2004 xc70 front hub ABS

[aPpYe]

2004 Volvo XC70 Automatic ...

About a month ago, I bought this vehicle and replaced the right-front cv shaft and hub/wheel bearing assemblies. About a week (and perhaps 400 miles) later, I was getting the ABS light, the little orange triangle with an exclamation point in the middle, and a message about ABS and anti-skid system. I don’t remember the exact verbiage of the message, as I don’t have the car with me at the moment. I also noticed some pulsing and grabbing of the brakes at low speeds, especially when coming to a stop. The AWD system would also come on and off, mostly during hard turns at low speed. I pulled the wheel and found the center axle bolt had backed completely out. Either I did not torque it at all or did not torque it enough. I torqued it down nice and tight with some thread locker, and cleaned a small amount of soot and grease from the sensor. I reassembled it and the symptoms went away. All was good the dash had no warnings...

A week later, the same exact symptoms showed up. I pulled the wheel again. This time, I double checked that the bolt was tight, which it was. I tightened it a little more for good measure. I pulled the sensor again and blew out everything in and even remotely around the hub with compressed air until no more dust came away. I then sprayed some brake cleaner down into the sensor hole while rotating the hub to make sure the nibs that the sensor are supposed to see were squeaky clean. I reassembled and all the issues went away.

Another week later, the same symptoms came right back. This time I made sure to get a brand new axle bolt from Volvo (p/n 30640878) and torqued it to spec, 35Nm plus 90 degrees. I did not pull the wheel or the sensor this time. I drove the car and everything went away again, but only for about fifteen minutes before the symptoms reappeared, same AWD thing coming up, same occasional grab/pulse of the breaks when coming to a stop, same lights and the same warning on the dashboard.

Besides having a weekly ritual of wiping down the sensor, does anyone have any ideas on what I can do here? I am out of ideas ...

[petey]

The sensor is designed to see through an amount of noise so cleaning it can reduce the noise, just, to an acceptable level, but the sensor / abs reluctor ring could be fubar’d. Rust gets going underneath the ring, swells it and then it cracks. Looking at the signal through VIDA will give you a clue there I think.

But it’s just as possible that the wheel bearing is sub-prime and that excessive lateral movement is putting and undue load on the driveshaft bolt.

[aPpYe]

Originally Posted by petey

The sensor is designed to see through an amount of noise so cleaning it can reduce the noise, just, to an acceptable level, but the sensor / abs reluctor ring could be fubar’d. Rust gets going underneath the ring, swells it and then it cracks. Looking at the signal through VIDA will give you a clue there I think.

But it’s just as possible that the wheel bearing is sub-prime and that excessive lateral movement is putting and undue load on the driveshaft bolt.

It's a brand new sensor ring though. It comes with the cv axle shaft assembly. The hub/wheel bearing assembly is also new. Also, when I was cleaning it with the air and parts cleaner, I shined a light down in the sensor hole, I did not see any cracks ... I will double check. As for the wheel bearing, wouldn't the fact that cleaning it fixes the problem for a while rule that out?

[petey]

If the signal to noise ratio is just tolerable, cleaning, or tightening or some other small change might shift it one way or another...

[MarvinMitt]

Here's what my unlikely problem was after I removed the sensor once.
I had a very long threaded bolt holding the sensor down and it didn't want to go all the way in (felt tight but wasn't in) and not locking the sensor down.
I tightened it and broke it in the spindle. Had to weld a nut on top and barely got the remains of it out.
I put a shorter bolt in and it's fine.