ABS engages at low speed

[deni2s]

After switching rear right trailing arm together with driveshaft and sensors from another z3, ABS started to engage at very low speeds. Other car's owner claims he hasn't never experienced that.

Found this great general article explaining many possible causes of the issue:
https://www.picoauto.com/library/cas...peed-abs-fault

So overall suspects are:
1) Damaged reluctor
2) Changed distance from WSS to the reluctor ring (due to corrosion on other reason), causing one wheel signal to be weaker than the others (case from the article)
3) Dirty WSS?
4) Different tooth count on switched reluctor ring?
5) Bad connection?

I still have a few Z3 specific questions:

1) We have only two wheel speed sensors on our cars correct? On which wheels are they?
2) Could reading the Z3 ABS block faults give some hint about issues mentioned above? I have no idea what information Z3 ABS block might contain - I was never looking into it.

Any suggestions?

[328 Power 04]

This is on the single vanos 2.8's that you own?
You should definitely read codes, there may be some there anyways.

There are 4 wheel speed sensors.

[deni2s]

Yes, on yellow US automatic.

UPDATE: Talked again with the owner of other car and seems like he had ABS disabled (due to some engine swaps he has done) so he wouldn't notice if something was already wrong here. This makes more options to diagnose.

[BimmerBreaker]

Install the sensors original to your car instead of the ones on the new components?

[deni2s]

UPDATE from the shop: ABS block had one error on front left WSS. After clearing it, the issue was still there, but somehow mechanic was not able to connect to ABS block. They tried some different more advanced software (don't know which one) and it showed no signals from WSS at all. (I understand the car was on a lift and they rotated the wheels to see signals.) So they called someone else to help them diagnose, who didn't appeared today, so postponed until Tuesday.

Install the sensors original to your car instead of the ones on the new components?

Unfortunately originals are gone. The shop doesn't want to install new ones before understanding the cause of the issue, they are afraid it will be "work and parts wasted".

[Randy Forbes]

UPDATE from the shop: ABS block had one error on front left WSS. After clearing it, the issue was still there, but somehow mechanic was not able to connect to ABS block. They tried some different more advanced software (don't know which one) and it showed no signals from WSS at all. (I understand the car was on a lift and they rotated the wheels to see signals.) So they called someone else to help them diagnose, who didn't appeared today, so postponed until Tuesday.



Unfortunately originals are gone. The shop doesn't want to install new ones before understanding the cause of the issue, they are afraid it will be "work and parts wasted".

With no signal, I'd be looking for broken__individual conductor**__wires. This is becoming more and more common; considerably more widespread than that pesky ol' trunkfloor/diff mount thing people have been going on about...

At the junction box in the wheelwell, they connect a known-good sensor (pulled off one of your other wheels) and by merely flicking a screwdriver back and forth across the end, mimic the wheel movement. At least that would verify if a clear path exists to the controller. Conversely, the offending WSS can be plugged into the junction at another wheelwell to prove/condemn its condition.

Point: I have yet to replace a non-functioning wheel speed sensor (owning my own Z3/Ms 3+ yrs for the Coupe, and >16 & 17 yrs for the two Rdstrs, plus the 100-some cars I've worked on). If I ever do have to replace one, I'll lay odds right now that it will be because of physical or impact damage to the case/housing or cable/connector getting smashed or ripped off.





** repeated flexing can fracture the individual copper strands inside the insulated sheath, and while the cabling may still appear to look normal, it may be no more effective at conducting an electrical signal than the tiny straw included with a can of WD-40 (or a Latvian equivalent aerosol product). This can make diagnosis tricky, and hence the suggestion of swapping sensors detailed above.

Good luck, and I hope they don't waste too much time going down a blind path!

[deni2s]

Randy, the problem is - they can't find the signal on any wheel...
And I am not sure if I myself had ever braked so hard on that car to engage the ABS. I had the car for like a couple years, one of them the car was sitting in a barn. And before it was used quite extensively even in snowy winters, lots of rust underneath, so I wouldn't wonder if all sensors would actually be dead.

[Randy Forbes]

Probably not, but if they can at least establish communications with the ABS module, that's a good sign, because they go toes up all too frequently too.

[deni2s]

Yes, let's see what they will come up with on Tuesday.

[deni2s]

SOLVED: Guy invited diagnosed issue with rear right WSS (the one on replaced trailing arm). Replacing WSS solved this issue.