Abs not working due to one wheel

[vrmm]

It snowed last night and I have to drive 45 miles to get home. It's thus far been quite sketchy! My front left brake just decides to come on when it feels like it as far as I can tell.
I have vagcom but not with me. I gernaded an axle a few months ago and ever since then my abs light has been on. The new axle has the wheel speed ring that's clean and I've taken my sensor out to check for any debris on it and it was clean. I've also reset the codes for it but it just comes back.

I know it's probably difficult to diagnose without having codes, but could I maybe just buy a new abs sensor or sumthin?

[MetalMan]

If you have VAG-COM, then diagnosing is definitely the best way to go. Throwing parts at it can be quite the crap shoot.

With VAG-COM you go into the ABS module. If you have Ross-Tech VCDS you can graphically display the measured speed of each wheel (while driving), and this can help you pinpoint [for sure] which wheel it is. Then you can check out that sensor, replace or push in as needed.

[Avant Nate]

The sensors can be really picky about the distance from the tone ring.

[redline380]

It is probably the sensor.

And keep in mind, Ebay sensors are awesome for the price.

[vrmm]

Thanks for the replies. Good to know ebay sensors work! Can I check my sensor gap somehow? I don't have my Bentley manual with me but does it say what it's depth is supposed to be?

[MetalMan]

There's not really a good way to measure sensor depth, even if there is a specification.
What I do, is push the sensor in so it starts to make light contact with the tone ring (verified by spinning the wheel). Then back it off maybe a millimeter and verify it no longer makes contact.
You can try this in the interim if you feel confident which wheel it is.

[Wrath And Tears]

There is a spec for sensor distance (something like .2mm to .8mm, can't remember off the top of my head). It involved using some heavy stock paper or like 3 pieces of printer paper. Put them in between the sensor and the ring, push on sensor till it contacts the paper, and then you are done. Don't know who came up with this method, probably the guys in the white coats. But it works with every adjustable sensor I've done.

The other way is the way MetalMan said. Push sensor till it contacts, rotate out slightly till there is no more contact.

[vrmm]

Good gravy that's not much gap. I probably put it in a decent amount which could be a cm for all I know. I pulled my abs fuse and made it home. My pipes froze and burst, so after I sort that out i'll jack it up and spin it/ push it it.
Thanks for the help! I would've just replaced it and probably still had the issue.

[Wrath And Tears]

Holy crap I hope the house and you are okay. I think when the sensor is too far away it gets tricked into thinking the wheel is just going balls out and so it keeps applying the brakes because it doesn't sense the wheel slowing down.

I use the push down then pull back until you can't hear any scraping from the sensor like Metalman last time I had to mess with them and it worked great, no problems.

[MetalMan]

My pipes froze and burst,

This is definitely a big issue! My wife's friend bought a house in Colorado about 1.5 years ago, came back to SoCal for Thanksgiving, and came home to a flooded house. Burst pipes... due to them not leaving the heater on (they're from California).
But they got a new kitchen, flooring, and other new things from insurance out of it.

[walky_talky20]

If it's *applying* the brakes, that is the EDL (electronic differential lock) kicking in. The EDL works to apply brakes to a spinning wheel, sending the power across the axle (left to right, or vice versa) to the wheel that is stopped (not spinning).

In your case, both wheels are turning, but one probably has a faulty (or too far spaced-out) sensor. As such, the EDL sees one "spinning" wheel and one "stopped" wheel and it KICKS IN (Yo!). So, most probably, it will be applying brakes to the wheel *across* from the one that has the sensor issue. So if it's applying brakes to the left front wheel randomly, the sensor issue is likely with the right front wheel.

Notably, EDL mostly works on take-off, up to 15mph (or was it 25mph?). So if it's pegging the brakes while you're cruising at 40, the above scenario is probably not accurate.

[vrmm]

I got my pipes fixed. Luckily it wasn't too bad. My roof is dumping water into my bathroom now! There was a wind storm last week in the great ol' Spokane and my house got hit with two trees. No power led to the frozen pipes. And a big branch went into my bathroom. Thought I fixed it... Guess not :p My 47 willys truck stopped the second massive tree and took it like a friggin champ! That's what I was most worried about ;) priorities...

Back to the sensor... I logged wheel speed and found that whichever is in the first column only read 1 km/h the whole time. Would that be drivers front? I wiggled the sensor out and it was a bit dirty but nothing crazy. I could never get it to stop my wheel from spinning. I used a rubber mallet and believe it is in as far as it can go. After testing it like that I pulled it out a bit more with no success.

Thanks for the info walky. Under 15 sounds reasonable. Given that there is no speed input to one sensor I would assume I shouldn't need to worry about the issue being on the opposite side?

I found the sensor on eBay for 9$. Sound about right or is that too 'ebay'?

[walky_talky20]

The measuring block data trumps all (especially Walky's nonsensical musings).

$9 sensor sounds like ebay "just right" to me.

[vrmm]

I'm assuming this will make it inoperative? I put the new one in but haven't driven yet...

[vrmm]

Well it didn't work. The abs axle ring is intact albiet dirty. Like worn down metal with a bit of black grease tossed into the mix. It's not excessively worn down. I would imagine it would work fine, but does any amount of contact cause it to not work?

I did not clear the abs code. I'm gonna assume it's recommended that I do? It was still staying at 1km/h on the drivers side. I'd imagine it would have output code or no code if properly functioning.

Walkyyyy Should I be looking into your 'opposite side' scenario or not since it straight up told me which sensor it is?

[walky_talky20]

No, you've found the problem based on the obvious damage there.

If there are metal particles in between the tone ring "teeth", then it can screw with the waveform. It's not going to see the all-important "edges" of the metal teeth. You should try to clean that out if you can. If you can't get a reading from the sensor via the module, a live scope of the sensor would be helpful to get to the bottom of things. Most people don't have a scope, but nowadays a smartphone can do basically everything:
https://retronics.wordpress.com/2012...-oscilloscope/
http://oa.onyx3.com/signals.html

Another quick method is to use a Digital Multimeter in A/C volts mode while spinning the wheel. Compare to a working sensor to get a baseline, then try the problem wheel. You don't get to see waveform quality or shape, but you get to see if you have some signal or not.