I’m curious as to exactly how ABS works. I understand how it works once ABS is triggered. What I’m curious about is how the signal initiates, where does it originate, and how it gets delivered to the ABS module.

I’m assuming it’s taking measurements from the axle and delivering it through the speed sensor. But how? Magnets? And how does the signal make it from the axle to the speed sensor? Thru what parts/mechanisms?

I ask because it determines what parts you buy for your car. Auto vs manual trans, the axle, the bearing, others.

 

ABS relies on the wheel sensors only on our vehicles.

The signal is a magnetic pickup and a toothed gear; usually a Hall Effect sensor at our vehicle's age, but there's other methods of doing the same thing.

And it relies on them being different enough speeds.

(For our vehicles, both rear wheels count as one.)

Consider. Rear axle and left front are spinning, but right front is not. Conclusion: Right front locked and sliding. Action: Interrupt brake pressure to right front until it spins again. Reallow brake pressure to right front. Lather, rinse, repeat.

And that's all it does on our vehicles.

(Some others it gets fancier; for instance, as I mentioned in another thread, on Fords with Traction Control, the ECU uses the ABS to fake for a limited slip by being able to apply brake pressure also; ours are a bit more primitive, and will interrupt brake fluid only.)

There's actually a discussion of ABS and how it works at How Anti-Lock Brakes Work | HowStuffWorks. As a bonus, the truck images could easily be a Dakota.

A quick discussion of the inputs is on page 5-8 of the 1994; I forget which year yours is, but it'll be in section 5.

RwP

 

And this toothed gear is in the differential?

 

For the rear wheels, yes.

After the 1st Gen it's also used as the vehicle speed sensor; but that's immaterial to us.

RwP

 

And are the signals sent through the PCM first then sent to the ABS module or does the signal go straight to the ABS module?

 

I don't know if the PCM even cares about front wheel speed, but, the ABS has it's own module to interpret the various signals.

 

Yours is a 1994, right?

Page AN-76 ( 8W-113 ) shows the sensors all three going straight into the Kelsey-Hayes 4WAL system.

Pages AN-25 and -26 ( 8W-62 and -63 ) show the rear sensor only goes to the RWAL controller.

So no, straight to the ABS.

RwP

 

‘93 but I think it’s the same difference.

So then if I were having ABS issues, I can immediately discount the PCM.

I’ll have to look at those pages if I can ever get a break from working on these damn vehicles.

Thanks Ralph.

I discovered the front wheel bearings on the wife’s front wheel drive make a difference with regard to the ABS. I guess there’s a magnetic encoder in the ABS bearings. Trying to figure that out in my mind. The bearing housing does not spin so it must be the “reader” that reads the axle rotation??

Wait, maybe it does spin.

 

Being as the ABS is an important safety braking feature. I think you'll find that about all vehicles send wheel speed information to the ABS module first then the ABS module converts it to MPH and broadcasts to the PCM and so on. This way eliminating failure points in the safety system.

Wheel bearings with ABS sensors generally have a tone wheel some some kind built into the bearing housing. For me if having issues its always easy to just pull the sensor and check for metal material build up. Since the the sensor is basically a magnetic pickup metal build up will reduce its ability to read.

You first gen dakota guys have 4wheel sensors? I thought 4wabs didnt come around until 2000-2001, atleast that was in the big trucks

 

I’m not sure when the first 4WABS came out but it was before 2000.

Does that tone wheel in the bearing housing run top to bottom in the bearing or is it placed on only one end? I ask because I’m curious if a bearing can be installed/pressed in backwards.