Abs

Strange thing - I have changed front struts and bearings, wheel hubs and bearings, inner CV joints, dust sleeves... now, when I brake slowly, on low speeds (5 - 15mph), before vehicle stops, ABS is engaging! No wheels sliding, dry road. Sometimes it turns on like I'm on the ice, but mostly before vehicle stops very slowly, or at the end of the hard braking, before vehicle stops. I'm tired already of this, because I have no idea what's going on. Brakes were not touched, lines or anything connected to them (not counting dismantling the brake pads). Help!

One or more of the wheel speed sensors is dropping out at low speed and the ABS sees this as lockup and acts appropriately.

You can watch them on a scanner if you have one to see the offending sensor, or in lieu of that reinspect all sensors. Maybe one got pulled out of it's socket just a little which will weaken the signal and cause the ABS to think lockup. Any aberration in the tone rings can cause problems too.

I am assuming you have no codes which will point to a signal error. ABS controller failures are common too, and I have had many rebuilt, but due to the timing of the failure I would doubt that in this case.

^ this - one of the sensors is dropping prematurely and the ABS sees it as a locked wheel. scan with VCDS as you come to a stop. watch all 4 wheel speeds and you will find the culprit as the one that goes to 0 kph first.

Oook. The rings of the wheel hub are brand new, bought the hub with them already installed.

Yeah, but what is the gap from the sensor to the reluctor rings?

Don't guess. - scan the car with a scanner capable of showing the abs speed sensor live data.

One wheel will drop faster than the rest - that's where you need to investigate the sensor, reluctor, wheel bearing play etc. It can even be high resistance (corrosion) on the sensor connector etc.

These days I'll check all the sensors for grease, rust, etc... unfortunately, still have no VCD (it's my bad). Before the new parts installing, all was ok, so we'll see what's the problem. Thanks a lot.

EDIT: I pulled off the left rear ABS sensor connector... car is braking normal! Connected back again, ABS is on! I'll check tomorrow the sensor itself.

If a sensor is disconnected, the ABS system will shut off and set a code / lamp.

That is why the car started to brake normally.

Yeah, I figured out that... little bit later, but at least I did it Thanks

Vehicle checked. The guy said all four sensors are static (and???), there will be no problems if I drive it like this (but, I don't want that!)... so I left the garage w/o solving the problem (and no charge!). I can disconnect one of the sensors and car will brake normal, but with no ABS!... or leave all the sensors on and... I don't know what to do... sigh...
(The front wheels sensors gaps are 4-5 mm).

Static??? Did he map the speed signals and compare? I am sure one is dropping out. While it is not unusual for abs controllers to fail I think the problem is sensor related due to the failure after being near them.

My Mantra:

"When in doubt, check what you messed with"

I think it applies here.

pdq import repair said:

...While it is not unusual for abs controllers to fail I think the problem is sensor related due to the failure after being near them.

Click to expand...

Sorry, I do not understand this. Near what?
Windex, I can't even sleep, where did I do the mistake?... maybe the new hub sensor rings are not magnetized, huh?

LLL said:

Sorry, I do not understand this. Near what?
Windex, I can't even sleep, where did I do the mistake?... maybe the new hub sensor rings are not magnetized, huh?

Click to expand...

What he's saying is that the problem will be found where you have been working, i.e. if you were working near (or around) something and now that system has a fault, the likely place to look is where you were working at the time.

Hope this helps

Steve

Yeah, that helped
That's why I went to check the car with VAG... and hoped that I'll find out where is the problem and eventually to replace the failed part(sensor?). But...

Hi,

Have you resolved this issue yet ? I've just read the post (2 Times !) and you say you have air gaps of 4-5mm on the ABS sensors ? That for a start is far too much, I would recheck that first, air gaps on ABS sensors are usually below 1mm.

No, I didn't resolve it. I think the gaps on my car were always like that, 4-5 mm, with no issues before.

Yours is a European model and a little older than the MK4 platform I’m more familiar with, but most of the wheel speed sensors I’m familiar with have a gap of 2-3mm. It’s possible one or both of your new hubs has a ring that isn’t quite correct.

You said the rings came with the hubs. Were the hubs aftermarket from a reputable source?

I probably read it, but did you check the wiring to the sensors themselves for obvious abrasions or damage?

Wiring was checked very carefuly, no problems found. The hubs are Optimal, german brand. One more time - the ABS is on ONLY before the car stops, instant before it stops! This week I'm noticing some buzzing from the place ABS pump is located (only at the veeeeery low speed, 1-2 mph)... very briefly depressing the pedal stops the buzz. I'm losing my mind...

Scan the car with VCDS and see which sensor is dropping out.

It's not that hard. Used to do this all the time on Saabs - they had adjustable sensor gaps on some models, and would exhibit this symptom (ABS activation on dry pavement in the last few feet of a stop) often.

Fix was easy - scan the car, isolate the wheel which dropped out first and adjust it.

Alternatively, tighten up each of the sensor gaps evenly, and ohm out the circuit resistance at the ABS ECU for each of the wheel circuits to ensure the sensors and wiring are good.

C'mon man - you know what you need to do.

In one of my previous posts I wrote that I was at the mastershop and checked the sensors... with no result.
What I'm confused about is how to adjust the gaps? I'm pretty sure that the gaps of the old, replaced hubs was the same!... and more, the rings were kind of bent, they didn't rotate evenly and all was working perfect!

You said the "mastershop" told you all sensors were static??? I quizzed you then about the interpretation of that term as it is one I am not accustomed to. Did they actually log and compare speed readings at near stopped conditions. I am pretty sure one sensor is dropping out at low speed.

The problem arose after work on the car, so something changed then. Are the tone rings on the new parts identical? Did you do a tooth count to be sure?

You really need to find someone with a scanner that can read wheel speeds and compare them, preferably VCDS.

threads like this are frustrating.

1/2 hour with the car and VCDS and I would have the solution. ABS sensors dropping out are a common issue, and easily fixed with the right scanner and a person who knows how to use it and interpret the readings.

Live ABS wheel speed scanner -> all four wheel speeds on one page -> Driving straight, all wheel speeds should be identical or within 1kph of each other -> when coming to a stop, one of them will drop to 0 early - that is the one causing the unintentional ABS activation.

a "Static" test of the ABS will not tell you what you need to know

Below is a typical situation - coming to a stop, the Left Rear sensor reading drops to zero a little earlier than the rest. The ABS computer misinterprets this as a locked wheel, and activates ABS to "unlock" Once all the rest of the wheels drop to actual zero, the ABS event stops.


Wheel Speed TIME -------------------------------->
LF 20 20 19 19 18 17 15 13 11 9 8 7 6 5 4 3 2 1 0 0 0 0 0 0
RF 20 20 19 19 18 17 15 13 11 9 8 7 6 5 4 3 2 1 0 0 0 0 0 0
LR 20 20 19 19 18 17 15 13 11 9 8 6 5 3 1 0 0 0 0 0 0 0 0 0 <-------Problem wheel
RR 20 20 19 19 18 17 15 13 11 9 8 7 6 5 4 3 2 1 0 0 0 0 0 0
ABS activation-----------------------starts^ Stops^
VCDS will allow you to see this live, to identify the sensor that has the issue - either with the sensor itself, the wiring, or the gap / reluctor.

pdq import repair, I really don't know what does "all sensors are static" phrase mean! Once I find time off, I'll go to the master again.
Yes Windex, I agree, the VCDS will solve the problem, especialy if somebody can read it properly... I was supposed last year to buy one (VAG-COM), but... I know, my bad...