slightly out of it's home causing erradic readings. If you have access to diag equipment, you can figure out what sensor is dropping out and go from there. I'd just pull the ABS fuse for the time being so you don't kill a soccer mom and the kids in a packed mini-van.

 

[Brian757]

If you have a multimeter, you can check the Ohm(resistance) of each of the sensors to determine whether they're faulty or not.

A healthy ABS wheel speed sensor will have a reading of ~1.6 Ohms, IIRC.

If the meter reading displays higher than normal resistance, or shows "O.L.", then the internal circuit is electrically open. Similarly, if the reading is lower than it should be, or zero Ohms, then you have a shorted internal circuit. In either situation, the sensor would require replacement.

You can check the resistance by disconnecting the quick disconnect clip of the ABS sensor circuit to the cars wiring harness, and attaching the prongs of the mulitmeter to the two pins inside the ABS sensor harness, not the cars wiring harness. There is no power required for a resistance test, as the wiring within the circuit is tested with power from the multimeter. (I state this because many people believe that you need voltage to test resistance, these people are wrong.)

Good luck diagnosing!

 

[duckrunner541]

I would suspect the rear sensors for sure particularly since the rear bearings were recently done. I have an 01 and the sensors are held in with one bolt. Just remove the bolt and pull the sensor out and check the wiring also.

Are the sensors easy to get to? Do i need a jack? I dont have access to one at the moment.

 

[nobb]

Ive had that happen before. Turns out the rear axle seal was leaking, casing grease to soak into the rear drum brakes and causing them to grab. Check that.

 

[Brian757]

Are the sensors easy to get to? Do i need a jack? I dont have access to one at the moment.

Yes. They're relatively easy to get to in the front. The rear is somewhat difficult. You won't need jack stands for this job unless you want to remove the tires to make access easier. Tire removal is not required. I've replaced two sensors and have just turned the wheels away from whichever side I had to work on.

(See figure below) The reference point for the "ABS wheel speed sensor" will have one bolt holding it onto the hub. The reference point of the "electrical control unit" will be a wiring harness quick release. Pop them apart to expose the 2 prongs on the sensor wiring harness. Perform the resistance test being (+) & (-) on either sides of these prongs with your multimeter set to the Ohm reading.

If it still doesn't make sense, I can take photos of my truck and show specific examples. Just let me know.