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Intermitent ABS Sensor Cutting Out

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Old 07-13-2018, 07:10 PM
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Intermitent ABS Sensor Cutting Out

Ever since I fixed my RAS motor, changed the diff seal, cleaned up the ABS sensor, and tone wheel. My ABS has been acting up at slow speeds when I engage the brakes. It loses a signal, so the ECU thinks the wheel is locked up. This triggers the ABS, VDC, SLIP, and RAS lights to come on. It then disables the RAS/ABS until I restart the car.

I was pretty delicate cleaning the tone wheel and sensor. I only used electrical contact cleaner. I'll obviously do another visual inspection, however, I'm wondering what's the best way to troubleshoot an intermittent ABS sensor?

I'm assuming this is a digital sensor, since it has a supply and signal wire. The FSM is basically just telling me to check for continuity between the ABS/VDC unit and sensor. I'm looking for a better way to check the sensor itself. I do have an entry level Picoscope that I have yet to use, so I'm assuming I should be able to hook that up and see a decent square wave correct?
 
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Old 07-14-2018, 01:50 AM
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I'd just inspect the **** out of the harness, if it's intermittent it's probably just something wiggling around due to vibration.
 
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Old 07-14-2018, 01:58 AM
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Yea, I'll try to do that tomorrow. It's just weird because it's only the side that I worked on and the rear kinda sucks. Both sides are connected to the same connector, so if I need to buy one; I will have to replace both. Maybe I'll get lucky and something is just grounding out like you're suggestion.
 
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Old 07-14-2018, 02:02 PM
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Yeah I'd just look at it really good, take off the little bolt and visually inspect it. Unfortunately what probably happened was the contact cleaner somehow got INSIDE the unit and contaminated it. On any type of magneto/pickup like that the best thing to do is simply wipe them down with a rag and not actually use any cleaner on them. Cam/crank/wheel speed/etc, those sensors are basically just a magnet inside and as the magnetic field changes every time the high parts of the pickup ring run in front of the sensor the ECU sees the data oscillate. Contact cleaner is some INCREDIBLY harsh solvent designed to chemically alter the carbon buildup on electrical contact points and that's really it's only recommended use.
 
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Old 07-16-2018, 10:42 AM
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Are you talking about the rear sensors on the differential that pickup the gears on the output shafts, or a front sensor that works with an actual tone ring on the wheel bearings?

If you dropped the sensor (or tossed harshly onto your work bench), it might have gotten damaged and needs to be replaced. I would start with simple checks though, make sure the ring is clean and has no damage. Then check that you mounted the sensor properly and that the gap between the sensor and the ring is within spec.

I had an issue with my front tone ring getting damaged because a mechanic jammed a screw driver in that area, I had chunks of rubber tone ring missing. The ABS does a self test at low speeds, so the pump would pulse and trigger the same lights you see after the first stop sign in my neighborhood. Then after I replaced the wheel bearing, I didn't align the sensor into its "slot" properly, causing it to be tweaked a little. It's embarrassing how many tries it took to notice that little thing.
 
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Old 07-16-2018, 10:44 AM
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Knowing what to look for is so important. It's why us DIY guys usually take a couple extra hours to get the job done.
 
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Old 07-16-2018, 11:52 AM
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It's one of the rear sensors near the flange on the diff. I didn't get a chance to take a look this weekend, but I think somehow I might have contaminated it. I fixed both my rear wheel bearings, RAS motor, and replaced the rear diff axle seal, since it was leaking bad. The sensor and tone ring were covered in dirt, grime, and gear oil. I used some eletrical contact cleaner on a rag to clean everything up and I was pretty gentle with all of it.

Now, when I am braking the signal is intermittent, so the ABS thinks my wheel is locked up due to a signal drop. I'm going to double check everything again, before I condemn the sensor. It just sucks, since the harness has both sensors attached I need to replace both of them and it costs more.
 
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Old 07-16-2018, 01:25 PM
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Since it ONLY happens when you touch the brake pedal I would first check it's fluid level, I thought it was doing this intermittently but not related to the actual brake pedal.
 
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Old 07-16-2018, 01:53 PM
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That was one of the first things I checked was the fluid and it's right where it needs to be. I had to replace the e-brake hardware, compress the caliper, and clean up the rear caliper holder while I was back there.

This is what usually happens and now it's more frequent.

1. I start the car everything works as it should.
2. I'm driving slow < 35MPH usually and hit the brakes due to a red light or another car.
3. The ABS starts engaging and making it harder for me to stop.
4. The ABS, VDC, and SLIP light all come on for the duration I keep the car running.
5. One or two minutes later the RAS light comes on due to the ABS fault.
6. I restart the car everything works like normal.

If I monitor my ABS sensors after the fault three of them show speeds the other (one I messed with) doesn't show anything. I've attached my Autel while everything is working and all four sensors will work normally.

So, common sense tells me either: I accidentally messed up the sensor, the tone ring is messed up, the gap is wrong, or something like that.
 
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Old 07-16-2018, 01:59 PM
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Ahhh yeah that does sound like the sensor unfortunately, I doubt the gap is the problem since it can't exactly be adjusted on the differential. the tone ring is a giant chunk of iron and you would visibly SEE if something was wrong with it. I'm definitely thinking it's the sensor, the good news is it's not a common wear item and I personally would be fine running a used one from a pick-n-pull vehicle or even an ebay one. The used OEM ones are all under $50 on ebay.
 
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Old 07-16-2018, 02:01 PM
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Also, if you opened the harness and cleaned it as well I would take it back apart and visually inspect for anything damaged. If you sprayed cleaner into the harness get some compressed air on it and try to blow it out. If you used electrical anti-oxidation grease in the harness use the contact cleaner to spray it all out. Etc.

Also inspect for a visibly damaged wire.
 
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